CWDRSSTC
Dr. Dark Current, Fri Nov 11 2011, 07:53PMHi all,
I was planning to build a large SSTC, running on halfwave rectified unfiltered two-phase voltage (400 Vac), in the several kW range. However then I realised that the switching losses would probably be huge and left the idea.
But then came to my mind, what if I used a tuned primary with primary current feedback and zero current switching, which would greatly decrease switching losses? The primary impedance would, of course, need to be increased over a standard DRSSTC.
Before I start putting it together, do you think this is even practically doable?
Here are some preliminary data on the test setup:
The average input power was chosen at 4 kW, which means approximately 16 kW peak power at 560 V bus voltage. The coil would use an IGBT half bridge, so the output current at input power peak is 16 kW / 280 V = 57 amps. I selected a Q of 10 (this is the part I'm most unsure about) and calculated the primary cap to have 2800 volt peak on it at 80 kHz, which turned out to be 57 nanofarads.
The primary impedance is adjusted by tuning the primary on the lower slope of the resonant peak (=tuning and detuning below secondary resonance).
Does this sound right?
Re: CWDRSSTC
Steve Conner, Fri Nov 11 2011, 08:01PM
The first DRSSTC I ever saw was a CW one built by Richie B, with MOSFETs. The pulsed ones are a new thing!
He used his usual SSTC bridge with a 4.7nF high voltage ceramic tube cap from a transmitter. It lasted about 10 seconds before exploding.
DRSSTCs "sort of work" with a wide range of tank LC ratios. I've tried 10, 50 and 100nF at 220kHz. It doesn't seem to be terribly critical: 57nF at 80kHz would probably work.
Steve Conner, Fri Nov 11 2011, 08:01PM
The first DRSSTC I ever saw was a CW one built by Richie B, with MOSFETs. The pulsed ones are a new thing!
He used his usual SSTC bridge with a 4.7nF high voltage ceramic tube cap from a transmitter. It lasted about 10 seconds before exploding.
DRSSTCs "sort of work" with a wide range of tank LC ratios. I've tried 10, 50 and 100nF at 220kHz. It doesn't seem to be terribly critical: 57nF at 80kHz would probably work.
Re: CWDRSSTC
Goodchild, Fri Nov 11 2011, 09:47PM
You know basically what you are building is QCW type coil.
The tank impedance is high to help keep the current low, you are also using unfiltered half wave aka "the ramp"
VTTC run is basically the same operation supper high tank Z and a CW or half wave operation.
Goodchild, Fri Nov 11 2011, 09:47PM
You know basically what you are building is QCW type coil.
The tank impedance is high to help keep the current low, you are also using unfiltered half wave aka "the ramp"
VTTC run is basically the same operation supper high tank Z and a CW or half wave operation.
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 01:02AM
Hi guys
Let me throw some of my experimental data. I've been playing with and improving my CW coil over time and am also about to take this route. But it's not as much because of switching losses alone, but the problem of having to drive such extreme reactive power through the low inductance primary.
My coil is so far still a normal SSTC and uses a full bridge of HGTG30N60A4D IGBT's which keep proving their extraordinary resistance to punishment, having ran up to 10kW input. They actually outlived all expectations and I have almost used up all my available socket current - yet it produces such amazing CW plasma that I just keep wanting more.
I installed a current limiter into my coil which monitors the bridge peak current, and it hit over 80A (in CW, so RMS current had to be around 60 amps!)
I didn't want to continue with any more of this completely unnecessary punishment for the IGBT's (well, I also had a problem of my cooling fans slowing to a crawl from interference, and have to replace them with AC fans). So I decided it would be time to consider a resonant primary topology. I had several considerations:
1. LCLR - I dismissed this soon after learning that it will actually not lead to much improvement in power factor compared to now.
2. Current fed inverter - Good PF, but unexplored topology for this power level and would require significant alterations on my current installations in this coil. I would very much like to explore this further though, especially for high frequency coils.
3. Series impedance matching network base feed - basically a DRSSTC, but with direct base feed of the secondary from the primary tank! The bridge drives a serier LC circuit of fairly high impedance, and the base of the secondary goes in between L and C. While this might look troublesome, a beefy inductor is actually far easier to design than a HV transformer (you can also finally put to use all those AM radio ferrite rods you have around). You could probably just use an air cored inductor like a spool of wire too, and C could be so small that you can use a big air variable cap for it (if you have one!). Would be very fun to try in my opinion.
4. High impedance series resonant - basically what you intend to do, use a high voltage MMC and a primary with many turns so you can drive this directly from your bridge. I was considering this heavily until I done some experiments - using a 100nF MMC which resonated well with my current primary, my input current skyrocketed even with just 20-30V input. So I don't think a 50 or so nF MMC will do for you, you will want more like the 3.3nF Steve mentioned earlier!
Note that this is also a tradeoff between your desired coupling and the tank impedance - the more weakly coupled primary will load your inverter harder, and you'll need to increase it's impedance to counter this. If you're building a coil in this topology from start, I assume you don't want the pointless risk with overcoupling (like it's done with normal SSTC's) so that is another thing to consider.
Only a good FEMM simulation or practical experimentation can lead you to the answer on what parameters you need.
The big drawback will definitely be the costly and cumbersome primary coil which will need to have several tens of turns of high current conductor (prefferably copper pipe) which you need to design into low enough profile so that it doesn't present a flashover risk.
I would try using several helical coils inside each other, possibly the inner ones being cable and only the outer one made of pipe to reduce the bending trouble.
The benefit here is that you can use transmitting doorknob caps for your tank cap instead of constructing a cumbersome MMC.
5. Low impedance series resonant: This is what I intend to do with my coil right now. Basically it's the same as above, but keeps the fairly normal primary and a DRSSTC-sized tanks cap. And instead of driving this directly, the bridge is coupled over a big step-down ferrite transformer. I got a nice ferrite toroid for this today and might carry out some quick experiments tomorrow.
With unity power factor on my bridge, it will be carrying only like 25 amps instead of those 60! So theoretically I will have no trouble pushing 20kW or even more considering I now have zero current switching... and you think that will make me satisfied? Not really - the next thing I'll probably want to do is replace the igbt's with some of those 1200V, 72A puppies whose dies are so large that no diode can fit in... but small external diodes can be used now since they have to carry only the transformer magnetizing current! The good thing is that garage breakers are unscrewable type, so I could temporarily hack in some 25A ones!
And when I truly end up with no more power available, that won't stop me ofcourse... I'll nicely ask at college for a high current 3 phase lab feed, and truly give an epic ending to this without caring if it turns into one big arc flash in the end. (BTW, No joke at all, I'm dead serious about seeing 50kW from that thing, if at least for few seconds. *epic laugh with arms raised and enormous CW sparks in background*
Anyways, enough of motivational talk - kilovolt, it's on you to decide what topology to choose (or whether you want to make your coil experimental and try them all!) Make sure to refer to Big Bad, the CW coil thread for more information!
One other challenge you'll probably face is also that there's not even a good, reliable SSTC driver circuit around that can keep lock with full and half wave rectified input well. I'm in process of designing one too!
Cheers, Marko
Marko, Sat Nov 12 2011, 01:02AM
Hi guys
Let me throw some of my experimental data. I've been playing with and improving my CW coil over time and am also about to take this route. But it's not as much because of switching losses alone, but the problem of having to drive such extreme reactive power through the low inductance primary.
My coil is so far still a normal SSTC and uses a full bridge of HGTG30N60A4D IGBT's which keep proving their extraordinary resistance to punishment, having ran up to 10kW input. They actually outlived all expectations and I have almost used up all my available socket current - yet it produces such amazing CW plasma that I just keep wanting more.
I installed a current limiter into my coil which monitors the bridge peak current, and it hit over 80A (in CW, so RMS current had to be around 60 amps!)
I didn't want to continue with any more of this completely unnecessary punishment for the IGBT's (well, I also had a problem of my cooling fans slowing to a crawl from interference, and have to replace them with AC fans). So I decided it would be time to consider a resonant primary topology. I had several considerations:
1. LCLR - I dismissed this soon after learning that it will actually not lead to much improvement in power factor compared to now.
2. Current fed inverter - Good PF, but unexplored topology for this power level and would require significant alterations on my current installations in this coil. I would very much like to explore this further though, especially for high frequency coils.
3. Series impedance matching network base feed - basically a DRSSTC, but with direct base feed of the secondary from the primary tank! The bridge drives a serier LC circuit of fairly high impedance, and the base of the secondary goes in between L and C. While this might look troublesome, a beefy inductor is actually far easier to design than a HV transformer (you can also finally put to use all those AM radio ferrite rods you have around). You could probably just use an air cored inductor like a spool of wire too, and C could be so small that you can use a big air variable cap for it (if you have one!). Would be very fun to try in my opinion.
4. High impedance series resonant - basically what you intend to do, use a high voltage MMC and a primary with many turns so you can drive this directly from your bridge. I was considering this heavily until I done some experiments - using a 100nF MMC which resonated well with my current primary, my input current skyrocketed even with just 20-30V input. So I don't think a 50 or so nF MMC will do for you, you will want more like the 3.3nF Steve mentioned earlier!
Note that this is also a tradeoff between your desired coupling and the tank impedance - the more weakly coupled primary will load your inverter harder, and you'll need to increase it's impedance to counter this. If you're building a coil in this topology from start, I assume you don't want the pointless risk with overcoupling (like it's done with normal SSTC's) so that is another thing to consider.
Only a good FEMM simulation or practical experimentation can lead you to the answer on what parameters you need.
The big drawback will definitely be the costly and cumbersome primary coil which will need to have several tens of turns of high current conductor (prefferably copper pipe) which you need to design into low enough profile so that it doesn't present a flashover risk.
I would try using several helical coils inside each other, possibly the inner ones being cable and only the outer one made of pipe to reduce the bending trouble.
The benefit here is that you can use transmitting doorknob caps for your tank cap instead of constructing a cumbersome MMC.
5. Low impedance series resonant: This is what I intend to do with my coil right now. Basically it's the same as above, but keeps the fairly normal primary and a DRSSTC-sized tanks cap. And instead of driving this directly, the bridge is coupled over a big step-down ferrite transformer. I got a nice ferrite toroid for this today and might carry out some quick experiments tomorrow.
With unity power factor on my bridge, it will be carrying only like 25 amps instead of those 60! So theoretically I will have no trouble pushing 20kW or even more considering I now have zero current switching... and you think that will make me satisfied? Not really - the next thing I'll probably want to do is replace the igbt's with some of those 1200V, 72A puppies whose dies are so large that no diode can fit in... but small external diodes can be used now since they have to carry only the transformer magnetizing current! The good thing is that garage breakers are unscrewable type, so I could temporarily hack in some 25A ones!
And when I truly end up with no more power available, that won't stop me ofcourse... I'll nicely ask at college for a high current 3 phase lab feed, and truly give an epic ending to this without caring if it turns into one big arc flash in the end. (BTW, No joke at all, I'm dead serious about seeing 50kW from that thing, if at least for few seconds. *epic laugh with arms raised and enormous CW sparks in background*
Anyways, enough of motivational talk - kilovolt, it's on you to decide what topology to choose (or whether you want to make your coil experimental and try them all!) Make sure to refer to Big Bad, the CW coil thread for more information!
One other challenge you'll probably face is also that there's not even a good, reliable SSTC driver circuit around that can keep lock with full and half wave rectified input well. I'm in process of designing one too!
Cheers, Marko
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 08:25AM
Hi Marko,
thank you for your comprehensive reply.
The problem with the small tank caps is huge voltage on them. The bridge output current cannot be changed and is a function of output power - in my case I calculated 57 amps RMS, if I used a 4700pf cap, there would be 24 kV RMS (34 kV peak) on it at 80 kHz !!!
I think the big increase in current with small voltage you observed is normal. The current should rise close to the maximum value at low supply voltage, and with increasing the voltage it should remain almost constant.
The key here is to properly tune the primary - if it's too far away from secondary resonance, the current will indeed rise to huge values. Tuning closer to secondary resonance (always tune below secondary resonance) makes it "suck" more energy from the tank, decreasing the current. The tank circuit must be tuned so that just the right energy transfer happens, and from VTTC experience this "sweet spot" is rather narrow. But I haven't tried this so far so I can't say for sure.
Dr. Dark Current, Sat Nov 12 2011, 08:25AM
Hi Marko,
thank you for your comprehensive reply.
The problem with the small tank caps is huge voltage on them. The bridge output current cannot be changed and is a function of output power - in my case I calculated 57 amps RMS, if I used a 4700pf cap, there would be 24 kV RMS (34 kV peak) on it at 80 kHz !!!
I think the big increase in current with small voltage you observed is normal. The current should rise close to the maximum value at low supply voltage, and with increasing the voltage it should remain almost constant.
The key here is to properly tune the primary - if it's too far away from secondary resonance, the current will indeed rise to huge values. Tuning closer to secondary resonance (always tune below secondary resonance) makes it "suck" more energy from the tank, decreasing the current. The tank circuit must be tuned so that just the right energy transfer happens, and from VTTC experience this "sweet spot" is rather narrow. But I haven't tried this so far so I can't say for sure.
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 11:52AM
Hi kilovolt,
well, yeah, it won't be easy to find the right RF doorknob caps for the application and will surely cost a lot. You might in the end resort to using an MMC, but it will be bulky as well with many small value caps in series per string. That's another reason why I opted for the transformer option.
Oh, I also didn't mention that I'm still using secondary feedback, and will continue doing so while tuning the primary slightly inductive.
I think Steve ward ran his QCW coil in true CW with about 20nF cap, but he used what I would think of as insanely high coupling (which apparently didn't matter anymore once the spark growth was actively controlled... but arcover in CW would certainly be a disaster)
By the way, I just bought some of these caps for my MMC, they are a great bargain (just don't buy them all! :) )
Oh, and have you even considered trying a base feed by a matching network? It sounds like a lot of fun and will be also confusing for most people to see a coil sprouting big sparks without a primary coil around it..
And heck, if you think this all is troubling and just want to have a reliable 4kW coil with no ominous greed for more power... just wind a secondary for about 100kHz and use a full bridge of HGTG30N60A4D's, I've tested this up to at least 10kW already! But I must warn you... 4kW CW sparks don't actually look so amazing as you would think, unless you make a smallish tabletop coil.
Marko
Marko, Sat Nov 12 2011, 11:52AM
Hi kilovolt,
well, yeah, it won't be easy to find the right RF doorknob caps for the application and will surely cost a lot. You might in the end resort to using an MMC, but it will be bulky as well with many small value caps in series per string. That's another reason why I opted for the transformer option.
Oh, I also didn't mention that I'm still using secondary feedback, and will continue doing so while tuning the primary slightly inductive.
I think Steve ward ran his QCW coil in true CW with about 20nF cap, but he used what I would think of as insanely high coupling (which apparently didn't matter anymore once the spark growth was actively controlled... but arcover in CW would certainly be a disaster)
By the way, I just bought some of these caps for my MMC, they are a great bargain (just don't buy them all! :) )
Oh, and have you even considered trying a base feed by a matching network? It sounds like a lot of fun and will be also confusing for most people to see a coil sprouting big sparks without a primary coil around it..
And heck, if you think this all is troubling and just want to have a reliable 4kW coil with no ominous greed for more power... just wind a secondary for about 100kHz and use a full bridge of HGTG30N60A4D's, I've tested this up to at least 10kW already! But I must warn you... 4kW CW sparks don't actually look so amazing as you would think, unless you make a smallish tabletop coil.
Marko
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 12:16PM
Marko,
I need to use a two phase (400 volt) supply for the coil, the 230 volts here only go up to 16 amps (which means at most a 2.6 kW coil supplied from a half wave rectifier).
For this reason I bought 1200 V IGBTs, which are somewhat slower than 600 V ones, so I want to use a ZCS topology to decrease their switching losses.
I'm not sure about the base fed topology, as I need to use primary current feedback and here the tank would need to be tuned exactly to secondary resonance, I think detuning with spark loading would be a problem here.
Dr. Dark Current, Sat Nov 12 2011, 12:16PM
Marko,
I need to use a two phase (400 volt) supply for the coil, the 230 volts here only go up to 16 amps (which means at most a 2.6 kW coil supplied from a half wave rectifier).
For this reason I bought 1200 V IGBTs, which are somewhat slower than 600 V ones, so I want to use a ZCS topology to decrease their switching losses.
I'm not sure about the base fed topology, as I need to use primary current feedback and here the tank would need to be tuned exactly to secondary resonance, I think detuning with spark loading would be a problem here.
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 03:31PM
Hi Jan,
Well I've just made some of the first successful runs of the CWDRSSTC. I used a 27:9 transformer and a 125nF MMC, and actually this already seems quite reasonable for the coupling I'm using. So if you insist on using a half bridge and are OK with typical SSTC-like coupling your idea of direct drive with a 57 nF MMC might not be so far fetched at all, especially if you'll be minimizing your average power by using half or full wave input rectification.
I have a big 3 phase variac so varying supply voltage is not a problem for me, but you could still use 600V IGBT's and perhaps a buck transformer in the input if you're afraid of slamming full 560V onto them.
On the other hand, I intend to switch on 1200V IGBT's too, particularly these ones
and run them up to 1000V supply... they have much steeper temperature/current de-rating than 30N60's but are actually quite fast.
One thing I might in the end do is re-tying my H bridge to work as two half bridges in parallel, and leave out the transformer altogether for the most insanely powered runs... but we'll see what comes of this in the end. My camera has charged so going to do some more runs and post pics and vids here.
Cheers,
Marko
Marko, Sat Nov 12 2011, 03:31PM
Hi Jan,
Well I've just made some of the first successful runs of the CWDRSSTC. I used a 27:9 transformer and a 125nF MMC, and actually this already seems quite reasonable for the coupling I'm using. So if you insist on using a half bridge and are OK with typical SSTC-like coupling your idea of direct drive with a 57 nF MMC might not be so far fetched at all, especially if you'll be minimizing your average power by using half or full wave input rectification.
I have a big 3 phase variac so varying supply voltage is not a problem for me, but you could still use 600V IGBT's and perhaps a buck transformer in the input if you're afraid of slamming full 560V onto them.
On the other hand, I intend to switch on 1200V IGBT's too, particularly these ones
and run them up to 1000V supply... they have much steeper temperature/current de-rating than 30N60's but are actually quite fast.
One thing I might in the end do is re-tying my H bridge to work as two half bridges in parallel, and leave out the transformer altogether for the most insanely powered runs... but we'll see what comes of this in the end. My camera has charged so going to do some more runs and post pics and vids here.
Cheers,
Marko
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 03:51PM
Cool Looking forward to the pictures.
The idea of this was to keep the coil simple, so no buck converters for me. Just a directly amplifying 74HC14 feedback and two gate drivers. The IGBTs I have are these I intend to put 2 and 2 in parallel for the 4 kW half bridge.
Dr. Dark Current, Sat Nov 12 2011, 03:51PM
Cool
Looking forward to the pictures.The idea of this was to keep the coil simple, so no buck converters for me. Just a directly amplifying 74HC14 feedback and two gate drivers. The IGBTs I have are these
I intend to put 2 and 2 in parallel for the 4 kW half bridge.Re: CWDRSSTC
Marko, Sat Nov 12 2011, 05:46PM
Hi sir kilovolt,
First, a video of a run at about 8kW, set the current limiter to some 40A peak and it never tripped until the full 400V input (limited by my DC link cap rating, really wouldn't want those to blow up)
Now, some pics of the setup.
Here's visible the ferrite transformer, as well as 125nF DRSSTC MMC I used as a tank cap. While the transformer did get warm in operation, the caps didn't even break a sweat at this power level (so the MMC is indeed fairly overkill)
The mess of the front of the coil, still with the driver board I'm unhappy with.
The whole system, ready to be fired up
Now, some more comments.
Jan - firstly, don't even think about using the 74HC14 input thing in your driver, it sucks. This is a Steve Ward's "invention" that has been plaguing people for years. I actually ended up feeding the base current sense CT directly into the inputs of UCC's, and this seemed to work quite well. But when I tried to recreate the same with other gate drivers, or using 74HC14 as a buffer, this resulted in overwhelmingly bizzare driver behavior and blown switching devices.
With the huge hysteresis 74HC14 introduces in the system, you need a powerful stimulus from a starting oscillator or interrupter to start it up (especially with half wave pulsed supply) and this seems to introduce huge problems because of later interference of the oscillator with the operation.
My coil still uses UCC's with a TL494 for startup (see the schematic in project thread) but it only ever seemed to work well in CW and after I introduced the dual resonance now the things got far worse, I can barely start the coil up and in short it's no good at all.
To make things worse, Steve ward designed a PLL driver which also had problems with modulated input (which he solved using the bilateral switch hack). It seemed to work for a while, but apparently he dismantled the coil because his mosfets kept blowing up (I think this is mainly due to use of isotop devices, to make worse, mosfets, which couldn't take the thermal load).
One could wonder, if we opt out using the Steve's PLL circuit, is there even a single reliable CW/HWR/FWR driver in existence?
The problem with the upper approach is the fact that input of 74HC14 is extremely poorly biased for small signals. We really want a circuit that will start at few mV input, and this is really a job for a good fast comparator. This is what Steve's very first SSTC's actually used.
For start, we can connect + pin of the comparator to say +5V and between it and another insert a CT shunt resistor between + and - . We're now having a huge loop gain compared to before and even tiny signals could trigger the oscillation. If noise is not enough, a very weakly coupled starting oscillator should do the trick.
External trimmers may be used to null the offset even further if necessary.
I'm in process of designing such a driver, and it will replace the old one from this coil in the scheduled refit.
Cheers - Marko
Marko, Sat Nov 12 2011, 05:46PM
Hi sir kilovolt,
First, a video of a run at about 8kW, set the current limiter to some 40A peak and it never tripped until the full 400V input (limited by my DC link cap rating, really wouldn't want those to blow up)
Now, some pics of the setup.
Here's visible the ferrite transformer, as well as 125nF DRSSTC MMC I used as a tank cap. While the transformer did get warm in operation, the caps didn't even break a sweat at this power level (so the MMC is indeed fairly overkill)
The mess of the front of the coil, still with the driver board I'm unhappy with.
The whole system, ready to be fired up
Now, some more comments.
Jan - firstly, don't even think about using the 74HC14 input thing in your driver, it sucks. This is a Steve Ward's "invention" that has been plaguing people for years. I actually ended up feeding the base current sense CT directly into the inputs of UCC's, and this seemed to work quite well. But when I tried to recreate the same with other gate drivers, or using 74HC14 as a buffer, this resulted in overwhelmingly bizzare driver behavior and blown switching devices.
With the huge hysteresis 74HC14 introduces in the system, you need a powerful stimulus from a starting oscillator or interrupter to start it up (especially with half wave pulsed supply) and this seems to introduce huge problems because of later interference of the oscillator with the operation.
My coil still uses UCC's with a TL494 for startup (see the schematic in project thread) but it only ever seemed to work well in CW and after I introduced the dual resonance now the things got far worse, I can barely start the coil up and in short it's no good at all.
To make things worse, Steve ward designed a PLL driver which also had problems with modulated input (which he solved using the bilateral switch hack). It seemed to work for a while, but apparently he dismantled the coil because his mosfets kept blowing up (I think this is mainly due to use of isotop devices, to make worse, mosfets, which couldn't take the thermal load).
One could wonder, if we opt out using the Steve's PLL circuit, is there even a single reliable CW/HWR/FWR driver in existence?
The problem with the upper approach is the fact that input of 74HC14 is extremely poorly biased for small signals. We really want a circuit that will start at few mV input, and this is really a job for a good fast comparator. This is what Steve's very first SSTC's actually used.
For start, we can connect + pin of the comparator to say +5V and between it and another insert a CT shunt resistor between + and - . We're now having a huge loop gain compared to before and even tiny signals could trigger the oscillation. If noise is not enough, a very weakly coupled starting oscillator should do the trick.
External trimmers may be used to null the offset even further if necessary.
I'm in process of designing such a driver, and it will replace the old one from this coil in the scheduled refit.
Cheers - Marko
Re: CWDRSSTC
Goodchild, Sat Nov 12 2011, 06:16PM
You know one of the things that makes the QCW so wonderful is the fact that it's tank voltage is rather low. A result of the lower current.
The tank Z is not that high, usually it resides in the 25 to 30 ohm range although this seems high, when you are only running 150A to 200A this still only equates to 6Kv peak at the worst case! The tank in my QCW usually doesn't see more than 4.5Kv under normal operation.
I have also run my QCW in CW mode and the results are less that exciting. Other than a big bush of plasma and a lot of power draw and melted tungsten breakout point (yes tungsten
) I have conclude that CW is not all that great. I think the highish duty cycles say in the 30% to 60% range are the butter zone.
Running a coil like the one you are talking about building using half rec mains would be a very similar operation and run at about 50% duty. This should make for very nice operation. The current should stay under 100A in most cases keeping your tank cap happy.
The QCW is a great type coil because it's mode of operation is very controlled. lots of control means longer sparks and less power draw.
Hope this helps,
Eric
Goodchild, Sat Nov 12 2011, 06:16PM
You know one of the things that makes the QCW so wonderful is the fact that it's tank voltage is rather low. A result of the lower current.
The tank Z is not that high, usually it resides in the 25 to 30 ohm range although this seems high, when you are only running 150A to 200A this still only equates to 6Kv peak at the worst case! The tank in my QCW usually doesn't see more than 4.5Kv under normal operation.
I have also run my QCW in CW mode and the results are less that exciting. Other than a big bush of plasma and a lot of power draw and melted tungsten breakout point (yes tungsten
) I have conclude that CW is not all that great. I think the highish duty cycles say in the 30% to 60% range are the butter zone.Running a coil like the one you are talking about building using half rec mains would be a very similar operation and run at about 50% duty. This should make for very nice operation. The current should stay under 100A in most cases keeping your tank cap happy.
The QCW is a great type coil because it's mode of operation is very controlled. lots of control means longer sparks and less power draw.
Hope this helps,
Eric
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 06:44PM
Hey Goodchild
Well, as a result of accumulated brain damage I have a special fetish for true CW sparks... being so hot and powerful, as well as the sheer excitement of 10's of kW going in with nothing blowing up makes all the pulsed sparks look 'faux'... but yeah I'd definitely love to see what would QCW sparks look with 10kW input as well!
I don't think using a buck converter is necessary at all, just having a comparator fed with a ramp and it's output acting to control the current should do well enough in my opinion.
Marko
Marko, Sat Nov 12 2011, 06:44PM
Hey Goodchild
Well, as a result of accumulated brain damage I have a special fetish for true CW sparks... being so hot and powerful, as well as the sheer excitement of 10's of kW going in with nothing blowing up makes all the pulsed sparks look 'faux'... but yeah I'd definitely love to see what would QCW sparks look with 10kW input as well!
I don't think using a buck converter is necessary at all, just having a comparator fed with a ramp and it's output acting to control the current should do well enough in my opinion.
Marko
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 06:45PM
Nice work, Marko.
Over the years, I came up with various start-up oscillators for direct amplification feedback. I'll attach a circuit I'm going to use for the coil. Without feedback, it works as a normal hysteresis oscillator, but after the tank current rings up a bit, the oscillation is blocked and the inverter starts amplifying the feedback signal. The diode thyngus is just a bipolar voltage clamp. Note that this circuit will need a real-world testing, but I'm convinced it will work.
Goodchild, did you observe what effect the tank capacitance had on the behavior of the coil?
Dr. Dark Current, Sat Nov 12 2011, 06:45PM
Nice work, Marko.
Over the years, I came up with various start-up oscillators for direct amplification feedback. I'll attach a circuit I'm going to use for the coil. Without feedback, it works as a normal hysteresis oscillator, but after the tank current rings up a bit, the oscillation is blocked and the inverter starts amplifying the feedback signal. The diode thyngus is just a bipolar voltage clamp. Note that this circuit will need a real-world testing, but I'm convinced it will work.
Goodchild, did you observe what effect the tank capacitance had on the behavior of the coil?
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 07:46PM
Hm, as far as I remember this is similar to what kim ladha used for his "OMG induction heater" from a while ago, and as far as I know it tended to blow up too.
I'm not sure what kind of instabilities might be happening there (perhaps someone with knowledge on nonlinear systems could help) but from the noises I could hear from the inverter itself I think they were pretty bad. I once had a fullbridge of 47N50 mosfets blow instantly upon switching the power on with no explanation at all.
From what I know, your oscillator has to have some sort of hysteresis regarding the signal that disables it too, otherwise it will fight it. So once the feedback signal appears it won't start back up until it has well fallen to zero. I've actually made a driver that does all this, but it turned overly complex and for some reason (probably poor thermal design) that coil was a mosfet eater so I abandoned the approach.
Finally, one simple advice that can save you countless IGBT lives - use overcurrent protection! No matter if you even use DR or go hard switched, a simple CT on the bridge output, a LM311 and a NE555 will prevent virtually any damaging overcurrent fault (apart from shoot through, but this will not be a problem if you use GDT's).
Steve Ward's original SSTC's seemed to work excellently with FWR and HWR inputs and were quite simple designs, yet hardly anyone makes reference to them today.
Marko
Marko, Sat Nov 12 2011, 07:46PM
Hm, as far as I remember this is similar to what kim ladha used for his "OMG induction heater" from a while ago, and as far as I know it tended to blow up too.
I'm not sure what kind of instabilities might be happening there (perhaps someone with knowledge on nonlinear systems could help) but from the noises I could hear from the inverter itself I think they were pretty bad. I once had a fullbridge of 47N50 mosfets blow instantly upon switching the power on with no explanation at all.
From what I know, your oscillator has to have some sort of hysteresis regarding the signal that disables it too, otherwise it will fight it. So once the feedback signal appears it won't start back up until it has well fallen to zero. I've actually made a driver that does all this, but it turned overly complex and for some reason (probably poor thermal design) that coil was a mosfet eater so I abandoned the approach.
Finally, one simple advice that can save you countless IGBT lives - use overcurrent protection! No matter if you even use DR or go hard switched, a simple CT on the bridge output, a LM311 and a NE555 will prevent virtually any damaging overcurrent fault (apart from shoot through, but this will not be a problem if you use GDT's).
Steve Ward's original SSTC's seemed to work excellently with FWR and HWR inputs and were quite simple designs, yet hardly anyone makes reference to them today.
Marko
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 07:55PM
Marko, I'm going to use this circuit for primary current feedback (with two cascaded CTs). Once the primary current rings up, the signal should be strong and nothing (eg. ground arcs) can disturb it. (note a too short arc can cause the primary current to ring up to a dangerous value)
I would never use a direct amplification feedback for secondary base or antenna input, as the signal is lost during ground arcs and the feedback loop can start to act erratic.
Dr. Dark Current, Sat Nov 12 2011, 07:55PM
Marko, I'm going to use this circuit for primary current feedback (with two cascaded CTs). Once the primary current rings up, the signal should be strong and nothing (eg. ground arcs) can disturb it. (note a too short arc can cause the primary current to ring up to a dangerous value)
I would never use a direct amplification feedback for secondary base or antenna input, as the signal is lost during ground arcs and the feedback loop can start to act erratic.
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 08:17PM
Well, what I meant was more like a comparator with hysteresis (or at least another HC14 gate used as one) to independently sense the amplitude of primary current and turn off the oscillator, until the current drops too low again. If you let the primary current "fight" the oscillator for gain in linear region the current may never ring up to fully override it and cause assorted problems (IIRC, this circuit was particularly fiddly in induction heaters ad and had to be tuned finely to resonant frequency of the tank). It's not that I want to pester you, but I just had a lot of bad experience with these circuits.
BTW, I can upload my version of this thing if you're interested (and If I can find it!).
I still think that using a sensitive fast comparator (for 100kHz range, LM119 and like will be more than good enough) instead of going through all this trouble.
Or heck, why not just use PLL (Copy Steve Ward's circuit, add overcurrent protection and better heatsinking to the devices!).
Marko
Marko, Sat Nov 12 2011, 08:17PM
Dr. Kilovolt wrote ...
Marko, I'm going to use this circuit for primary current feedback (with two cascaded CTs). Once the primary current rings up, the signal should be strong and nothing (eg. ground arcs) can disturb it. (note a too short arc can cause the primary current to ring up to a dangerous value)
I would never use a direct amplification feedback for secondary base or antenna input, as the signal is lost during ground arcs and the feedback loop can start to act erratic.
Marko, I'm going to use this circuit for primary current feedback (with two cascaded CTs). Once the primary current rings up, the signal should be strong and nothing (eg. ground arcs) can disturb it. (note a too short arc can cause the primary current to ring up to a dangerous value)
I would never use a direct amplification feedback for secondary base or antenna input, as the signal is lost during ground arcs and the feedback loop can start to act erratic.
Well, what I meant was more like a comparator with hysteresis (or at least another HC14 gate used as one) to independently sense the amplitude of primary current and turn off the oscillator, until the current drops too low again. If you let the primary current "fight" the oscillator for gain in linear region the current may never ring up to fully override it and cause assorted problems (IIRC, this circuit was particularly fiddly in induction heaters ad and had to be tuned finely to resonant frequency of the tank). It's not that I want to pester you, but I just had a lot of bad experience with these circuits.
BTW, I can upload my version of this thing if you're interested (and If I can find it!).
I still think that using a sensitive fast comparator (for 100kHz range, LM119 and like will be more than good enough) instead of going through all this trouble.
Or heck, why not just use PLL (Copy Steve Ward's circuit, add overcurrent protection and better heatsinking to the devices!).
Marko
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 08:29PM
Of course the circuit I posted is for oscillator/resonant frequency feedback, not overcurrent protection. If I was going to use an overcurrent protection, I would put a shunt resistor between the two CTs (to eliminate the need for another CT) and use this to control a small SCR which would shut down the gate drivers through their enable pins.
Sure you can upload the circuit if you want, I'll take a look.
Dr. Dark Current, Sat Nov 12 2011, 08:29PM
Marko wrote ...
Well, what I meant was more like a comparator with hysteresis (or at least another HC14 gate used as one) to independently sense the amplitude of primary current and turn off the oscillator, until the current drops too low again.
Well, what I meant was more like a comparator with hysteresis (or at least another HC14 gate used as one) to independently sense the amplitude of primary current and turn off the oscillator, until the current drops too low again.
Of course the circuit I posted is for oscillator/resonant frequency feedback, not overcurrent protection. If I was going to use an overcurrent protection, I would put a shunt resistor between the two CTs (to eliminate the need for another CT) and use this to control a small SCR which would shut down the gate drivers through their enable pins.
Sure you can upload the circuit if you want, I'll take a look.
Re: CWDRSSTC
Marko, Sat Nov 12 2011, 09:32PM
Nah, this would be a separate comparator, nothing to do with overcurrent protection, just turning off the oscillator properly.
Here is the pic of what I used, it's a bit weirder than I expected but has a point... the 74HC14 gates provide enough hysteresis to decouple the oscillator from the feedback signal well. Though I'm not sure if all this is necessary when a comparator could simply be used that might not even require any oscillators to start.
Marko, Sat Nov 12 2011, 09:32PM
Dr. Kilovolt wrote ...
Of course the circuit I posted is for oscillator/resonant frequency feedback, not overcurrent protection. If I was going to use an overcurrent protection, I would put a shunt resistor between the two CTs (to eliminate the need for another CT) and use this to control a small SCR which would shut down the gate drivers through their enable pins.
Sure you can upload the circuit if you want, I'll take a look.
Marko wrote ...
Well, what I meant was more like a comparator with hysteresis (or at least another HC14 gate used as one) to independently sense the amplitude of primary current and turn off the oscillator, until the current drops too low again.
Well, what I meant was more like a comparator with hysteresis (or at least another HC14 gate used as one) to independently sense the amplitude of primary current and turn off the oscillator, until the current drops too low again.
Of course the circuit I posted is for oscillator/resonant frequency feedback, not overcurrent protection. If I was going to use an overcurrent protection, I would put a shunt resistor between the two CTs (to eliminate the need for another CT) and use this to control a small SCR which would shut down the gate drivers through their enable pins.
Sure you can upload the circuit if you want, I'll take a look.
Nah, this would be a separate comparator, nothing to do with overcurrent protection, just turning off the oscillator properly.
Here is the pic of what I used, it's a bit weirder than I expected but has a point... the 74HC14 gates provide enough hysteresis to decouple the oscillator from the feedback signal well. Though I'm not sure if all this is necessary when a comparator could simply be used that might not even require any oscillators to start.
Re: CWDRSSTC
Dr. Dark Current, Sat Nov 12 2011, 09:52PM
Yes now I see what you're up to with the circuit, sorry I thought you were referring to the overcurrent comparator.
The circuit I posted should lock at very low bus voltage, and it should be stable once it locks. There *might* be a problem with turning the TC on at the positive peak of mains voltage, but I should get around this problem somehow if it happens (like a zero cross detecting SSR or something like that). But I believe this will not be necessary.
I have used similar design of start up oscillator for an electronic feedback for "zvs" driver and "zvs" Tesla coil, and there were no problems with it, though the feedback was voltage not current.
Dr. Dark Current, Sat Nov 12 2011, 09:52PM
Yes now I see what you're up to with the circuit, sorry I thought you were referring to the overcurrent comparator.
The circuit I posted should lock at very low bus voltage, and it should be stable once it locks. There *might* be a problem with turning the TC on at the positive peak of mains voltage, but I should get around this problem somehow if it happens (like a zero cross detecting SSR or something like that). But I believe this will not be necessary.
I have used similar design of start up oscillator for an electronic feedback for "zvs" driver and "zvs" Tesla coil, and there were no problems with it, though the feedback was voltage not current.
Re: CWDRSSTC
Marko, Sun Nov 13 2011, 01:26AM
Hm, even though it's a bit offtopic (or not, as I guess it categorizes as CWDRSSTC too) - but, OMG pix/schemz/vidz?
Marko
Marko, Sun Nov 13 2011, 01:26AM
"zvs" Tesla coil
Hm, even though it's a bit offtopic (or not, as I guess it categorizes as CWDRSSTC too) - but, OMG pix/schemz/vidz?
Marko
Re: CWDRSSTC
Goodchild, Sun Nov 13 2011, 07:44AM
Marko,
I simulated the control method you are talking about for the QCW using the comparator, but after it's all said and done with loading and such you get very bad ripple in the primary current even with delta sigma modulation of the primary current. Really the only way this control method could possible work is with a system that had a crazy high tank Z like say > 60ohms. Here is a result of one of my sims with a tank Z of around 25ohm:
45 ohm Z:
The reason for this is because at the start of the burst when the current is slow you have to skip so many cycles that it leaves a large ripple in the current. This ripple would be at a nasty low freq as well (<5KHz). The only way to solve this is with a really low coupling and a high Z to slow the rise and fall of the current and damp out the ripple effect. Although the bus modulation is by far the way to go, but it doesn't necessary have to be a buck! It could be as simple as half rectified mains or a level shift circuit like what a VTTC uses.
Pure CW is not my thing, I like making the biggest sparks with the least power. Everyone has there own likes I guess. hehe
On the ZVS topic, I simulated that as well. It came out rather well in simulation and I even came close to building a full bridge ZVS drive for my QCW. I never ended up doing it though because ZVS doesn't work all that well with IGBTs because of there tail currents. If you were to build it with some large MOSFETs however a phase shift ZVS full bridge would be a rather fancy nice approach for a pure CW coil. All the modulation could be accomplished through phase shifting the two half bridges.
Goodchild, Sun Nov 13 2011, 07:44AM
Marko,
I simulated the control method you are talking about for the QCW using the comparator, but after it's all said and done with loading and such you get very bad ripple in the primary current even with delta sigma modulation of the primary current. Really the only way this control method could possible work is with a system that had a crazy high tank Z like say > 60ohms. Here is a result of one of my sims with a tank Z of around 25ohm:
45 ohm Z:
The reason for this is because at the start of the burst when the current is slow you have to skip so many cycles that it leaves a large ripple in the current. This ripple would be at a nasty low freq as well (<5KHz). The only way to solve this is with a really low coupling and a high Z to slow the rise and fall of the current and damp out the ripple effect. Although the bus modulation is by far the way to go, but it doesn't necessary have to be a buck! It could be as simple as half rectified mains or a level shift circuit like what a VTTC uses.
Pure CW is not my thing, I like making the biggest sparks with the least power.
Everyone has there own likes I guess. hehe On the ZVS topic, I simulated that as well. It came out rather well in simulation and I even came close to building a full bridge ZVS drive for my QCW. I never ended up doing it though because ZVS doesn't work all that well with IGBTs because of there tail currents. If you were to build it with some large MOSFETs however a phase shift ZVS full bridge would be a rather fancy nice approach for a pure CW coil.
All the modulation could be accomplished through phase shifting the two half bridges. Re: CWDRSSTC
Dr. Dark Current, Sun Nov 13 2011, 08:16AM
Marko, the ZVS Tesla coil was a fail.
The first version was the standard push-pull circuit, with active feedback. Even though it did oscillate and I was getting some hundred volts per turn, I couldn't get it to make a longer discharge than 1cm. While increasing the tank frequency by tuning the tap, the frequency suddenly jumped from below secondary resonance to above secondary resonance. This was probably the result of high coupling, but I don't think a lower coupling would help.
The second version was with a self-oscillating ZVS fullbridge I designed. It oscillated, again producing some hundred volts per turn with the 4 sq.mm primary wire getting hot from the kVAR's, but the behavior was exactly the same as before. I just couldn't tune it.
PS. I just looked at your schematic again, and you have a Zener diode connected directly across the 74HC14 input. Zener diodes have high capacitance and are slow, that might be part of a reason why you got such a weak feedback.
Dr. Dark Current, Sun Nov 13 2011, 08:16AM
Marko, the ZVS Tesla coil was a fail.
The first version was the standard push-pull circuit, with active feedback. Even though it did oscillate and I was getting some hundred volts per turn, I couldn't get it to make a longer discharge than 1cm. While increasing the tank frequency by tuning the tap, the frequency suddenly jumped from below secondary resonance to above secondary resonance. This was probably the result of high coupling, but I don't think a lower coupling would help.
The second version was with a self-oscillating ZVS fullbridge I designed. It oscillated, again producing some hundred volts per turn with the 4 sq.mm primary wire getting hot from the kVAR's, but the behavior was exactly the same as before. I just couldn't tune it.
PS. I just looked at your schematic again, and you have a Zener diode connected directly across the 74HC14 input. Zener diodes have high capacitance and are slow, that might be part of a reason why you got such a weak feedback.
Re: CWDRSSTC
Steve Conner, Sun Nov 13 2011, 11:13AM
The secret of the sigma delta control method is to turn all of the IGBTs off when the current goes above the setpoint, and let the tank energy regenerate back into the DC bus, the bridge running as a rectifier. That way, the envelope must fall as fast as it rose: the energy transfer is the same magnitude, just the opposite direction.
The back-and-forth of energy stresses the diodes more, but I believe the extra losses in the diodes are no worse than the losses in the high-level modulators you use just now.
I've been doing it for years, that's how the OCD on my coils works, and when doing demos in undersized cages they'll happily bounce off it all day for weeks on end. I just haven't tried substituting the constant setpoint for a ramp.
My PLL driver was supposed to work with CW SSTCs and induction heaters too. It is designed with two inputs, so you can drive the PLL from the secondary base, and the current limiter from the primary.
In practice I found trouble getting the PLL to lock when first applying the DC bus on my induction heater. With half wave rectified supply, it probably needs a keep-alive current.
Steve Conner, Sun Nov 13 2011, 11:13AM
The secret of the sigma delta control method is to turn all of the IGBTs off when the current goes above the setpoint, and let the tank energy regenerate back into the DC bus, the bridge running as a rectifier. That way, the envelope must fall as fast as it rose: the energy transfer is the same magnitude, just the opposite direction.
The back-and-forth of energy stresses the diodes more, but I believe the extra losses in the diodes are no worse than the losses in the high-level modulators you use just now.
I've been doing it for years, that's how the OCD on my coils works, and when doing demos in undersized cages they'll happily bounce off it all day for weeks on end. I just haven't tried substituting the constant setpoint for a ramp.
My PLL driver was supposed to work with CW SSTCs and induction heaters too. It is designed with two inputs, so you can drive the PLL from the secondary base, and the current limiter from the primary.
In practice I found trouble getting the PLL to lock when first applying the DC bus on my induction heater. With half wave rectified supply, it probably needs a keep-alive current.
Re: CWDRSSTC
Marko, Sun Nov 13 2011, 02:20PM
Hi guys
Kilovolt:
Interesting that you couldn't get the current fed inverter to work, as I planned on something similar (for very high frequency TC) any data on what you did would be valuable.
Goodchild: My sight is too poor to make out much of your schematic (higher resolution pic please?) but what I intended to do is basically this -
the most of the circuit is Steve Ward's original DRSSTC controller, except there's a comparator in place of interrupter. Current signal (shared with OCD comparator, which now acts as auxiliary protection) goes into the - input, and to + input I'll feed signal from audio output of my PC (It will be a micro tabletop coil for start).
There'll be no hysteresis on the comparator so after the igbt's go off, the only "memory" is provided by the output synchro-flip-flop: every next cycle IGBT's get a chance to get turned on back again if the current has dropped low enough. As far as I know, this is what Steve Conner used as his OCD.
Regarding the initial current ripple, why is it so troublesome after all? It doesn't look like anything that would trouble the inverter too much.
One must note that the whole model is probably not too ideal as any secondary will have a latency time before it breaks out (even with breakout point!) during which it will have much higher Q and load the primary system more heavily. I'm thinking this might actually help rectify the initial current ripple.
And after all I intend to play with my waveform shape instead of just keeping it a plain ramp (If the ripple appears, I could even adjust the waveform with "anti ripple" to actively counter it). In any case, before the start of the ramp I intended to keep a steady DC signal for some time until all transients level out, and only then starting to ramp it up slowly.
Ofcourse this puts my PC at danger, probably not too much with a mini coil but for larger ones I'd probably want to have an isolation amplifier in between or use an MP3 player or something to produce the signal.
Still the original design was intended as normal DRSSTC, hence the tank impedance is low (10ohms or so) and I'd like to have it higher for QCW operation just for the sake of inverter losses. But for this I'd need about 3 CDE caps of 47nF or so and2-3kV rating, anyone happen to have some? :(
Anyway, to get back to the true CWDRSSTC's and not just part-time ones... I'm really serious about those 50kW (or 40-ish, what I think might be required for 1 meter CW arc, my initial goal), just not sure where to get them... bringing the coil to college would be fun but impractical, so my old school might be the next best option... it has it's own transformer station nearby (actually just few meters from the lab)... wonder what kind of feeds do they have there... Large arc flashes anyone? :O
Marko
Marko, Sun Nov 13 2011, 02:20PM
Hi guys
Kilovolt:
Interesting that you couldn't get the current fed inverter to work, as I planned on something similar (for very high frequency TC) any data on what you did would be valuable.
Goodchild: My sight is too poor to make out much of your schematic (higher resolution pic please?) but what I intended to do is basically this -
the most of the circuit is Steve Ward's original DRSSTC controller, except there's a comparator in place of interrupter. Current signal (shared with OCD comparator, which now acts as auxiliary protection) goes into the - input, and to + input I'll feed signal from audio output of my PC (It will be a micro tabletop coil for start).
There'll be no hysteresis on the comparator so after the igbt's go off, the only "memory" is provided by the output synchro-flip-flop: every next cycle IGBT's get a chance to get turned on back again if the current has dropped low enough. As far as I know, this is what Steve Conner used as his OCD.
Regarding the initial current ripple, why is it so troublesome after all? It doesn't look like anything that would trouble the inverter too much.
One must note that the whole model is probably not too ideal as any secondary will have a latency time before it breaks out (even with breakout point!) during which it will have much higher Q and load the primary system more heavily. I'm thinking this might actually help rectify the initial current ripple.
And after all I intend to play with my waveform shape instead of just keeping it a plain ramp (If the ripple appears, I could even adjust the waveform with "anti ripple" to actively counter it). In any case, before the start of the ramp I intended to keep a steady DC signal for some time until all transients level out, and only then starting to ramp it up slowly.
Ofcourse this puts my PC at danger, probably not too much with a mini coil but for larger ones I'd probably want to have an isolation amplifier in between or use an MP3 player or something to produce the signal.
Still the original design was intended as normal DRSSTC, hence the tank impedance is low (10ohms or so) and I'd like to have it higher for QCW operation just for the sake of inverter losses. But for this I'd need about 3 CDE caps of 47nF or so and2-3kV rating, anyone happen to have some? :(
Anyway, to get back to the true CWDRSSTC's and not just part-time ones... I'm really serious about those 50kW (or 40-ish, what I think might be required for 1 meter CW arc, my initial goal), just not sure where to get them... bringing the coil to college would be fun but impractical, so my old school might be the next best option... it has it's own transformer station nearby (actually just few meters from the lab)... wonder what kind of feeds do they have there... Large arc flashes anyone? :O
Marko
Re: CWDRSSTC
Goodchild, Sun Nov 13 2011, 06:20PM
First off I have to say this thread has become very interesting indeed Don't see a lot of this anymore on 4hv.
Steve, I have tried what you are talking about with shutting off all the IGBTs and re triggering them on the next cycle. (this is what's happening in that simulation) Now under normal operation of a low impedance DR tank this works, because the current in most cases only takes a couple cycles to recycle back into the tank. But in the land of QCW with it's high impedance tank, it can take 10's of cycles to get the current to drop down. This makes for a nasty low frequency ripple at low power because it has to skip many cycles to lower the current enough to turn back on again.
Marko, Sorry I don't have a bigger pic, It's rather simple simulation, just a standard DRSSTC high impedance tank driven with a basic DRSSTC driver. The changed part was the addition of the integrator before the comparator and another flip flop to make the delta sigma modulator. It's basically what you are talking about doing only it has an integrator with the comparator. That hysteresis is very necessary in my mind. If the hysteresis wasn't there you would retrigger on the next cycle only to have the tank ring right back up again, some times higher than the point that you stopped it at the first time! This could lead to even more nasty oscillations.
Unlike cycle by cycle limiting in say a boost converter or other none resonant SMPS the current is sinusoidal, making cycle by cycle limiting kinda impossible without the addition of hysteresis. This is because with a boost you can hard switch and turn off right at the current you want to stop at. Where with the DR we have to turn on and off at the zero crossings. So we have to remember what the current was and add some delay so that doesn't keep re triggering.
The ripple can be bad for a couple of reasons, first off at 5KHz it would be audible! Also as Steve ward pointed out to me a while back, a ripple of that size will most likely effect spark growth. I very well could be wrong though, I still love the idea of only using the bridge for modulation, but practically I have yet to find a solution...
I hope my rambling wasn't to bad
Dr. Kilovolt, may I ask how the full bridge ZVS was implemented? The way I would do it is with a small amount of extra C in parallel with each switch and a small amount of delay in each gate drive. this makes the switch turn on when the current in the primary LC is at say 5% of it's peak value. That current will be flowing through the C on each switch and the switches internal C as well. So when the switch is turning on and off the voltage across each switch is 0. spice is a great way to get ZVS tuned right before building the real thing. Because you are right with an unturned ZVS is can be even worse than just regular ZCS!
Eric
Goodchild, Sun Nov 13 2011, 06:20PM
First off I have to say this thread has become very interesting indeed
Don't see a lot of this anymore on 4hv. Steve, I have tried what you are talking about with shutting off all the IGBTs and re triggering them on the next cycle. (this is what's happening in that simulation) Now under normal operation of a low impedance DR tank this works, because the current in most cases only takes a couple cycles to recycle back into the tank. But in the land of QCW with it's high impedance tank, it can take 10's of cycles to get the current to drop down. This makes for a nasty low frequency ripple at low power because it has to skip many cycles to lower the current enough to turn back on again.
Marko, Sorry I don't have a bigger pic, It's rather simple simulation, just a standard DRSSTC high impedance tank driven with a basic DRSSTC driver. The changed part was the addition of the integrator before the comparator and another flip flop to make the delta sigma modulator. It's basically what you are talking about doing only it has an integrator with the comparator. That hysteresis is very necessary in my mind. If the hysteresis wasn't there you would retrigger on the next cycle only to have the tank ring right back up again, some times higher than the point that you stopped it at the first time! This could lead to even more nasty oscillations.
Unlike cycle by cycle limiting in say a boost converter or other none resonant SMPS the current is sinusoidal, making cycle by cycle limiting kinda impossible without the addition of hysteresis. This is because with a boost you can hard switch and turn off right at the current you want to stop at. Where with the DR we have to turn on and off at the zero crossings. So we have to remember what the current was and add some delay so that doesn't keep re triggering.
The ripple can be bad for a couple of reasons, first off at 5KHz it would be audible! Also as Steve ward pointed out to me a while back, a ripple of that size will most likely effect spark growth. I very well could be wrong though, I still love the idea of only using the bridge for modulation, but practically I have yet to find a solution...
I hope my rambling wasn't to bad
Dr. Kilovolt, may I ask how the full bridge ZVS was implemented? The way I would do it is with a small amount of extra C in parallel with each switch and a small amount of delay in each gate drive. this makes the switch turn on when the current in the primary LC is at say 5% of it's peak value. That current will be flowing through the C on each switch and the switches internal C as well. So when the switch is turning on and off the voltage across each switch is 0. spice is a great way to get ZVS tuned right before building the real thing. Because you are right with an unturned ZVS is can be even worse than just regular ZCS!
Eric
Re: CWDRSSTC
Marko, Sun Nov 13 2011, 06:49PM
Hi goodchild,
Well, it seems that the forum is being taken up by a new generation of newbies, I feel like a very old outsider here now!
Not sure if I was clear about that, but the "hysteresis" is provided by the 74HC74 flip flop (or whatever) that is normally used for soft turn-off in DRSSTC's, and the delay circuit which can be tuned to prevent turn on in one or more following cycles, so you may not even need any hysteresis on the comparator (I remember asking Steve Conner about exactly the same thing of not using any hysteresis at all for his OCD scheme, which is very similar concept from what I can tell).
I haven't figured out what the integrator is for though, just generating a ramp from the input signal, or is it actually in feedback path?
I'm pretty definitely going to try something like this in next few months on a small DRSSTC I have in parts here, after I redo the control board to include a comparator.
I might add a bit of optional + feedback hysteresis too just in case, but otherwise I want to see if this uber simple scheme will work if I can have good control over input waveform. (hopefully without frying my PC)
Kilovolt: How did you implement your full bridge current-fed inverter? It's troublesome since it needs gate drive overlap instead of deadtime, so you'd need isolated gate drives controlled by optocouplers. But advantage would be zero voltage switching as well as no diode conduction and unity power factor which is attractive for HFSSTC's.
Marko
Marko, Sun Nov 13 2011, 06:49PM
Hi goodchild,
Well, it seems that the forum is being taken up by a new generation of newbies, I feel like a very old outsider here now!
Not sure if I was clear about that, but the "hysteresis" is provided by the 74HC74 flip flop (or whatever) that is normally used for soft turn-off in DRSSTC's, and the delay circuit which can be tuned to prevent turn on in one or more following cycles, so you may not even need any hysteresis on the comparator (I remember asking Steve Conner about exactly the same thing of not using any hysteresis at all for his OCD scheme, which is very similar concept from what I can tell).
I haven't figured out what the integrator is for though, just generating a ramp from the input signal, or is it actually in feedback path?
I'm pretty definitely going to try something like this in next few months on a small DRSSTC I have in parts here, after I redo the control board to include a comparator.
I might add a bit of optional + feedback hysteresis too just in case, but otherwise I want to see if this uber simple scheme will work if I can have good control over input waveform. (hopefully without frying my PC)
Kilovolt: How did you implement your full bridge current-fed inverter? It's troublesome since it needs gate drive overlap instead of deadtime, so you'd need isolated gate drives controlled by optocouplers. But advantage would be zero voltage switching as well as no diode conduction and unity power factor which is attractive for HFSSTC's.
Marko
Re: CWDRSSTC
Dr. Dark Current, Sun Nov 13 2011, 07:01PM
All,
here is my CFPR fullbridge circuit. It oscillates as expected, but the IGBTs got hot at 200 kHz and I got just a small spark. Maybe it would help to use MOSFETs, but their higher voltage drop could be a problem for the circuit. Note: 21V is regulated from LM317.
Dr. Dark Current, Sun Nov 13 2011, 07:01PM
All,
here is my CFPR fullbridge circuit. It oscillates as expected, but the IGBTs got hot at 200 kHz and I got just a small spark. Maybe it would help to use MOSFETs, but their higher voltage drop could be a problem for the circuit. Note: 21V is regulated from LM317.
Re: CWDRSSTC
Marko, Mon Nov 14 2011, 02:35AM
Jan, that circuit looks like a plain shock horror to me. Have you tried using a "normal" SSTC driver circuit instead (this is what I'm considering, but will likely have to do with isolated gate drivers in the end)
Marko
Marko, Mon Nov 14 2011, 02:35AM
Jan, that circuit looks like a plain shock horror to me. Have you tried using a "normal" SSTC driver circuit instead (this is what I'm considering, but will likely have to do with isolated gate drivers in the end)
Marko
Re: CWDRSSTC
Goodchild, Mon Nov 14 2011, 04:21AM
Marko, I also wasn't being totally clear now that I read my own post again. The hysteresis and integrator are one in the same. There is no hysteresis on the comparator it's self but rather hysteresis of the whole feedback loop, provided by the integrator. So yes the integrator is a big part of the feedback loop!
The feedback from my system goes though the integrating amplifier and is then squared up by the comparator. This signal along with the feedback clock is then fed to the flip flop latch that turns the bridge on and off at the zero crossings. This method of control is better known as delta sigma modulation.
Let me give an example why you need the hysteresis: So your tank just rang up to say 100A and your reference is set at 100A. With a normal OCD the controller would stop the bridge right there and wait for the next interrupter pulse to enable the flip flop again. But for the QCW we would want to re-enable the driver after the current has dropped to an acceptable level. So we uses the clock to re-enable the flip flop. But with no hysteresis in the feedback loop the system would turn back on again after only one cycle! This would result in ringing because the current in the primary LC is not going to ring down a whole lot in just one cycle. So the addition of the hysteresis in the feedback loop makes the system react a little bit more sluggish, avoiding the oscillations. This is at least how I have looked at it so far. hmmm maybe sluggish is not the right word....
I'm not sure if Steve McConner uses the same OCD configuration? I'm familiarized with Steve Ward's driver. Steve do you use the same OCD as Steve Ward? (wow lots of Steve's in there )
Steve perhaps you could elaborate on how you would implement the delta sigma modulation in a little more detail? I'm curios to the difference if any bettwen the way I'm looking at it and how you would do it.
Dr. kilovolt, That driver looks a lot like a flyback ZVS driver
I agree with Marko in that you should isolate both the feedback and the gate drive. As I said in my other post IGBTs are going to suck hard with ZVS!!! This is because of the charge stored in the device being wasted in heat at turn off. This is probably where most of your heat is coming from! MOSFETs would be a better choice for a ZVS design because of there lack of a tail current.
I "had" a great paper on IGBTs and MOSFETs in ZVS and ZCS systems.... If I find it again I will be sure to link it.
Eric
[EDIT]
Well would you look at that I found that paper after all
Goodchild, Mon Nov 14 2011, 04:21AM
Marko, I also wasn't being totally clear now that I read my own post again. The hysteresis and integrator are one in the same. There is no hysteresis on the comparator it's self but rather hysteresis of the whole feedback loop, provided by the integrator. So yes the integrator is a big part of the feedback loop!
The feedback from my system goes though the integrating amplifier and is then squared up by the comparator. This signal along with the feedback clock is then fed to the flip flop latch that turns the bridge on and off at the zero crossings. This method of control is better known as delta sigma modulation.
Let me give an example why you need the hysteresis: So your tank just rang up to say 100A and your reference is set at 100A. With a normal OCD the controller would stop the bridge right there and wait for the next interrupter pulse to enable the flip flop again. But for the QCW we would want to re-enable the driver after the current has dropped to an acceptable level. So we uses the clock to re-enable the flip flop. But with no hysteresis in the feedback loop the system would turn back on again after only one cycle! This would result in ringing because the current in the primary LC is not going to ring down a whole lot in just one cycle. So the addition of the hysteresis in the feedback loop makes the system react a little bit more sluggish, avoiding the oscillations. This is at least how I have looked at it so far. hmmm maybe sluggish is not the right word....
I'm not sure if Steve McConner uses the same OCD configuration? I'm familiarized with Steve Ward's driver. Steve do you use the same OCD as Steve Ward? (wow lots of Steve's in there
) Steve perhaps you could elaborate on how you would implement the delta sigma modulation in a little more detail? I'm curios to the difference if any bettwen the way I'm looking at it and how you would do it.
Dr. kilovolt, That driver looks a lot like a flyback ZVS driver
I agree with Marko in that you should isolate both the feedback and the gate drive. As I said in my other post IGBTs are going to suck hard with ZVS!!! This is because of the charge stored in the device being wasted in heat at turn off. This is probably where most of your heat is coming from! MOSFETs would be a better choice for a ZVS design because of there lack of a tail current. I "had" a great paper on IGBTs and MOSFETs in ZVS and ZCS systems.... If I find it again I will be sure to link it.
Eric
[EDIT]
Well would you look at that I found that paper after all
Re: CWDRSSTC
Dr. Dark Current, Mon Nov 14 2011, 08:05AM
Hi,
yes the circuit is the standard "zvs flyback driver" adapted to full bridge with smaller gate resistors to make it run at a higher frequency, also it adds bias to the gates, to make sure they turn off properly. This circuit would probably not work with FETs at higher power, if their drop rises to the point the other transitor which should be off starts turning on, something pops.
The idea of this was just to make something simple and foolproof. Of course I could use active feedback, in which case we could use MOSFETs. I'm just not into complex circuits, and I try to avoid as much control ICs as possible, as they are most often the source of unexpected behavior, are disturbed by interference etc...
Dr. Dark Current, Mon Nov 14 2011, 08:05AM
Hi,
yes the circuit is the standard "zvs flyback driver" adapted to full bridge with smaller gate resistors to make it run at a higher frequency, also it adds bias to the gates, to make sure they turn off properly. This circuit would probably not work with FETs at higher power, if their drop rises to the point the other transitor which should be off starts turning on, something pops.
The idea of this was just to make something simple and foolproof. Of course I could use active feedback, in which case we could use MOSFETs. I'm just not into complex circuits, and I try to avoid as much control ICs as possible, as they are most often the source of unexpected behavior, are disturbed by interference etc...
Re: CWDRSSTC
Steve Conner, Mon Nov 14 2011, 09:34AM
Hi all
As seen in the envelope domain, the resonator already integrates the output from your inverter. (A constant input of RF voltage gives a ramp up of the envelope.)
Therefore, if you just close a feedback loop around it, you have a first-order sigma-delta modulator. If you add an integrator as Eric did, it becomes a second-order modulator. The first-order is guaranteed stable.
Spark loading turns the resonator (again in envelope terms) from a pure integrator to a low-pass filter, since the ramp no longer goes up indefinitely, but settles to some steady value where power input equals power output.
In an epic post on sigma-delta theory, our own Dr. Slack said that higher-order modulators need all their integrators but one "broken back" in this way, to meet the Nyquist criterion for the loop. I would do that to the "other" integrator in a second-order modulator anyway, to ensure it was stable before breakout. But I wouldn't be using the second-order one in the first place, unless the first-order failed to meet my perfomance goals.
And what's wrong with this? That's how my sigma-delta scheme works, it makes up the desired envelope by adding and subtracting "lumps" of one cycle (or more) worth of RF. I don't see the problem, unless it tried to dish out half a cycle, or three-quarters or whatever. But the synchronized sampling forbids that.
Here's a picture from the early development, where the OCD setpoint is just a constant, and the DC bus voltage isn't much greater than needed to overcome losses at that current level. Note the waveform is the exact opposite of what Eric's sim predicts: slow rise, fast fall.
My driver is different to Steve Ward's. We agreed to disagree on the best way to drive Tesla coils. Figuring out which one of us is right is left as an exercise for the student.
As to ripple on the envelope: Sure it will be worse than with a high-level modulator. The question is whether it'll be bad enough to affect spark growth. Personally I think the main effect will be to make the spark go "whee!" instead of "pop", which might even be seen as an advantage.
Steve Conner, Mon Nov 14 2011, 09:34AM
Hi all
As seen in the envelope domain, the resonator already integrates the output from your inverter. (A constant input of RF voltage gives a ramp up of the envelope.)
Therefore, if you just close a feedback loop around it, you have a first-order sigma-delta modulator. If you add an integrator as Eric did, it becomes a second-order modulator. The first-order is guaranteed stable.
Spark loading turns the resonator (again in envelope terms) from a pure integrator to a low-pass filter, since the ramp no longer goes up indefinitely, but settles to some steady value where power input equals power output.
In an epic post on sigma-delta theory, our own Dr. Slack said that higher-order modulators need all their integrators but one "broken back" in this way, to meet the Nyquist criterion for the loop. I would do that to the "other" integrator in a second-order modulator anyway, to ensure it was stable before breakout. But I wouldn't be using the second-order one in the first place, unless the first-order failed to meet my perfomance goals.
wrote ...
But with no hysteresis in the feedback loop the system would turn back on again after only one cycle!
But with no hysteresis in the feedback loop the system would turn back on again after only one cycle!
And what's wrong with this? That's how my sigma-delta scheme works, it makes up the desired envelope by adding and subtracting "lumps" of one cycle (or more) worth of RF. I don't see the problem, unless it tried to dish out half a cycle, or three-quarters or whatever. But the synchronized sampling forbids that.
Here's a picture from the early development, where the OCD setpoint is just a constant, and the DC bus voltage isn't much greater than needed to overcome losses at that current level.
Note the waveform is the exact opposite of what Eric's sim predicts: slow rise, fast fall.My driver is different to Steve Ward's. We agreed to disagree on the best way to drive Tesla coils. Figuring out which one of us is right is left as an exercise for the student.
As to ripple on the envelope: Sure it will be worse than with a high-level modulator. The question is whether it'll be bad enough to affect spark growth. Personally I think the main effect will be to make the spark go "whee!" instead of "pop", which might even be seen as an advantage.
Re: CWDRSSTC
Uspring, Mon Nov 14 2011, 10:30AM
Dr. Kilovolt wrote:
According to my (preliminary) calculations, I think there is an upper bound to the primary tanks impedance of Z=k*sqrt(Lpr/Cpr) if
you use zero current switching. That makes it difficult to get currents down reasonably.
Marko wrote:
I like this idea, but a quick analysis tells me, that this behaves much like a standard DRSSTC but with a coupling of k=sqrt(Lpr/Lsec) which is tiny in most cases.
Uspring, Mon Nov 14 2011, 10:30AM
Dr. Kilovolt wrote:
The primary impedance is adjusted by tuning the primary on the lower slope of the resonant peak (=tuning and detuning below secondary resonance).
According to my (preliminary) calculations, I think there is an upper bound to the primary tanks impedance of Z=k*sqrt(Lpr/Cpr) if
you use zero current switching. That makes it difficult to get currents down reasonably.
Marko wrote:
3. Series impedance matching network base feed - basically a DRSSTC, but with direct base feed of the secondary from the primary tank! The bridge drives a serier LC circuit of fairly high impedance, and the base of the secondary goes in between L and C.
I like this idea, but a quick analysis tells me, that this behaves much like a standard DRSSTC but with a coupling of k=sqrt(Lpr/Lsec) which is tiny in most cases.
Re: CWDRSSTC
Steve Conner, Mon Nov 14 2011, 11:51AM
Correct. Richie and I analysed the base feed by L-match (which is what this is) and came to the same conclusion. You are throwing away the transformer action, so have to make the voltage step-up by resonant rise instead. That means a higher Q of the primary circuit, so more losses.
There's nothing to stop you combining the best parts of base feed and transformer coupling, by wiring it as an Oudin coil, with the magnetic and direct couplings reinforcing each other.
Steve Conner, Mon Nov 14 2011, 11:51AM
Uspring wrote ...
I like this idea, but a quick analysis tells me, that this behaves much like a standard DRSSTC but with a coupling of k=sqrt(Lpr/Lsec) which is tiny in most cases.
I like this idea, but a quick analysis tells me, that this behaves much like a standard DRSSTC but with a coupling of k=sqrt(Lpr/Lsec) which is tiny in most cases.
Correct. Richie and I analysed the base feed by L-match (which is what this is) and came to the same conclusion. You are throwing away the transformer action, so have to make the voltage step-up by resonant rise instead. That means a higher Q of the primary circuit, so more losses.
There's nothing to stop you combining the best parts of base feed and transformer coupling, by wiring it as an Oudin coil, with the magnetic and direct couplings reinforcing each other.
Re: CWDRSSTC
Marko, Mon Nov 14 2011, 01:24PM
Hi guys,
Well here be the schematic, I'm not sure if anything is missing yet, but it's got the main picture... I might only put a little hysteresis on the comparator itself to prevent it from sitting in linear region, and that's basically it - I feed it input from my PC soundcard, and hope for this to work... I really can't be bothered with anything more complex right now (have yet another big DRSSTC pracitcally finished but sitting around in parts (stole the MMC from it) without clue what to do with it)
Well, that is all true, but I argued that this might be a more suitable option than say, a base feed transformer, simply due to fact that a hefty HV inductor is much easier to construct than a ferrite transformer (with or without ferrite core). If ferrite rod is used as core one could even use it as a magnifier again and have the significantly lower magnetizing current that pure air cored design.
Still the original proposal both makes your coil look pretty (and flashover-proof) as well as making the inverter happy with unity power factor. Some additional losses in a big inductor should be easier to manage than losses in semiconductors!
The much bigger drawback in my opinion is that the coil would never be well isolated from mains, so the output would be live, dangerous and ground arcs might blow the system (not sure to what extent would DC blocking caps on resonator base help)
If we could have an isolated power supply which we can mid-point ground it would be perfect for this (afaik people in USA tend to have 120-0-120 V Supply which could be used this way)
PS. Also one more question about my CW coil - Since conditions seem optimal for that I definitely intend on rewiring my H bridge as two parallel H bridges and omitting the ferrite transformer, which is apparently what Jmartis intends to do as well. So I think I definitely need some advice on how to do that properly -
how bad idea would it be to simply connect the inverters together? Datasheet doesn't say anything particular about paralleling but VCEsat can be seen to drop with increase in temperature which is not a good sign. My brain also starts kind of exploding when I try to figure out whether or not splitting the MMC would actually help at all here.
I realize the best way to assure current sharing is using a common mode choke between the inverters - yet how to calculate it's required parameters? (I thought on using something like a large flyback core with 5 - 10 turns of fat wire on each side or something around this).
Marko
Marko, Mon Nov 14 2011, 01:24PM
Hi guys,
Well here be the schematic, I'm not sure if anything is missing yet, but it's got the main picture... I might only put a little hysteresis on the comparator itself to prevent it from sitting in linear region, and that's basically it - I feed it input from my PC soundcard, and hope for this to work... I really can't be bothered with anything more complex right now (have yet another big DRSSTC pracitcally finished but sitting around in parts (stole the MMC from it) without clue what to do with it)
I like this idea, but a quick analysis tells me, that this behaves much like a standard DRSSTC but with a coupling of k=sqrt(Lpr/Lsec) which is tiny in most cases.
Well, that is all true, but I argued that this might be a more suitable option than say, a base feed transformer, simply due to fact that a hefty HV inductor is much easier to construct than a ferrite transformer (with or without ferrite core). If ferrite rod is used as core one could even use it as a magnifier again and have the significantly lower magnetizing current that pure air cored design.
Still the original proposal both makes your coil look pretty (and flashover-proof) as well as making the inverter happy with unity power factor. Some additional losses in a big inductor should be easier to manage than losses in semiconductors!
The much bigger drawback in my opinion is that the coil would never be well isolated from mains, so the output would be live, dangerous and ground arcs might blow the system (not sure to what extent would DC blocking caps on resonator base help)
If we could have an isolated power supply which we can mid-point ground it would be perfect for this (afaik people in USA tend to have 120-0-120 V Supply which could be used this way)
PS. Also one more question about my CW coil - Since conditions seem optimal for that I definitely intend on rewiring my H bridge as two parallel H bridges and omitting the ferrite transformer, which is apparently what Jmartis intends to do as well. So I think I definitely need some advice on how to do that properly -
how bad idea would it be to simply connect the inverters together? Datasheet doesn't say anything particular about paralleling but VCEsat can be seen to drop with increase in temperature which is not a good sign. My brain also starts kind of exploding when I try to figure out whether or not splitting the MMC would actually help at all here.
I realize the best way to assure current sharing is using a common mode choke between the inverters - yet how to calculate it's required parameters? (I thought on using something like a large flyback core with 5 - 10 turns of fat wire on each side or something around this).
Marko
Re: CWDRSSTC
Dr. Dark Current, Mon Nov 14 2011, 03:11PM
Marko,
there is generally no problem with paralelling IGBTs, as long as they are from the same batch and located close to each other on the heatsink (so their case temperatures are the same).
However paralelling the gates directly could possibly lead to some problems, so use individual gate resistors.
Dr. Dark Current, Mon Nov 14 2011, 03:11PM
Marko,
there is generally no problem with paralelling IGBTs, as long as they are from the same batch and located close to each other on the heatsink (so their case temperatures are the same).
However paralelling the gates directly could possibly lead to some problems, so use individual gate resistors.
Re: CWDRSSTC
Marko, Mon Nov 14 2011, 03:37PM
Well, it was formerly a fullbridge so they'll be on different heatsinks, also gate drive is individual GDT for every IGBT.
Marko
Marko, Mon Nov 14 2011, 03:37PM
Well, it was formerly a fullbridge so they'll be on different heatsinks, also gate drive is individual GDT for every IGBT.
Marko
Re: CWDRSSTC
Goodchild, Mon Nov 14 2011, 06:16PM
Steve, thanks for adding that depth on the delta sigma modulator! I'm rather new to them myself and it always helps to have some one clarify!
Your scope shots are making my mind really think now! I may have to re-simulate and try with no integrator to make it a first order delta sigma modulator.
The ripple in your scope shots was also a little less that 10KHz by the look of it. That seems a bit much. My simulation with it's 5KHz ripple was probably a result of it's high tank impedance and extra integration in the control loop. I guess the only way to find out whether that low freq ripple will be bad is to try it in real life. The only thing I'm not real wild about is that low freq is going to sound nasty! I liked the QCW for how quiet it was. I don't know maybe it won't be as bad as I think...
Now that you explain it, the ringup of the primary current in it's self is integration as it it takes time to ring up. So the tank impedance kinda sets the integration speed, low Z tank = fast integration and high Z tank = slow integration. I love it when I understand stuff
Marko, Your driver looks good! Nice and simple. My only though would be that the LM311 comparator may be a little slow at QCW frequency (>350KHz). I also love your 3 input diode logic gate!
I will throw in my two sense about paralleling IGBTs but take it with a grain of salt hehe
If you keep them on the same heatsink and also the two IGBTs that are paralleled right next to each other, they should auto balance even with a negative tempCo. The 60N60s I know work in this manner and have been successfully paralleled without a current sharing transformer. When you think about it, it's really hard to get two IGBTs on the same heatsink to drift in temp more then like 10C before they starts heating up the other and balancing out the current.
Although It would probably be a good idea to add the current sharing transformers in case of a shorted bridge scenario! If one of the IGBTs fail the current sharing transformer will limit the short current to the other IGBTs giving the OCD circuit more time to kick in and shut it down. Although I have no idea how to calculate the value of these things. I assume it would be based on the universal xformer equation and frequency.
Eric
Goodchild, Mon Nov 14 2011, 06:16PM
Steve, thanks for adding that depth on the delta sigma modulator! I'm rather new to them myself and it always helps to have some one clarify!
Your scope shots are making my mind really think now! I may have to re-simulate and try with no integrator to make it a first order delta sigma modulator.
The ripple in your scope shots was also a little less that 10KHz by the look of it. That seems a bit much. My simulation with it's 5KHz ripple was probably a result of it's high tank impedance and extra integration in the control loop. I guess the only way to find out whether that low freq ripple will be bad is to try it in real life. The only thing I'm not real wild about is that low freq is going to sound nasty! I liked the QCW for how quiet it was. I don't know maybe it won't be as bad as I think...
Now that you explain it, the ringup of the primary current in it's self is integration as it it takes time to ring up. So the tank impedance kinda sets the integration speed, low Z tank = fast integration and high Z tank = slow integration. I love it when I understand stuff
Marko, Your driver looks good! Nice and simple. My only though would be that the LM311 comparator may be a little slow at QCW frequency (>350KHz). I also love your 3 input diode logic gate!
I will throw in my two sense about paralleling IGBTs but take it with a grain of salt hehe
If you keep them on the same heatsink and also the two IGBTs that are paralleled right next to each other, they should auto balance even with a negative tempCo. The 60N60s I know work in this manner and have been successfully paralleled without a current sharing transformer. When you think about it, it's really hard to get two IGBTs on the same heatsink to drift in temp more then like 10C before they starts heating up the other and balancing out the current.
Although It would probably be a good idea to add the current sharing transformers in case of a shorted bridge scenario! If one of the IGBTs fail the current sharing transformer will limit the short current to the other IGBTs giving the OCD circuit more time to kick in and shut it down. Although I have no idea how to calculate the value of these things. I assume it would be based on the universal xformer equation and frequency.
Eric
Re: CWDRSSTC
Marko, Wed Nov 16 2011, 04:29AM
Hey guys
Jan - how's your project progressing? I'm pretty sure a normal non-resonant SSTC would work for you at that power level, just put some fat IGBT's onto nonisolated heatsinks and use a proper driver circuit (with mandatory OCD) and you should be fine.
I'm still trying to ideate a right topology myself, and it's not easy. I've talked with Steve Conner about this for a while and came to some realizations:
- Predikter circuit using inductor is very hardly tunable, requires a finicky two-sided RC delays afterwards and picks up magnetic fields easily. What opted for (and was also recommended by Steve) was to use an opamp based differentiator, in which phase shift can be easily adjusted in wide range using a pot in parallel with the capacitor. Susceptibility to noise can be reduced by adding a resistor in series with C and thus reducing the gain at high frequencies.
This is then fed into a comparator to produce a square wave drive signal. Comparators like LM311 are good enough because their delay can be compensated out by the differentiator circuit too.
I'm considering building a circuit that can run as DRSSTC, as well as normal CW and QCW (so it can be used in basically any type of the coil - no matter if interrupted or not or whether fed with modulated supply voltage).
If the interrupter is not used the problem that appears is starting up the oscillation.
It is possible that it might start from noise, but I'm not sure if this can be counted on.
The quality solution would be to use some kind of starting oscillator, but this is where I fall into another dilemma.
I could use an oscillator that is always weakly coupled to the input so that feedback signal easily overrides it. This would be easy to implement but might reduce the loop gain too much.
The alternative would be an oscillator free driving the output, which is switched by hard logic to feedback once the signal becomes strong enough.
We would need to find the criteria of the "feedback signal becoming strong enough", though.
If I was to use a simple scheme akin to schematic I posted earlier, I would somehow need to bias the comparator to ensure it's output is certainly at 0 when no feedback signal is present - this could be done either by biasing the predikter input or by using a bias pot between pins 5 and 6 on LM311, I'm not sure which would be better. In any case the bias would again remove a bit of sensitivity but I think this would be acceptable.
Some delay before and after the feedback signal appears could be introduced as well - few cycle start delay may actually remove the initial phase offset problem (provided the oscillator is tuned well enough) by allowing the differentiator to get to the steady state before switching to feedback.
After this is solved, I think we might be getting at the picture of the ultimate non-PLL SSTC driver.
Eric: actually I'm going to use LM311 at >500kHz and I'm pretty sure it'll be fine - Steve used LM393 in his OCD circuit!
If one half bridge failed the OCD might not even do anything to save the other one though; it only senses output current and some sort of differential protection (another ocd sensing current from a few turns of wire on the differential choke) for this, although probably overkill since OCD is there to prevent any failure in first place.
BTW, do you happen to know any relatively cheap sources of good, beefy igbt's? 30N60A4D's are somewhat hard to get, found them on ebay for $6.5 and I'm wondering whether there is a better source.
27N120's are actually cheaper but lack a diode. Any suggestions for a diode (very prefferably a DPAK one) would be welcome, needs to have rating of 1200V and at least 10 or so amps. FOund these on ebay but they are extremely expensive, and I wondered if there's a better bargain around!
PS. found some interesting semiconductor items on ebay...
Some very nice mosfets! Could be even considered instead of igbt's for the application...
ISOPLUS IGBT's - if you can' find two separate heatsinks...
BIMOSFETS! Would like to try a base fed coil with those one day...
Marko
Marko, Wed Nov 16 2011, 04:29AM
Hey guys
Jan - how's your project progressing? I'm pretty sure a normal non-resonant SSTC would work for you at that power level, just put some fat IGBT's onto nonisolated heatsinks and use a proper driver circuit (with mandatory OCD) and you should be fine.
I'm still trying to ideate a right topology myself, and it's not easy. I've talked with Steve Conner about this for a while and came to some realizations:
- Predikter circuit using inductor is very hardly tunable, requires a finicky two-sided RC delays afterwards and picks up magnetic fields easily. What opted for (and was also recommended by Steve) was to use an opamp based differentiator, in which phase shift can be easily adjusted in wide range using a pot in parallel with the capacitor. Susceptibility to noise can be reduced by adding a resistor in series with C and thus reducing the gain at high frequencies.
This is then fed into a comparator to produce a square wave drive signal. Comparators like LM311 are good enough because their delay can be compensated out by the differentiator circuit too.
I'm considering building a circuit that can run as DRSSTC, as well as normal CW and QCW (so it can be used in basically any type of the coil - no matter if interrupted or not or whether fed with modulated supply voltage).
If the interrupter is not used the problem that appears is starting up the oscillation.
It is possible that it might start from noise, but I'm not sure if this can be counted on.
The quality solution would be to use some kind of starting oscillator, but this is where I fall into another dilemma.
I could use an oscillator that is always weakly coupled to the input so that feedback signal easily overrides it. This would be easy to implement but might reduce the loop gain too much.
The alternative would be an oscillator free driving the output, which is switched by hard logic to feedback once the signal becomes strong enough.
We would need to find the criteria of the "feedback signal becoming strong enough", though.
If I was to use a simple scheme akin to schematic I posted earlier, I would somehow need to bias the comparator to ensure it's output is certainly at 0 when no feedback signal is present - this could be done either by biasing the predikter input or by using a bias pot between pins 5 and 6 on LM311, I'm not sure which would be better. In any case the bias would again remove a bit of sensitivity but I think this would be acceptable.
Some delay before and after the feedback signal appears could be introduced as well - few cycle start delay may actually remove the initial phase offset problem (provided the oscillator is tuned well enough) by allowing the differentiator to get to the steady state before switching to feedback.
After this is solved, I think we might be getting at the picture of the ultimate non-PLL SSTC driver.
Eric: actually I'm going to use LM311 at >500kHz and I'm pretty sure it'll be fine - Steve used LM393 in his OCD circuit!
If one half bridge failed the OCD might not even do anything to save the other one though; it only senses output current and some sort of differential protection (another ocd sensing current from a few turns of wire on the differential choke) for this, although probably overkill since OCD is there to prevent any failure in first place.
BTW, do you happen to know any relatively cheap sources of good, beefy igbt's? 30N60A4D's are somewhat hard to get, found them on ebay for $6.5 and I'm wondering whether there is a better source.
27N120's are actually cheaper but lack a diode. Any suggestions for a diode (very prefferably a DPAK one) would be welcome, needs to have rating of 1200V and at least 10 or so amps. FOund these on ebay but they are extremely expensive, and I wondered if there's a better bargain around!
PS. found some interesting semiconductor items on ebay...
Some very nice mosfets! Could be even considered instead of igbt's for the application...
ISOPLUS IGBT's - if you can' find two separate heatsinks...
BIMOSFETS! Would like to try a base fed coil with those one day...
Marko
Re: CWDRSSTC
Dr. ISOTOP, Wed Nov 16 2011, 05:27AM
This IGBT seems like a good choice; it similar to the 40N60A4D, but quite a bit faster.
Dr. ISOTOP, Wed Nov 16 2011, 05:27AM
This IGBT seems like a good choice; it similar to the 40N60A4D, but quite a bit faster.
Re: CWDRSSTC
Marko, Wed Nov 16 2011, 05:34AM
Funny, I don't know why are there so many "faux" 30N60's and 40N60's around there. They tend to have rating of 70 - 80 A at 25C but is always severely derated with increase of temperature and always inferior pulsed current ratings which indicates smaller die sizes... (that igbt does though have a nice high speed).
I still haven't found any 600V IGBT that beats the HGTG30N60A4D which is derated only to 60 amps - even it's B3D version is significantly inferior.
Would love to find some affordable igbt's though that are packaged like those mosfets I found above and rated 100amps +...
Marko
Marko, Wed Nov 16 2011, 05:34AM
bwang wrote ...
This IGBT seems like a good choice; it similar to the 40N60A4D, but quite a bit faster.
This IGBT seems like a good choice; it similar to the 40N60A4D, but quite a bit faster.
Funny, I don't know why are there so many "faux" 30N60's and 40N60's around there. They tend to have rating of 70 - 80 A at 25C but is always severely derated with increase of temperature and always inferior pulsed current ratings which indicates smaller die sizes... (that igbt does though have a nice high speed).
I still haven't found any 600V IGBT that beats the HGTG30N60A4D which is derated only to 60 amps - even it's B3D version is significantly inferior.
Would love to find some affordable igbt's though that are packaged like those mosfets I found above and rated 100amps +...
Marko
Re: CWDRSSTC
Goodchild, Wed Nov 16 2011, 07:19AM
These suckers are fairly nifty APT45GP120B fast and high current rating.
Although there lack of a co-pack diode is incredibly annoying...
Goodchild, Wed Nov 16 2011, 07:19AM
These suckers are fairly nifty APT45GP120B fast and high current rating.
Although there lack of a co-pack diode is incredibly annoying...
Re: CWDRSSTC
Marko, Wed Nov 16 2011, 01:00PM
Woo, nice. I realized though that if I end up wanting to use external diodes and yet use sigma-delta QCW, the little 18A ones might not be enough! :|
Kind of hard to find a diode I can easily incorporate into my existing bridge design as I don't have any more space on the heatsinks...
Marko
Marko, Wed Nov 16 2011, 01:00PM
Goodchild wrote ...
These suckers are fairly nifty APT45GP120B fast and high current rating.
Although there lack of a co-pack diode is incredibly annoying...
These suckers are fairly nifty APT45GP120B fast and high current rating.
Although there lack of a co-pack diode is incredibly annoying...
Woo, nice. I realized though that if I end up wanting to use external diodes and yet use sigma-delta QCW, the little 18A ones might not be enough! :|
Kind of hard to find a diode I can easily incorporate into my existing bridge design as I don't have any more space on the heatsinks...
Marko
Re: CWDRSSTC
Dr. Dark Current, Wed Nov 16 2011, 01:40PM
Marko,
The school is eating up some of my free time, so the progress is somewhat slower. I have the control circuit and bridge lashed up, just need to wind a primary with taps and I can start testing the coil.
However something a bit "stinky" turned out about the IGBTs... I fired up the controller at 80 kHz (no bridge power), and the two TO220 gate drivers got very hot running the four IGBT gates! There is a figure of 3200 pf gate capacitance in the datasheet, but the real energy-related capacitance is probably around 4 times that.
Hard switched design is out of question, even though the swithing times look pretty good on paper, the switching losses don't and I'm pretty sure I would be burning huge amounts of power on switching. And add to that the huge gate capacitances and resulting slower rise/fall times...
I can't see any theoretical problem with this primary driving method, we'll se how it works in real life.
Dr. Dark Current, Wed Nov 16 2011, 01:40PM
Marko,
The school is eating up some of my free time, so the progress is somewhat slower. I have the control circuit and bridge lashed up, just need to wind a primary with taps and I can start testing the coil.
However something a bit "stinky" turned out about the IGBTs... I fired up the controller at 80 kHz (no bridge power), and the two TO220 gate drivers got very hot running the four IGBT gates! There is a figure of 3200 pf gate capacitance in the datasheet, but the real energy-related capacitance is probably around 4 times that.
Hard switched design is out of question, even though the swithing times look pretty good on paper, the switching losses don't and I'm pretty sure I would be burning huge amounts of power on switching. And add to that the huge gate capacitances and resulting slower rise/fall times...
I can't see any theoretical problem with this primary driving method, we'll se how it works in real life.
Re: CWDRSSTC
Marko, Wed Nov 16 2011, 01:50PM
Hi kilovolt,
I'm actually pretty sure the IGBT's will work in hard switched too because I have ran 10kW continuous at over 100kHz so far that way without problems. Still running resonant will make it all far easier on devices and is not a big problem to do once you get the tank caps.
I initially used UCC's in my design with mosfets, and alter just swapped for 30N60 igbt's and they kept working... I don't know how hot they actually get but they seem to survive, some TO220 gate drivers are internally biased so that they waste a lot of power even when not driving anything and that's actually the main source of heating - put them onto a big heatsink and they should be fine. Also make sure your GDT is not saturating, you'll need quite a lot of turns (20..30) oon it to prevent it from doing so. UC3710's especially sucked with this self heating as I found out.
If you don't use overcurrent protection and keep insisting on HC14 feedback though, I think there are big chances your design will blow up anyway. SO better watch for the developments in this thread! (Heck, you have enough info already!)
Marko
Marko, Wed Nov 16 2011, 01:50PM
Hi kilovolt,
I'm actually pretty sure the IGBT's will work in hard switched too because I have ran 10kW continuous at over 100kHz so far that way without problems. Still running resonant will make it all far easier on devices and is not a big problem to do once you get the tank caps.
I initially used UCC's in my design with mosfets, and alter just swapped for 30N60 igbt's and they kept working... I don't know how hot they actually get but they seem to survive, some TO220 gate drivers are internally biased so that they waste a lot of power even when not driving anything and that's actually the main source of heating - put them onto a big heatsink and they should be fine. Also make sure your GDT is not saturating, you'll need quite a lot of turns (20..30) oon it to prevent it from doing so. UC3710's especially sucked with this self heating as I found out.
If you don't use overcurrent protection and keep insisting on HC14 feedback though, I think there are big chances your design will blow up anyway. SO better watch for the developments in this thread! (Heck, you have enough info already!)
Marko
Re: CWDRSSTC
Dr. Dark Current, Wed Nov 16 2011, 03:11PM
Marko,
the GDT is not saturating, I checked by disconnecting the gates and the input power dropped 6 times.
It seems to me that the 1200v IGBTs are kinda "sluggish", even though they claim fast switching times in the datasheet. The total switching loss at 600V Vce and 40 amp current, is 3.4 mJ. If the bridge did run filtered CW at 120 kHz, each IGBT would be burning 408 Watts from switching losses! Running halfwave unfiltered power, the value is 2.82 times less (assuming constant current sink) or 4 times less (for resistive load), but still this is too much. The tuning and/or feedback with hard switched TC is also more problematic.
The gate drivers are TC4422/4421 and they don't heat up without GDT connected.
I just checked by replacing the gates with 5 nF caps, and the input power dropped to half! So this means that the effective input capacitance of the devices is in the order of 10 nF, or maybe even a bit more.
Dr. Dark Current, Wed Nov 16 2011, 03:11PM
Marko,
the GDT is not saturating, I checked by disconnecting the gates and the input power dropped 6 times.
It seems to me that the 1200v IGBTs are kinda "sluggish", even though they claim fast switching times in the datasheet. The total switching loss at 600V Vce and 40 amp current, is 3.4 mJ. If the bridge did run filtered CW at 120 kHz, each IGBT would be burning 408 Watts from switching losses! Running halfwave unfiltered power, the value is 2.82 times less (assuming constant current sink) or 4 times less (for resistive load), but still this is too much. The tuning and/or feedback with hard switched TC is also more problematic.
The gate drivers are TC4422/4421 and they don't heat up without GDT connected.
I just checked by replacing the gates with 5 nF caps, and the input power dropped to half! So this means that the effective input capacitance of the devices is in the order of 10 nF, or maybe even a bit more.
Re: CWDRSSTC
Marko, Wed Nov 16 2011, 04:52PM
Hi kilovolt,
I had the same grim predictions for 30N60's, but they outlived all expectations, I guess partly because of they really switch only the magnetizing current and not the full peak value, as well as that my heatsinking was quite well done with very low case-heatsink thermal resistance.
The predictions of very conduction losses of mosfets turned out far more grim actually, at those 60 amps the igbt's have been withstanding they would dissipate over 700W of conduction losses alone, while for igbt's this was less than 100W or so!
I don't know about the 1200V igbt's, they may be slower after all and if I was to build a 4kW coil I'd use 30N60's because I know they would work (and buck the voltage a bit with a buck transformer). So in reality no matter how bad the igbt's might be the mosfets are actually far worse.
But I guess implementing a resonant cap is always a good idea and would be neat to compare losses with one and another setup. If anything you could run at lower power temporarly with hard switching to set everything up before your tank caps arrive.
BTW, have you ordered your tank caps and where from?
Marko
Marko, Wed Nov 16 2011, 04:52PM
Hi kilovolt,
I had the same grim predictions for 30N60's, but they outlived all expectations, I guess partly because of they really switch only the magnetizing current and not the full peak value, as well as that my heatsinking was quite well done with very low case-heatsink thermal resistance.
The predictions of very conduction losses of mosfets turned out far more grim actually, at those 60 amps the igbt's have been withstanding they would dissipate over 700W of conduction losses alone, while for igbt's this was less than 100W or so!
I don't know about the 1200V igbt's, they may be slower after all and if I was to build a 4kW coil I'd use 30N60's because I know they would work (and buck the voltage a bit with a buck transformer). So in reality no matter how bad the igbt's might be the mosfets are actually far worse.
But I guess implementing a resonant cap is always a good idea and would be neat to compare losses with one and another setup. If anything you could run at lower power temporarly with hard switching to set everything up before your tank caps arrive.
BTW, have you ordered your tank caps and where from?
Marko
Re: CWDRSSTC
Dr. Dark Current, Wed Nov 16 2011, 06:24PM
Marko,
I need to use a two-phase power because the standard 230 Vac sockets here only go up to 16 amps, and I would need around 25 amps rms. Our three phase socket has no neutral, just protective earth so I must draw power from between the phases.
So far my tank cap is made from Arcotronics MMKP caps, .082 uf at 1600 volts, they are 8 in series and 2 parallel strings. I have no idea if they're going to survive the full power. If they won't take the power, I'll need to start looking for other caps. I have 50 pcs of .015 2kV MKP caps, maybe something can be made out of these.
Dr. Dark Current, Wed Nov 16 2011, 06:24PM
Marko,
I need to use a two-phase power because the standard 230 Vac sockets here only go up to 16 amps, and I would need around 25 amps rms. Our three phase socket has no neutral, just protective earth so I must draw power from between the phases.
So far my tank cap is made from Arcotronics MMKP caps, .082 uf at 1600 volts, they are 8 in series and 2 parallel strings. I have no idea if they're going to survive the full power. If they won't take the power, I'll need to start looking for other caps. I have 50 pcs of .015 2kV MKP caps, maybe something can be made out of these.
Re: CWDRSSTC
Dr. Dark Current, Thu Nov 17 2011, 02:31PM
The feedback circuit I designed was just tested and works perfectly. The circuit tunes to resonance at maybe 1-2 volts (!!) on the bridge, and the ZCS is almost perfect. So far I made some little sparks with it (~20 cm) and nothing blew up, even with the driver not shielded.
Dr. Dark Current, Thu Nov 17 2011, 02:31PM
The feedback circuit I designed was just tested and works perfectly. The circuit tunes to resonance at maybe 1-2 volts (!!) on the bridge, and the ZCS is almost perfect. So far I made some little sparks with it (~20 cm) and nothing blew up, even with the driver not shielded.
Re: CWDRSSTC
Marko, Thu Nov 17 2011, 04:54PM
Hi kilovolt,
nice work, but please do add overcurrent protection (just an LM311 and a NE555) no matter how unnecessary you think it might be.
I assume this run is already a tuned one with primary feedback? Please do some pics in any case
Marko
Marko, Thu Nov 17 2011, 04:54PM
Dr. Kilovolt wrote ...
The feedback circuit I designed was just tested and works perfectly. The circuit tunes to resonance at maybe 1-2 volts (!!) on the bridge, and the ZCS is almost perfect. So far I made some little sparks with it (~20 cm) and nothing blew up, even with the driver not shielded.
The feedback circuit I designed was just tested and works perfectly. The circuit tunes to resonance at maybe 1-2 volts (!!) on the bridge, and the ZCS is almost perfect. So far I made some little sparks with it (~20 cm) and nothing blew up, even with the driver not shielded.
Hi kilovolt,
nice work, but please do add overcurrent protection (just an LM311 and a NE555) no matter how unnecessary you think it might be.
I assume this run is already a tuned one with primary feedback? Please do some pics in any case
Marko
Re: CWDRSSTC
Dr. Dark Current, Thu Nov 17 2011, 05:29PM
Marko,
yes the run is with the resonant primary, otherwise the feedback would not work.
So far I went up to some 30 cm sparks, with the bridge supply input peaking at 230 volts and primary peak current 50 amps. The bridge peak voltage at full power will be around 560 volts. The tank circuit is close to a constant current sink, with increasing bridge voltage the current first quickly rises then the rate of rise slows down greatly. The caps so far hold up fine and don't heat up.
For the sake of keeping things simple, I omitted the over current protection. The gate drivers don't have enable pins and the only gate drivers with enable I have are in DIP packages. I have used 4 of the 16 IGBTs, so if they pop its not a big deal, but of course its better not to pop them.
I can post pictures if you want, but I warn you that my room is a real mess...
Edit: Some data which may be of interest
Dr. Dark Current, Thu Nov 17 2011, 05:29PM
Marko,
yes the run is with the resonant primary, otherwise the feedback would not work.
So far I went up to some 30 cm sparks, with the bridge supply input peaking at 230 volts and primary peak current 50 amps. The bridge peak voltage at full power will be around 560 volts. The tank circuit is close to a constant current sink, with increasing bridge voltage the current first quickly rises then the rate of rise slows down greatly. The caps so far hold up fine and don't heat up.
For the sake of keeping things simple, I omitted the over current protection. The gate drivers don't have enable pins and the only gate drivers with enable I have are in DIP packages. I have used 4 of the 16 IGBTs, so if they pop its not a big deal, but of course its better not to pop them.
I can post pictures if you want, but I warn you that my room is a real mess...
Edit: Some data which may be of interest
Re: CWDRSSTC
Marko, Thu Nov 17 2011, 08:19PM
Hi kilovolt,
Yes OMGPIX plz!
The way to implement the OCD is to use two 1k resistors in series with your gate drive inputs, and two small mosfets which clamp the inputs to ground (which inturn are controlled by NE555; see Steve Ward's DRSSTC 1 schematic)
I've also found out that current tends to settle to almost constant value once the spark loading has commenced. I'm wondering what kind of consequences this might have - so doubling the voltage may do more like doubling the power output than quadrupling it... which means I might not even be able to reach the ultimate power level I wanted using a half bridge - we'll see.
Marko
Marko, Thu Nov 17 2011, 08:19PM
Hi kilovolt,
Yes OMGPIX plz!
The way to implement the OCD is to use two 1k resistors in series with your gate drive inputs, and two small mosfets which clamp the inputs to ground (which inturn are controlled by NE555; see Steve Ward's DRSSTC 1 schematic)
I've also found out that current tends to settle to almost constant value once the spark loading has commenced. I'm wondering what kind of consequences this might have - so doubling the voltage may do more like doubling the power output than quadrupling it... which means I might not even be able to reach the ultimate power level I wanted using a half bridge - we'll see.
Marko
Re: CWDRSSTC
Goodchild, Thu Nov 17 2011, 08:45PM
This thread inspired me to bring my QCW back to life after staying dormant for a couple months. I'm going to need a larger primary so that I can use my 10nF MICA. The 25nF teflon cap is currently the only thing that will tune with the 14 turn primary. The secondary is being driven at 320KHz upper pole.
I have it all set up so anything to try out on it while I have it setup do suggest
Goodchild, Thu Nov 17 2011, 08:45PM
This thread inspired me to bring my QCW back to life after staying dormant for a couple months. I'm going to need a larger primary so that I can use my 10nF MICA. The 25nF teflon cap is currently the only thing that will tune with the 14 turn primary. The secondary is being driven at 320KHz upper pole.
I have it all set up so anything to try out on it while I have it setup do suggest
Re: CWDRSSTC
Dr. Dark Current, Thu Nov 17 2011, 08:50PM
Edit: Goodchild posted while I was typing. Nice picture, the sparks look very similar to a VTTC. My sparks branch a bit...
Note that this is all temporary proof-of-concept test setup, for the final version everything will be rebuilt and a new secondary will be wound.
So this is the mess. The PC PSU case contains a breadboard with the controller on it, and a power supply. The tank capacitor was rewired for 20.5 nF instead of the calculated 57 nF, because the frequency had to be increased from 80 to 125 kHz, higher voltage also allows higher Q. One CT is for current measurement, the "cascade" is for feedback.
The bridge. Heatsinking is inadequate but ok for short runs, a fan blows some air through the bridge. The bypass caps are 5uF 400V each, connected 2x2. Two transistors and one diode on each heatsink.
The topload is a film reel case...
Dr. Dark Current, Thu Nov 17 2011, 08:50PM
Edit: Goodchild posted while I was typing. Nice picture, the sparks look very similar to a VTTC. My sparks branch a bit...
Note that this is all temporary proof-of-concept test setup, for the final version everything will be rebuilt and a new secondary will be wound.
So this is the mess. The PC PSU case contains a breadboard with the controller on it, and a power supply. The tank capacitor was rewired for 20.5 nF instead of the calculated 57 nF, because the frequency had to be increased from 80 to 125 kHz, higher voltage also allows higher Q. One CT is for current measurement, the "cascade" is for feedback.
The bridge. Heatsinking is inadequate but ok for short runs, a fan blows some air through the bridge. The bypass caps are 5uF 400V each, connected 2x2. Two transistors and one diode on each heatsink.
The topload is a film reel case...
Re: CWDRSSTC
Marko, Thu Nov 17 2011, 08:52PM
Eric, some questions -
Firstly, I've heard that those russian teflon caps suck heavily. How long did you find yours to last in a QCW or CW duty, and how much current can they stand without overheating?
Secondly, I'm highly puzzled now how can you get any output with such large primary impedance, compared to what I observed. Have you atempted to run your system true CW without modifications, and what was the power drawn by the coil, compared to the peak power of cour QCW bang?
Marko
PS. kilovolt nice work :) No sparkpics though?
I guess you might be getting away with lower primary impedance because of low coupling provided by your huge, tall secondary. I think you might actually und up dissapoint with power level/sparks if you don't switch to full bridge or use a lower primary reactance.
Marko
Marko, Thu Nov 17 2011, 08:52PM
Eric, some questions -
Firstly, I've heard that those russian teflon caps suck heavily. How long did you find yours to last in a QCW or CW duty, and how much current can they stand without overheating?
Secondly, I'm highly puzzled now how can you get any output with such large primary impedance, compared to what I observed. Have you atempted to run your system true CW without modifications, and what was the power drawn by the coil, compared to the peak power of cour QCW bang?
Marko
PS. kilovolt nice work :) No sparkpics though?
I guess you might be getting away with lower primary impedance because of low coupling provided by your huge, tall secondary. I think you might actually und up dissapoint with power level/sparks if you don't switch to full bridge or use a lower primary reactance.
Marko
Re: CWDRSSTC
Goodchild, Thu Nov 17 2011, 09:20PM
hehe the secondary is actually rather short! only 9 inches tall. The coupling as of now is at 0.369k and the peak primary current is low, only about 75Apeak. The OCD is set at 100Apk but is not tripping.
The Russian teflon caps do indeed suck hard! Running at about 10pps with the on time about 10mS the cap heats up to about 100C in about 10 mins. I have a supper awesome MICA that is just begging to be used but the performance is sucky because I can only tune the primary to about 350KHz rather than the 320KHz that the secondary is at.
The QCW is not about peak current! That's what a lot of people don't get. The spark is built on top of it's self and and grown or stretched out over a long time. In order to make a spark in a very short amount of time like a regular DR you need a LOT of voltage to do it, which means high tank currents! But with the QCW you build the sparks over a long period of time using very little top volts <50KV in the sparks pic shown above. This equates to low tank currents and a high Z tank. That is why it's not flashing over with the supper high coupling.
The highest Ive run the QCW at is about 80% duty cycle. During this run the power draw was easily 10A from the 120VAC mains and was only making about 1 foot of spark. This was at a relatively low power as those 60N60s can only take so much! In the photo above the coil is at about 40% duty cycle and is pulling about 4A off of the 120VAC mains.
Goodchild, Thu Nov 17 2011, 09:20PM
Marko wrote ...
Eric, some questions -
Firstly, I've heard that those russian teflon caps suck heavily. How long did you find yours to last in a QCW or CW duty, and how much current can they stand without overheating?
Secondly, I'm highly puzzled now how can you get any output with such large primary impedance, compared to what I observed. Have you atempted to run your system true CW without modifications, and what was the power drawn by the coil, compared to the peak power of cour QCW bang?
Marko
PS. kilovolt nice work :) No sparkpics though?
I guess you might be getting away with lower primary impedance because of low coupling provided by your huge, tall secondary. I think you might actually und up dissapoint with power level/sparks if you don't switch to full bridge or use a lower primary reactance.
Marko
Eric, some questions -
Firstly, I've heard that those russian teflon caps suck heavily. How long did you find yours to last in a QCW or CW duty, and how much current can they stand without overheating?
Secondly, I'm highly puzzled now how can you get any output with such large primary impedance, compared to what I observed. Have you atempted to run your system true CW without modifications, and what was the power drawn by the coil, compared to the peak power of cour QCW bang?
Marko
PS. kilovolt nice work :) No sparkpics though?
I guess you might be getting away with lower primary impedance because of low coupling provided by your huge, tall secondary. I think you might actually und up dissapoint with power level/sparks if you don't switch to full bridge or use a lower primary reactance.
Marko
hehe the secondary is actually rather short! only 9 inches tall. The coupling as of now is at 0.369k and the peak primary current is low, only about 75Apeak. The OCD is set at 100Apk but is not tripping.
The Russian teflon caps do indeed suck hard! Running at about 10pps with the on time about 10mS the cap heats up to about 100C in about 10 mins. I have a supper awesome MICA that is just begging to be used but the performance is sucky because I can only tune the primary to about 350KHz rather than the 320KHz that the secondary is at.
The QCW is not about peak current! That's what a lot of people don't get. The spark is built on top of it's self and and grown or stretched out over a long time. In order to make a spark in a very short amount of time like a regular DR you need a LOT of voltage to do it, which means high tank currents! But with the QCW you build the sparks over a long period of time using very little top volts <50KV in the sparks pic shown above. This equates to low tank currents and a high Z tank. That is why it's not flashing over with the supper high coupling.
The highest Ive run the QCW at is about 80% duty cycle. During this run the power draw was easily 10A from the 120VAC mains and was only making about 1 foot of spark. This was at a relatively low power as those 60N60s can only take so much! In the photo above the coil is at about 40% duty cycle and is pulling about 4A off of the 120VAC mains.
Re: CWDRSSTC
Dr. Dark Current, Thu Nov 17 2011, 09:20PM
Marko,
by extrapolating the peak current and spark length curves, I am getting close to the "design" values at 560 volt peak input.
I know the coupling is low. The new secondary will be fatter and shorter. This is just a proof of concept setup, and I've proved it works
Dr. Dark Current, Thu Nov 17 2011, 09:20PM
Marko,
by extrapolating the peak current and spark length curves, I am getting close to the "design" values at 560 volt peak input.
I know the coupling is low. The new secondary will be fatter and shorter. This is just a proof of concept setup, and I've proved it works
Re: CWDRSSTC
Goodchild, Thu Nov 17 2011, 09:25PM
That looks a lot like the QCW sparks when running it from a strait DC bus rather than the class D modulator! The two key things that make the QCW look like VTTC sparks is the high res freq 300KHz+ and also the slow linear ramp of the power over the burst length.
Goodchild, Thu Nov 17 2011, 09:25PM
Dr. Kilovolt wrote ...
Marko,
by extrapolating the peak current and spark length curves, I am getting close to the "design" values at 560 volt peak input.
I know the coupling is low. The new secondary will be fatter and shorter. This is just a proof of concept setup, and I've proved it works
Marko,
by extrapolating the peak current and spark length curves, I am getting close to the "design" values at 560 volt peak input.
I know the coupling is low. The new secondary will be fatter and shorter. This is just a proof of concept setup, and I've proved it works
That looks a lot like the QCW sparks when running it from a strait DC bus rather than the class D modulator! The two key things that make the QCW look like VTTC sparks is the high res freq 300KHz+ and also the slow linear ramp of the power over the burst length.
Re: CWDRSSTC
Dr. Dark Current, Thu Nov 17 2011, 09:38PM
Goodchild,
The coil runs from half wave rectified unfiltered power. There is no interrupter or other modulator. The low switching frequency was chosen because of the IGBTs... I'm not going for anything fancy, just a way to increase inverter efficiency over a normal non-tuned primary, but still not going to the point of "standard" DRSSTCs with long thin sparks and extreme noise.
Note: The coil is running at relatively low power now, I'm going for 2-3 times longer sparks in the final design. This might change the spark appearance too.
Dr. Dark Current, Thu Nov 17 2011, 09:38PM
Goodchild,
The coil runs from half wave rectified unfiltered power. There is no interrupter or other modulator. The low switching frequency was chosen because of the IGBTs... I'm not going for anything fancy, just a way to increase inverter efficiency over a normal non-tuned primary, but still not going to the point of "standard" DRSSTCs with long thin sparks and extreme noise.
Note: The coil is running at relatively low power now, I'm going for 2-3 times longer sparks in the final design. This might change the spark appearance too.
Re: CWDRSSTC
Goodchild, Thu Nov 17 2011, 10:13PM
Just for giggles I turned up the power a bit. The coil is now running from a 120VAC variac to a doubling transformer so 240VAC in but had it at about 200VAC for the photo. primary current was about 85Apk occasionally hit the 100A limiter once or twice. sparks are about 48 inches. No flash overs so voltage is still less than 60Kv ish.
I also put the 10nF MICA in there for this run, I'm tuned all the way at the bottom of the primary! Event with the coil 20KHz out of tune the thing still manged to push out 48 inches while under 600w
QCW is one awesome coil...
instead of making a new primary I will probably just wind a new secondary. another 9 x 4.5 but with 28 AWG wire rather than 30. This will put the upper pole in the 450KHz+ range and make the primary current tune to an even higher tank impedance. This should straiten out the sparks and also lower the primary current.
Also here is a video of some very close to CW action about 80% duty
Goodchild, Thu Nov 17 2011, 10:13PM
Just for giggles I turned up the power a bit. The coil is now running from a 120VAC variac to a doubling transformer so 240VAC in but had it at about 200VAC for the photo. primary current was about 85Apk occasionally hit the 100A limiter once or twice. sparks are about 48 inches. No flash overs so voltage is still less than 60Kv ish.
I also put the 10nF MICA in there for this run, I'm tuned all the way at the bottom of the primary! Event with the coil 20KHz out of tune the thing still manged to push out 48 inches while under 600w
QCW is one awesome coil...
instead of making a new primary I will probably just wind a new secondary. another 9 x 4.5 but with 28 AWG wire rather than 30. This will put the upper pole in the 450KHz+ range and make the primary current tune to an even higher tank impedance. This should straiten out the sparks and also lower the primary current.
Also here is a video of some very close to CW action about 80% duty
Re: CWDRSSTC
Dr. Dark Current, Thu Nov 17 2011, 10:29PM
Very nice
Is there any specific reason why you are tuning to the upper pole of the secondary?
Dr. Dark Current, Thu Nov 17 2011, 10:29PM
Very nice
Is there any specific reason why you are tuning to the upper pole of the secondary?
Re: CWDRSSTC
Goodchild, Thu Nov 17 2011, 10:37PM
Yeah there is. The secondary's F0 is in the 280KHz area this is to low for QCW operation, the sparks would branch and look a lot more like what your coil looks like. It would also be supper silly to make a secondary with a F0 at 350KHz because it would ether be really small or have very few turns. So I did a larger secondary and because the coupling is so high the upper pole is also very far away from the natural resonant freq making it a great choice for high frequency operation.
Richie's site explains this very well! Thanks again Richie! I was in the dark until I read that page.
Goodchild, Thu Nov 17 2011, 10:37PM
Dr. Kilovolt wrote ...
Very nice
Is there any specific reason why you are tuning to the upper pole of the secondary?
Very nice
Is there any specific reason why you are tuning to the upper pole of the secondary?
Yeah there is. The secondary's F0 is in the 280KHz area this is to low for QCW operation, the sparks would branch and look a lot more like what your coil looks like. It would also be supper silly to make a secondary with a F0 at 350KHz because it would ether be really small or have very few turns. So I did a larger secondary and because the coupling is so high the upper pole is also very far away from the natural resonant freq making it a great choice for high frequency operation.
Richie's site explains this very well!
Thanks again Richie! I was in the dark until I read that page. Re: CWDRSSTC
Marko, Fri Nov 18 2011, 12:05AM
Hey goodchild, good work.
Actually, what stops you from ramping this up while you get some several meters worth of arcs? :O From what it looks to me all you need is more energy storage..
The peak power of your coil with 100A was like 30kW, while I had a bridge run 24kVA in CW - which means you could keep almost indefninte on times there...
I guess the only real danger is your ridiculously overcoupled primary flashing over I guess.
Marko
Marko, Fri Nov 18 2011, 12:05AM
Hey goodchild, good work.
Actually, what stops you from ramping this up while you get some several meters worth of arcs? :O From what it looks to me all you need is more energy storage..
The peak power of your coil with 100A was like 30kW, while I had a bridge run 24kVA in CW - which means you could keep almost indefninte on times there...
I guess the only real danger is your ridiculously overcoupled primary flashing over I guess.
Marko
Re: CWDRSSTC
Goodchild, Fri Nov 18 2011, 12:18AM
Yeah energy store is a big part of it. I currently have 10mF at 380VDC on the class D. The main thing that stops me from going to say like 30Kw is that bridge of 60N60 IGBTs. At 100A CW they would be in for a world of hurt event with water cooling. I run them pulsed right now so they will happily run at 100A with not to much trouble up to about 50% duty. Any higher than that and they tend to go boom....
Yeah the primary's high coupling is not to big of a problem, because if you are going CW and you still keep your ramps in there the peak top volts should still be low, you just get lots and lots of individual sparks that clump together into one big bush! So it can't go true CW but it can get dang close say 98%
Goodchild, Fri Nov 18 2011, 12:18AM
Marko wrote ...
Hey goodchild, good work.
Actually, what stops you from ramping this up while you get some several meters worth of arcs? :O From what it looks to me all you need is more energy storage..
The peak power of your coil with 100A was like 30kW, while I had a bridge run 24kVA in CW - which means you could keep almost indefninte on times there...
I guess the only real danger is your ridiculously overcoupled primary flashing over I guess.
Marko
Hey goodchild, good work.
Actually, what stops you from ramping this up while you get some several meters worth of arcs? :O From what it looks to me all you need is more energy storage..
The peak power of your coil with 100A was like 30kW, while I had a bridge run 24kVA in CW - which means you could keep almost indefninte on times there...
I guess the only real danger is your ridiculously overcoupled primary flashing over I guess.
Marko
Yeah energy store is a big part of it. I currently have 10mF at 380VDC on the class D. The main thing that stops me from going to say like 30Kw is that bridge of 60N60 IGBTs. At 100A CW they would be in for a world of hurt event with water cooling. I run them pulsed right now so they will happily run at 100A with not to much trouble up to about 50% duty. Any higher than that and they tend to go boom....
Yeah the primary's high coupling is not to big of a problem, because if you are going CW and you still keep your ramps in there the peak top volts should still be low, you just get lots and lots of individual sparks that clump together into one big bush! So it can't go true CW but it can get dang close say 98%
Re: CWDRSSTC
Marko, Sat Nov 19 2011, 09:50PM
Hi guys
Anyone interested into a group buy of these igbt's? They are pretty awesome and cost about $20 on digikey.
Could be a good resource for future QCW work.
Marko
Marko, Sat Nov 19 2011, 09:50PM
Hi guys
Anyone interested into a group buy of these igbt's? They are pretty awesome and cost about $20 on digikey.
Could be a good resource for future QCW work.
Marko
Re: CWDRSSTC
Goodchild, Sat Nov 19 2011, 10:26PM
Ahhh looks like the IGBT's I linked earlier but with a co-pack diode. I didn't think they made them...
They are basically the exact same speed as the 60N60s.
Pulsed rating is lower, but max dissipation is higher it also looks like they have a positive tempco at 90A or more (nice for paralleling).
Gosh I would probably buy some, but I'm flat broke right now...
Goodchild, Sat Nov 19 2011, 10:26PM
Marko wrote ...
Hi guys
Anyone interested into a group buy of these igbt's? They are pretty awesome and cost about $20 on digikey.
Could be a good resource for future QCW work.
Marko
Hi guys
Anyone interested into a group buy of these igbt's? They are pretty awesome and cost about $20 on digikey.
Could be a good resource for future QCW work.
Marko
Ahhh looks like the IGBT's I linked earlier but with a co-pack diode. I didn't think they made them...
They are basically the exact same speed as the 60N60s.
Pulsed rating is lower, but max dissipation is higher it also looks like they have a positive tempco at 90A or more (nice for paralleling).
Gosh I would probably buy some, but I'm flat broke right now...
Re: CWDRSSTC
Steve Ward, Mon Nov 21 2011, 10:32PM
Kilovolt asked why operate at the upper pole. The reason I did this was initially an accident, but what i later realized was that there is a benefit to this. The main thing is that it reduces the stored energy in the system because the primary and secondary magnetic fields are cancelling each other out. This allows the use of a "large" resonator (4.5" x 9" seems awfully popular now) but still obtaining a high operating frequency which appears to be quite useful for spark propagation with lower top voltage.
I suspect you could likely use a smaller, higher F resonator and tune it for the lower pole if you wanted... and i havent tried this, maybe there is something to it.
Steve Ward, Mon Nov 21 2011, 10:32PM
Kilovolt asked why operate at the upper pole. The reason I did this was initially an accident, but what i later realized was that there is a benefit to this. The main thing is that it reduces the stored energy in the system because the primary and secondary magnetic fields are cancelling each other out. This allows the use of a "large" resonator (4.5" x 9" seems awfully popular now) but still obtaining a high operating frequency which appears to be quite useful for spark propagation with lower top voltage.
I suspect you could likely use a smaller, higher F resonator and tune it for the lower pole if you wanted... and i havent tried this, maybe there is something to it.
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