Re: 20KW DRSSTC
thedatastream, Mon Mar 26 2007, 10:20AM

Very, very impressive, that's a nice piece of engineering - congratulations. I look forward to seeing the results on a really good run.
I think I shall give up coiling and go and live in a hole somewhere

I would also be interested to see your primary fullbridge and decoupling layout for something of that power.

Give yourself a pat on the back
James

p.s. I wonder what the driver of that car thought?

Re: 20KW DRSSTC
Steve Conner, Mon Mar 26 2007, 11:09AM

Awesome! Madness!

Re: 20KW DRSSTC
Bennem, Mon Mar 26 2007, 11:55AM

Ermmm.....what can i say , but,....... WOW !!!


ps....lol at driver deciding to turn right.

Re: 20KW DRSSTC
Andrew L., Mon Mar 26 2007, 12:35PM

Holy mother of all Tesla coils It makes my puny Tesla coils look so small and insagnificant .

Re: 20KW DRSSTC
Marko, Mon Mar 26 2007, 12:40PM

Holy Thor! The best owned picture ever:

And.. you are driving CM1000's with a GDT? It must be a monster

..I don't know what anymore to say!

Re: 20KW DRSSTC
Sulaiman, Mon Mar 26 2007, 01:14PM

Jaw-dropingly WOW!

Great coil and photo's

Congratulations

Re: 20KW DRSSTC
vasil, Mon Mar 26 2007, 01:17PM

Cool! Impressive project!

Re: 20KW DRSSTC
Steve Ward, Mon Mar 26 2007, 04:45PM

Nice to see that driver work like it *should* this time .

That toroid is pretty cool looking, and now it explains the toroid on the large SGTC on the hvguy site. Pretty clever, and surely cheaper than getting the *big* corrugated tube (my 9" OD stuff was like 35 bucks).

How can you expect 20' with the toroid only ~10 feet above the ground?! Do you have a taller stand in the works?

Id really like you to take an "official" measurement of the primary current. Last i remember, i was running about 1500Apk for only the 10-12 foot sparks, so 2kA for your 16 foot spark would seem about right.

Oh, and your workspace is really organized .

Re: 20KW DRSSTC
hvguy, Mon Mar 26 2007, 05:34PM

Thanks for the comments everyone. It's always nice when a project of this scale finally gets off the ground.

Steve, believe it or not all the problems I was having at the thon were because of a half dead UCC part. My gate waveforms were all screwed up which created some odd transits and took out one of the driver IGBTs and fried that sand block. I think the part was dead before it was even installed….

It may seem like expecting +20’ from a coil of this height is unlikely but I base this off of two things. 1) My 10” coil hit 16’ and is about 9’ tall 2) The longest arc last night was not caught on video but was 18’ out to the red cart on the left, so why not another two feet or so. Also, I can raise my toroid up another 12” using the all thread it is mounted on.

If the weather holds up I will run again tonight and take a primary current measurement.

Thanks... It’s not always that clean though, this project took so much space I had to keep it as clean as possible for everything to fit.

Re: 20KW DRSSTC
sparky, Mon Mar 26 2007, 06:53PM

20' discharge ...hum mm- sounds like it would be quite possible. I'd adjust your top load for sure, and maybe add another turn of copper tube?? I wonder what a Tesla magnifier system would do on your monster power supply!! ....anyways this makes me really want to build one just for the hell of it!

I just found a brand new Powerex CM1200HA-24J ... I wonder if these kind are okay for a Tesla Coil ???
and even one DIM2400ESM17A00 .... I'm not sure if these are very useful??

Tell me... why are you running 2 IGBTs together?? ...maybe a stupid question but I'm a little new to this kind of coil system.

Re: 20KW DRSSTC
Marko, Mon Mar 26 2007, 09:20PM

As fas as I see, he's running a most basic fullbridge with a beefy GDT powered directly from mains.

In a way, it's more reliable than isolated driver since there's practically no way for both IGBT's in bridge to turn on simultaneously and for long.

I just found a brand new Powerex CM1200HA-24J ... I wonder if these kind are okay for a Tesla Coil ???
and even one DIM2400ESM17A00 .... I'm not sure if these are very useful??

Well.. if you can find a fullbridge of them, surely! Althoguh they are slow and huge overkill considering those IGBT's in this monster actually have a lower current rating.

Re: 20KW DRSSTC
sparky, Tue Mar 27 2007, 01:37AM

I wonder what current/volt rating those IGBT are that Aaron is using??
1200V 600A? Humm... curious.

Re: 20KW DRSSTC
Steve Ward, Tue Mar 27 2007, 04:51AM

Steve, believe it or not all the problems I was having at the thon were because of a half dead UCC part.

Here's one for ya. When my coil failed at DCs thon, and the gate driver card was "down"... guess what the problem was? The negative supply regulation zener! Took me so long to figure that one out as it seems so unlikely. So that, along with the obviously blown IGBT. So i will have to agree, your driver is probably a little easier to de-bug.

Re: 20KW DRSSTC
hvguy, Tue Mar 27 2007, 05:49AM

Well the weather didn’t just hold up for tonight, it was perfect; practically no wind, about 65 and 50% humidity. So given these ideal circumstances I expected to push the 20’ mark with relative ease. Everything was set up as the night before. I did a few test runs and nailed down the tuning. Things were looking good with a few ~10sec runs easily reaching out to 18’ at about 70% input power. I set up the camera and DSO to capture some primary current waveforms, ran the variac stack up to about 50% then… This happen:



Shit! I hope I didn’t kill an $1100 transistor (not that I paid that for it, but still…)! I checked the 60A ultra-fast semiconductor fuse on my PS and it was blown so I shut everything down and went to take a look. This is what I found:





After a few intense moments checking the IGBTs I found they were fine; looks like the TVSs, which all had holes blown out the sides, did their job. Everything else is tested and functional except the other TVS strings. They too showed signs of imminent failure (one string measured 1180ohms, both ways).

Ok, so why did 20 (1.5KE200 2 parallel strings of 5 in series on each IGBT) TVSs fail so catastrophically after only maybe 2min total run time? Upon closer inspection the solder had begun to melt off the undetonated strings on the other brick. This tends toward a fairly obvious answer of over-dissipation. This does not sound unreasonable as the total delay time at the switching transitions, because of these huge, slow, IGBTs it almost 1uS. While I did not get a scope shot of the primary current I did manage to read the screen before the failure. My guess was right on at nearly 2KA Pk on the primary, possibly less (lower Vin) when it failed; I was not watching the scope then.

So if TVSs can not handle the spikes what’s next? Well, I will be bench testing the system without the secondary in circuit to get an idea of the voltage spikes I am dealing with. Given the scale of things I would not be surprised if they were 2X the rail. All this with only 2” between the caps and the IGBT... I really think the delay in these things is causing this problem. The thing is I can’t fix the delay, its inherent to these IGBTs, so now I have to figure out how to deal with it. Any Ideas?

Steve, I hate problems like that, they just suck time like a black hole and it’s never that satisfying in the end. Mine is probably easier to trouble shoot but it’s still a Tesla coil; I’d swear it is impossible to build one of these things with out something failing…

Re: 20KW DRSSTC
sparky, Tue Mar 27 2007, 07:54AM

I'm wondering why all the complexity in your circuit...excuse me for being naive I've never built one of these systems before, but I can't help but notice the circuit controls, variacs, diode this diode that.

Why not boost the voltage using a rewired transformer and rectify it to a higher voltage - say 800-900VDC. Maybe this would be less of a problem a give a bigger bang size- cut down on the amount of silicon used between the IGBT and the TC primary???

Re: 20KW DRSSTC
Steve Conner, Tue Mar 27 2007, 10:44AM

Hi Aron,

The scale of this thing is really awesome! Even if you "only" ever got 18' you could still be pretty proud of it. Here are a few things I thought of:

1) Most even halfway recent IGBTs have avalanche capability. That is to say, the IGBT dies themselves have TVS-like behaviour. Even if they only have a very modest avalanche rating, I bet they can still take more abuse than those tiny TVS you burnt out. These IGBTs typically hard-switch hundreds of amps in industrial service, with maybe a snubber capacitor, but certainly no bolt-on TVS.

So the system may work fine with no TVS at all (but check the IGBT datasheet, or call the IGBT manufacturer, if you want to be sure) If you really want to use TVS, a series-parallel "MMC" array of them with forced air or oil cooling may be a good option.

2) As for coping with the turn-off delays of large IGBTs, my solution to that was the PLL driver, which can anticipate the current zero crossing and send the turn-off signal in advance. This was pretty much the only reason I spent so much effort on PLLs, because I thought they would scale to the biggest coils. Unfortunately I've been too busy with other stuff to try it out on IGBT bricks yet, it could be a total flop

Sparky: The bigger you make a solid-state coil, the more expensive the damage when it blows up. So, for bigger coils, it's worth spending more effort on protection circuitry. The power circuitry on Aron's coil is already as simple as a SSTC can be.

Re: 20KW DRSSTC
sparky, Tue Mar 27 2007, 04:27PM

I was thinking along the same lines too... having protection circuits is a bonus and I'd hate to see an expensive IGBT fry!!


I am really amazed by the sheer output of Arons Tesla Coil. I'd love to build one - though I must admit I've never really been one for Solid State devices --- guess I should buy a book on it. I'd would really like to learn more about these kinds of Tesla Coils...

Re: 20KW DRSSTC
Dr. Drone, Tue Mar 27 2007, 05:34PM

Re: 20KW DRSSTC
hvguy, Tue Mar 27 2007, 07:46PM

Steve C., you make a good point. I had not considered that as an option so I called Powerex and spoke to an applications engineer and, believe it or not, the entire CM series has NO avalanche capability! He said if you exceed Vce a few times the part will most likely fail. In light of this new information I may have to consider an active clamp. This would allow me to dissipate the spikes in something more conveniently heat sunk than a DO-201 diode or even recover the spike energy. Of course, pending the bench testing, this may be over kill. How about the good-ol RCD snubber and a big fan?

Also, you had mentioned a PLL. I experimented with this back in 98 when I was working with the SSTCs. I do not recall having much luck with it. Ultimately I did get a CD4046 based circuit working but it was such a hassle to adjust and so much more complicated than the feedback scheme of the time that I abandon the idea. Perhaps I should reconsider… Is your design as reliable as my current driver? I have a small DRSSTC that has logged more than 200hrs with this set up so I am a little leery to change to something new.

Sparky, these parts are rated 1000A 1700V. You can google the PN for a data sheet. Yes, I would much rather nuke $40 of TVSs than one IGBT.

Chris, thanks! One thing I learned from the 100 or so coils I have built is that a larger top=less primary and ground hits. If you look at the e-field shape of a coil with a wide top it becomes fairly obvious why this is. Further more if you place a break out point up and at an angle off this wide toroid the arcs have a tendency to reach a bit further before grounding. So, no, for now I will leave it at this height. If ground strikes become a problem at higher power levels I may reconsider. Far as the thon goes...I don't know. It depends on whether time/money permit.

Re: 20KW DRSSTC
Steve Conner, Tue Mar 27 2007, 09:16PM

Hi Aron,

I have no idea if my driver is as reliable as Steve Ward's, and a coil that throws 18' sparks is probably not a good place to start the testing! I have never had a single IGBT failure with it, and neither has Finn, AFAIK. But Steve's is pretty much the "industry" standard now.

If you want to know more about the PLL, see

Re: 20KW DRSSTC
Tom540, Wed Mar 28 2007, 01:56AM

Speaking of your PLL Steve. I was thinking of using it to rebuild one of my coils. I have a question about your schematic. What are the parts labeled as LCL1-5? They look like two series resistors and a cap.

Re: 20KW DRSSTC
Steve Ward, Wed Mar 28 2007, 03:00AM

Looks like you have gotten yourself in the same place i was at, with the inherent IGBT delays.

For the CM600 bridge i built for Chris R, i put RCD snubbers (using film and ceramics in parallel for the C) on each IGBT. This helped a little... The other thing was to slow down the turn ON of each IGBT, since its really the diode recovery that kills ya in this case (since the switching lags the "active" current, and takes place during the recycle portion of the current flow).

In the end, for my own coil, i decided that i just had to reduce the input voltage to a safer level, which meant no more than 700VDC on the 1200V silicon (and even that might be pushing it). Ever notice that all of the tests in the datasheets list the supply voltage at 600VDC? There must be a reason for all that headroom... and yes, that sucks.

Re: 20KW DRSSTC
hvguy, Wed Mar 28 2007, 07:16AM

Worst case voltage spike was found to be 2.3X the rail. This definitely explains why the 1KV TVS string exploded. Here is a scope shot at 170V:



Although the caps are already very close to the terminals, about 2”, that is still room for improvement. This design was not originally intended for 20KW or 800V. I actually built it for a commercial application at about 6KW at 350V. At that voltage and power level the leakage spikes where of no concern. However, that project fell though so I decided to use the driver as-is for the 16”.

I managed to get the fast spikes down by about 50% with an RCD snubber. The problem is the snubber was dissipating about 20W at 170V. That means well over 100W at full power, per IGBT, if I go that route; not cool. Unfortunately, I think a mechanical redesign will need to happen in order to get these spikes down. Hopefully this will allow me to hit the 800V/24’ mark. I will still include some form of protection, hopefully a TVS string will do it provided the spikes are occasional rather than almost constant as they are now.

I agree Steve; it is becoming ever more obvious why these large bricks are rated more than 1KV. BTW, what values did you use for your snubber? I ended up with 25ohms and 220nF.

Re: 20KW DRSSTC
Hazmatt_(The Underdog), Wed Mar 28 2007, 07:29AM

Can you not snub that HF transistion by putting 1 or 2 turns of one of your power busses through a huge ferrite core?

I might be way off, but that really HF switching noise is going to be almost impossible to eliminate unless you use some kind of choke.

Re: 20KW DRSSTC
Steve Ward, Wed Mar 28 2007, 04:43PM

Can you not snub that HF transistion by putting 1 or 2 turns of one of your power busses through a huge ferrite core?

I might be way off, but that really HF switching noise is going to be almost impossible to eliminate unless you use some kind of choke.

Eeek no! thats the exact opposite of what is needed since any added inductance (while lowering the frequency) would store more energy, making the spikes bigger yet. Chokes are fine if you need to keep noise from getting in, but they dont actually stop the noise from being generated. I cant blame you if you dont immediately understand the problem Aron has, since its more of a power electronics issue (something i certainly havent learned from school yet), and you probably wouldnt ever think about it unless you start messing with these big machines.

Aron, your spikes of 2.3X Vbus arent *that* bad really. I found that as i increased my voltage input, the spikes became less severe (of course, this might just be my cheap scope probes to blame). If you are only going to 700VDC on 1700V silicon, then you should be OK i would say. Anyway, if you snubber is dissipating that much power, you need to redesign it. For the CM600 snubbers, i think i had a .68uF film cap plus .1uF in ceramics. The resistors are not wired to completely drain the caps, but rather to reset their voltage to Vbus. If you wired your RCD to completely drain the caps every half-cycle, then yes, thats a lot of energy to dissipate. I found the performance of the 2 different schemes to be similar really, and the first scheme disspates very little power overall (i was able to use 5W resistors). Of course, unless your snubber has lower inductance than your buss structure, you wont gain anything.

Keep in mind that your 1uF cap, without any sort of diode in series, will allow ringing. Id be curious to see what happens if you removed that cap (in some cases i found it helpful!). I was also surprised to see how much a dinky .1uF ceramic disk cap (the ones that are the size of a nickel) did to help reduce the ringing... Of course, if you cant get your snubber down to <20nH, its almost useless .

Re: 20KW DRSSTC
Finn Hammer, Wed Mar 28 2007, 06:24PM

Aron,

You coil is awesome!
I have been working on a way to predict the zero crossing in advance, on a level where there is no PLL or other strange stuff involved.
I have gotten it to work in Orcad, this is how it is:

At the resonant frequency, let`s assume that when the current has fallen to 1/10th of the peak current, then there is 1µS to the zero crossing.
I`m not using real numbers here, just some nice numbers to help draw the picture.

Now, if you take the signal from a current transformer monitoring the primary current, and rectify it, you have a pulsating DC signal that indicates the primary current.
You use this signal to do 2 things:
You sample the signal, and hold it at the peak. Divide it with a 1:10 resistive divider. Feed this into a comparator.
The other input of the comparator receives the undivided signal from the transformer.
When this signal gets smaller than the 1/10th of the peak signal, the comparator changes state, and there you have your "let`s initiate the transition" signal.

From this signal, with RC delays and schmitt triggers, you can derive the precise timing for the turn on and turn off of the bricks.

In a real circuit, there are diode drops to compensate for, source followers to add servo boost etc. but this is just the basic idea.

In orcad, this has shown, that within reasonable tuning points, the IGBT`s are not forced to switch more than 45A even if the peak current is 1200A.

This scheme will not adapt to the change in resonant frequency caused by streamer loading, but it will allow you to tune the coil to produce beat envelopes should you desire so.

I can send you a Orcad file to play with, if you are interested.

Cheers, Finn Hammer

Re: 20KW DRSSTC
hvguy, Wed Mar 28 2007, 10:39PM

Thanks Finn. If you don't mind I would like that file, you can send it to **link**. I like the concept. Sounds like it should be simple and relatively effective. Compensation for the frequency shift would be nice, but a shift of only 2KHz or so should not have a significant affect on things since the ZC will only change by a 100ns or so.

While testing various snubber options I notice another problem. Every time my girl friend turned the hot water on in the house, 150’ away, my driver would begin to oscillate at about 2Hz and trash the GD waveforms. Ok, this was strange… I figured the most logical reason for the oscillation was due the lower line voltage when the 17KW instant hot water heater was on. I measured the line voltage, and sure enough, the voltage dropped from 117V (already a little low) to 113V. While this was clearly causing the problem I was confused as to why a 4% line voltage change would have such a profound affect on the logic. Just to verify the results I hooked the logic board to a variac and lowered the voltage until the problem started, which was at about 115V. My 15V rail is regulated and showed no change even at 90V on the line. The 160V rail for the GD dropped, as it should, but a change of 7v is divided down though the GDT to only 1V so there was no significant affect on the already 30V gate waveform. At this point it occurred to me the only non-regulated logic device on board is the PC817 opto coupler between the remote ON/OFF switch and the logic. I checked the voltage at its emitter and found it to be right at the turn on threshold for the 2N7000 it was driving; there’s my problem. Simple enough, I changed the 1K resistor on the PC817s LED to 560ohms and the 1K pull up on the 2N7000 gate to 4.7K. This fixed the problems but it makes me wonder how much of an affect this had on the TVS failure. I know the coil drops the line voltage out here in the shop by at least 10V at full power. This may have been the reason for the failure… Either way its fixed and the voltage spikes are still to excessive for my liking, so back to the bench.

BTW, Steve, it turns out my CDE snubbers where causing more harm than good. I am trying different combinations of caps to kill the HF spikes…. We’ll see how it goes.

Re: 20KW DRSSTC
Tom540, Thu Mar 29 2007, 03:15PM

Hey Aron,

Wow that coil is 1337. I'd love to see that thing run without a breakout point. Anyway I thought I would do some research here at work Before I quit in two weeks. We(Elgar)make some big beefy power supplies here. Some are in the 30KW region. I'm gonna browse through the schematics and see if I can find a super snubber design that can tackle your problem. Theres gotta be something.

Re: 20KW DRSSTC
hvguy, Thu Mar 29 2007, 09:25PM

That would be great, thanks. I recently purchased two surplus Elgar AC supplies to sell on eBay. I opened them up before listing them; very well built equipment.

I am currently redesigning the full bridge mechanical layout. I think the design I have come up with is about as low inductance as is humanly, or mechanically, possible. I will post an update with pictures and test results later today.

Re: 20KW DRSSTC
Mike, Thu Mar 29 2007, 11:13PM

Wow , very cool coil.

Eat your heart out Steve Ward!!! :P
Just kidding, but I think it is time for a tesla arc war!

Re: 20KW DRSSTC
Finn Hammer, Fri Mar 30 2007, 06:12AM


And from an angle, where we can see that novel gate transformer?

Cheers, Finn Hammer

Re: 20KW DRSSTC
hvguy, Fri Mar 30 2007, 07:47AM

I have spent most of the day experimenting with various snubbers trying to find the best solution for the new bridge assembly. Here’s what I came up with:



Not shown in this schematic are four parallel 20nF 1KV ceramic disc caps placed rail to rail on each half bridge section. All my testing was done with a buss voltage of 170V at about 2.3KW. With this snubber design I was able to reduce my transients to about 1.5X the rail. This is just fine by me as the most input voltage I had planned to run was 800V and these are 1.7KV parts; that’s 500V head room. The TVS strings are designed to begin conducting at ~1250V and clamp at ~1550V, just in case a spike does make in though the snubber. I’d still rather nuke a string of TVSs than an IGBT. Here are a few scope shots:

No snubber:
Snubber:


And, Finn, here is the GDT. The core has a 1.5” ID, primary is .125” flat Teflon wire secondary is 600V 14ga PVC. Nothing special really. Although I would HIGHY recommend anyone duplicating this shield the primary from the secondary, it will eliminate noise feedback though all the various capacitances in the gate driver.




Just check out this folder for all the new bridge pics:



I will set up for some high power testing either tomorrow or next week. There’s a hamfest in Raleigh this weekend...

Re: 20KW DRSSTC
Steve Conner, Fri Mar 30 2007, 10:40AM

Yay, that new bridge looks great! BTW, about shielding in gate drive transformers: on my last coil I made the GDT primaries from 1/8" miniature coax (RG-174 or whatever it's called) and grounded the braid at one end to act as a Faraday shield. The secondaries were just ordinary PVC insulated wire: I taped two wires together with the coax to make a bundle.

Re: 20KW DRSSTC
Marko, Fri Mar 30 2007, 11:59AM

The bridge looks just.. awesome
Beautiful low inductance design.

And those bricks can actually be unscrewed and opened just so easily.. they aren't actually hermetically sealed?

Re: 20KW DRSSTC
Steve Ward, Fri Mar 30 2007, 03:37PM

And those bricks can actually be unscrewed and opened just so easily.. they aren't actually hermetically sealed?

Yep. I have some CM300s like that, where i opened them to disable the "real time current limiter" for tesla use . Just have to be *damn* sure you snip the right bonding wire!

Aron, your snubber is now a capacitor on the output of the bridge? Interesting... i tried that too (small C of course), and it seemed to make things worse, as its in effect increasing the parasitic C of the CE junction. My thought at the time was that it would slow down the ringing, allowing the main snubber to be more effective (didnt quite work out). I havent run the calculations, but be sure not to exceed the current rating of the little cap (it will surely see some very high peak currents). But, your waveforms certainly do look better. I also like the bridge layout .

Something crossed my mind... what is the current rating on those electrolytics? Seems like an awful lot of power for those things!

Re: 20KW DRSSTC
hvguy, Fri Mar 30 2007, 05:55PM

Ya, I’m pretty happy with this design. As low L as possible and a little more space in my enclosure, plus I had all the materials on hand so it went together quickly.

Steve C. it’s funny you mention that. When I first realized a shielded primary was necessary I tried RG-174 in that exact configuration. The results were excellent but I needed bigger coax to accommodate the ~35A pk primary current. RG-58 worked well…

That’s a good question Steve. I have not been able to find data on these caps so I don't know the current rating. Chances are I would be exceeding it anyway. The good news is I pulled them from a 50KVA VFD. I would assume they can handle this type of operation having been installed a drive that large. Also, after running for several hours at ~2.5KVA (1.2KA pk on the pri) they were only slightly warm.

Yep, my snubber is nothing more than a cap on the low side IGBT. At first I assumed this would make things worse, and in fact, it did. Then I rebuilt the bridge and all the noise except the ~1.5MHz spike at turn off was gone. I though I’d give the CE cap another try since there was almost no L for it to ring with the new design. There is some ringing cause by the cap, but the amplitude is only 10% higher than the rail, much better than the 2.3X with out the cap.

The noise currents though the cap are high. With the tank at 1.2KA and the rail at 170V the peak current though that cap was 200A. This is definitely a lot of current, but the noise is so fast that the pulse falls within the datasheet spec for pulse current and the 1.6KV .22uF CDE film caps showed know heating after several hours of run time.

Re: 20KW DRSSTC
Tom540, Fri Mar 30 2007, 08:53PM

Well Elgar wasnt much help but I did find something interesting on an old schematic for a 10KW power supply. Don't know if any HVers have tried this one.

Since I made today my last day since I was only a temp and wanted a week off in between job. I didn't get the pdf I made of the schematic but I did draw this up of the snubber that was used across the igbt bridge. The cap was the only value I remember. I'm sure values differ depending on bridge voltage and frequency.

There was also another snubber with a higher parts count I found but it was an old power ten product and there was no schematic availibe. Problem with Elgar is theyve had so many diffrent people in and out doing layouts plus new managment about a year ago that all thier boards and schematics are half in Pcad and Half in PADS. Anyway enough rambling. Here is the drawing. You'll probably have to do a search on how to calculate it.

Re: 20KW DRSSTC
ragnar, Fri Mar 30 2007, 09:38PM

Whoa, being able to see their guts must be really handy if (IF, not when ;) it ever fails, for some diagnosis.

Re: 20KW DRSSTC
hvguy, Sat Mar 31 2007, 12:03AM

Thanks for checking. That circuit is what’s known as an RCD snubber and is fairly common in full bridge inverters. I did try that arrangement but found just the cap to be more affective.

Re: 20KW DRSSTC
hvguy, Wed Apr 04 2007, 02:15AM

The latest challenge:

I was planning on running the coil this evening so I though I’d grab some more waveforms before taking everything off the bench. All my testing is being done at 170V so I had not thought about this before but it looks like I still have a problem. I turned the bridge on for about 10sec, took a scope shot, and then shut it off. Just out of curiosity I reached over and felt a TVS string; it was HOT! This did not make sense as the peak voltage measured across the string (4 1.5KE250 is series) is only 285V. And that was measured with a calibrated TEK P5200 HV differential probe so I am confident the waveform was accurate. I hooked up the current probe and sure enough there was a nearly 600A 30nS wide current pulse through the TVS string occurring seemingly randomly. The string was turning on for no apparent reason. I thought about it for awhile then replaced the unidirectional string with a bi-directional string of the same parts. Sure enough this fixed the problem. As it turns out the forward voltage drop on these parts, which is not specified in the datasheet, is the standard .7V. This means that any reverse voltage is going to turn the string on and short the IGBT. This, I believe, was the real cause of my original failure.


I have some 1.5KE200CA parts (the bi-directional ones) on order and will use five of them in a string for the next run. Good news is, after all this testing and redesign I have a much superior bridge… Hopefully I will break the 20’ mark next run. I was also able to test the bridge up to 2.4KA with no problems.

Re: 20KW DRSSTC
Steve Ward, Wed Apr 04 2007, 03:16AM

Wow, thats a pretty serious discovery, and i will need to keep that in mind if i have any problems with my TVS. I suppose with only 4 in series, the 2.8V drop would be similar to the free-wheeling diode drop at a few hundred amps, so it makes sense. I think im using 7 (120VBR) TVS in series on my setup, so i should have some good headroom there, or i could always throw in a 30V bi-directional, but i havent had any trouble with them (though my strings of 4 of the 200V types DID melt on occasion!).

Re: 20KW DRSSTC
hvguy, Wed Apr 04 2007, 03:30AM

Ya, crazy problem to trouble shoot. Prior to this I had never even considered the free-wheeling diode drop. The good news is the problem was simple to fix. I will keep that in mind about the 4 200V TVSs melting, maybe I will parallel a few strings.

Re: 20KW DRSSTC
LarsE, Wed Apr 04 2007, 11:37AM

And you could also series more lover voltage TVS to increase the current capability and spread the heat.

Re: 20KW DRSSTC
hvguy, Wed Apr 04 2007, 05:49PM

I could, but I got a great deal on 800 or so 1.5KE200CA so I think they will be in everything I build for the next few years

Re: 20KW DRSSTC
Marko, Wed Apr 04 2007, 06:14PM

Hi guys...
Don't flame me for this... but is there actually a danger from DC bus ''voltage spikes'' in the end?
Steve blamed them as well in order just to find a more cardinal error in his gate driver!

Is there a DRSSTC (of any size) wich actually had a true and confirmed problem as such?


AFAIK free wheeling diode practically conducts only on the end of cycle, and not the full tank current, but still if the drop gets close to TVS's forward drop and they start to conduct, TVS will get hot much faster and their drop will go down.. very interesting problem you've found!

Re: 20KW DRSSTC
Steve Ward, Wed Apr 04 2007, 07:39PM

Don't flame me for this... but is there actually a danger from DC bus ''voltage spikes'' in the end?
Steve blamed them as well in order just to find a more cardinal error in his gate driver!

Good point! I sometimes forget that i dont actually have hard evidence to suggest that voltage spikes ever killed my IGBTs (except perhaps my earlier DRSSTCs that used secondary current feedback where hard switching and junk feedback caused the IGBTs to switch full peak current). Every failure ive had as of recently has turned up some other explanation (with very good supporting evidence) rather than voltage spikes... Of course, other people still have unexplained failures, which may very well be voltage spikes (or anything else, i suppose).

BUT, there is evidence (scope measurements) at low power that voltage spikes are very real, and that if they remained the same factor of the nominal bus voltage (2X or whatever) that they would be a real threat to the IGBTs. Though, as ive maybe mentioned before, these voltage spikes *appear* to diminish as i increased the bus voltage, and at a rate suggesting that they arent tooo bad. I stress "appear" because its possible that my scope probe (rated for 400VDC) is non-linearly attenuating the 10mhz switching transient noise that im speaking about (so they may in fact not be dimishing).

I still think the possibility of the bus voltage *resonating* up to higher voltage is a very real issue, but this is also very easy to see at low powers, and *doesnt* diminish as input powers climb. The cause of this (for any who arent aware) is the inductance of the bridge structure resonating with any low impedance poly-film or ceramic decoupling capacitors. It seems all to easy to get this L and C ringing up to dangerously high levels particularly if the bus inductance is large. But this problem is rather easily solved with an appropriate bus structure (i go with laminated copper sheets now).

Re: 20KW DRSSTC
HV Enthusiast, Wed Apr 04 2007, 08:31PM

I have a question. What exactly is a 20kW DRSSTC? Are you specifying that 20kW as input power, or actually power delivered to the primary?

You should probably clarify that. A 20kW input power DRSSTC isn't hard to build. A 20kW output power DRSSTC (at least assume that much power is actually getting switched into the primary) would be much more impressive.

Re: 20KW DRSSTC
..., Wed Apr 04 2007, 08:53PM

Is there really that huge of a difference? I mean, there can't be more than a kw going into that heat sink, and the caps can't be disapating more than a few % of the input power before they melt...

Re: 20KW DRSSTC
hvguy, Wed Apr 04 2007, 10:07PM

Dan, 20KW is the actual input power measured at the mains. Assuming 90% total line-primary efficiency that’s 18KW of through-put; unless you can think of a reason why the efficiency of this system is any less than that of other ZCS full bridge circuits...

I will be testing an IGBTs tolerance for over voltage shortly. I believe this and excessive over-current are the biggest killers of IGBTs in TCs so it will be nice to have some proof.

Re: 20KW DRSSTC
Steve Ward, Wed Apr 04 2007, 10:46PM

Why would a 20kW input not be impressive compared with 20kW output? Even if efficiency isnt that great, then would you say a 25kW input (20kW output) DRSSTC is impressive? Or have we all missed the point?

Re: 20KW DRSSTC
Dr. Drone, Thu Apr 05 2007, 12:01AM

Re: 20KW DRSSTC
HV Enthusiast, Thu Apr 05 2007, 01:01AM


Because input power is a very poor way to characterize a system unless you state efficiency as well. For example, I could take a mini-DRSSTC, load it down with lots of losses, and have 20kW DRSSTC.

I'm not saying 20kW is not impressive, but i think we need a bettery way to actually characterize the output power capability of a DRSSTC other than input power.

Re: 20KW DRSSTC
hvguy, Thu Apr 05 2007, 03:36AM

Thanks again Chris. I hope I don't have to keep tweaking this thing to keep it running; not that I mind that much but there just doesn't seem to be any reason why the system can't work right 100% of the time.

Dan, you’ve got a good point about power measurement. I can assume, based on my experience with other switchers, what the primary efficiency of this system is. However I have no real way of knowing what the line to arc efficiency really is. As far as I know no one does. Any ideas?

I did some testing on IGBT over voltage strain. Here is a short write up:

Re: 20KW DRSSTC
Ken M., Thu Apr 05 2007, 03:51AM

Very Nice,

Re: 20KW DRSSTC
Steve Conner, Thu Apr 05 2007, 08:13AM

I never bothered that much about the distinction between input and output, for the reasons mentioned. Our IGBTs and tank capacitors must be fairly efficient because they would catch fire or explode if they weren't. So if you can get 20kW to go into something, and it stays in one piece and sparks come out, I am impressed. Dan's afore mentioned mini DRSSTC would be a flaming stain on the bench in about 10 seconds.

As for how to measure the input power, the only real mistake you can make is measuring line voltage and line current into your coil, multiplying the two, and calling it "kW". The poor power factor of a rectifier with big filter caps leads to an inflated reading. You should say "kVA" unless you used a true power meter like a Kill-A-Watt. Aron mentioned power factor, so I assume he did this.

If you don't have one, you can measure DC bus voltage and DC bus current (average with a moving coil meter) and multiply them to get the power in kW.

Re: 20KW DRSSTC
Marko, Thu Apr 05 2007, 12:26PM

The biggest error with such a conclusion would be if we measured apparent power instead of effective power, but regarding power dissipation I agree with Steve as none could allow himself to disspiate large fraction of input power and live, especially on coil this big...

If that really is still controversal, could the coil be just called 16inch DRSSTC, or 20' arc DRSSTC?

Re: 20KW DRSSTC
Steve Ward, Thu Apr 05 2007, 07:57PM

That was an *excellent* test Aron! Only one problem, it doesnt apply to the bricks we use. But, that really is astounding to see it avalanche with no ill effects, especially since the dissipated power eventually got to 150W! It was a simple test but revealed much about the behavior of those smaller IGBTs.

When i go home this weekend, i will grab up my "half-dead" CM300DY-24H IGBTs and do a similar test when i can get some time. I will have to build a quick voltage divider for my scope to read the 1.2kV+ spikes. I will aim to re-create the 10MHz switching noise that i saw on my bridge. Of course, if you could perform this test, it would likely happen much sooner (university leaves me little spare time).

Re: 20KW DRSSTC
Finn Hammer, Thu Apr 05 2007, 09:28PM


Would someone pls. explain, or point to an explanation of IGBT avalanche.
I suppose the effect of it is seen on the scope traces, where the voltage flattops, and it appears that the IGBT takes on the action of a tranzorb. Perhaps even, there is a built in tranzorb in those little critters?
Anyone?

Cheers, Finn hammer

Re: 20KW DRSSTC
Marko, Thu Apr 05 2007, 09:47PM

Pretty much all semiconductors will avalanche when their voltage rating is exceeded to a limit.
Some that are 'avalanche rated' have an energy rating they can eat up without harm, some have no such rating.

Various TVS are either beefy zeners or avalanche diodes, with main difference of avalanche effect having a positive tempco, while zener having negative.

TVS just have the job of removing the avalanche effect from the IGBT onto themselves.


I'm curious to see how will various IGBT's (avalanche rated and unrated) behave in these conditions.. If 40N60 could dissipate 150W from avalanching alone I actually wouldn't be much of concerned with that problem!

Re: 20KW DRSSTC
hvguy, Fri Apr 06 2007, 12:38AM

Thanks Steve. Your right, it would be nice to perform this test on some bricks. The problem for me is all my bricks are either completely dead or good. I killed three 40N60s in the test and don’t really have any bricks I can nuke while testing If you’ve got some half dead ones go for it. I would expect the results to be similar to this test but who knows… That’s why we need to test one. Also, I like the idea of recreating the 10MHz ring. BTW that happens to be about the same Frequency as my buss spikes.

Finn, that’s basically it. An IGBT, MOSFET, or diode will “avalanche” if a certain reverse voltage is exceeded. At that point it begins to conduct, causing the waveform across the device to appear clipped. If this voltage is exceeded to quickly and/or with to much energy the junction will fail.

It is defiantly clear that these parts have some avalanche capability. Thing is a spike of significant magnitude will kill the device; it just takes a lot of power. I am really curious to see what happens when the spikes are very fast, like the 50ns or so rise times Steve and I are seeing.

Re: 20KW DRSSTC
hvguy, Wed Apr 11 2007, 04:51AM

Well another night of near perfect weather and another failed run. After all the modifications I though I’d break 20' for sure, as it turns out I killed an IGBT (ouch) and never put more than 400v into the bridge. I’m not sure what caused the error; the logic and gate drive are still functioning perfectly and the TVS strings show no signs of heating or damage. I will replace the brick and set up for some more bench testing to try and find the problem…

I do have this theory though: if you want your IGBTs to live a long, meaningful life run them within the data sheet specs. That means current rating too. I have done a lot of testing with small IGBTs and bricks and found them to be most reliable at ½ their voltage rating and equal to or less than their peak current rating. The only problem with this philosophy is that several people have coils that run outside of these ratings and have never failed, so there are still some unknowns. I will be calling Powerex tomorrow to see what they think the max pulsed current could be given the pulse width of these coils.

Obligatory cool blown IGBT pic

Re: 20KW DRSSTC
ShawnLG, Wed Apr 11 2007, 05:33AM

"Obligatory cool blown IGBT pic "
Thats not so cool.

Re: 20KW DRSSTC
..., Wed Apr 11 2007, 05:36AM

yikes, I wonder if you could find a way to remove that goo (man that is nasty stuff) and see if the individual dies still work... It looks like only a few of them exploded

Re: 20KW DRSSTC
Steve Ward, Wed Apr 11 2007, 06:50AM

Oooh! ooh! ooh!

Looks like the IGBT didnt completely let go like they did for many of my failures. I also experienced a very similar "low power" failure where only parts of the IGBT failed, and similar to your picture, some of the dies had only small patches that failed. This seems to be some "other" failure mode (where the ENTIRE IGBT is obliterated) than what is most commonly experienced, i think. Or perhaps it just has to do with probability and chance... My similar failure was also at reduced input voltage, and i was very confused at the time. I was running the CM300DU-24F modules with the RTC (real time current limit) and i blamed the failure on the IGBT falling out of saturation (since i didnt disable the RTC at this time). I think the current was high enough (over 600A) for me to even consider that conclusion, but the current was also low enough that i didnt think it would have caused a problem.

If only we worked for powerex, we could test these failure modes all day while making big sparks .

Re: 20KW DRSSTC
hvguy, Wed Apr 11 2007, 06:57AM

That’s basically were I’m at. It’s technically possible the IGBT fell out of saturation or there was some other "this is what happens when out side of SOA" failure. These failures are so damn frustrating… BTW the current limit was set at ~2.4KA. Glad I’m no the only one who gets the strange fauilures

Now that would be a hell of a job! If only I got paid to destroy silicon.

Re: 20KW DRSSTC
Steve Conner, Wed Apr 11 2007, 09:44AM

Richie Burnett and I argued a lot over this some years ago. We came to the same conclusion Aron did: if you want reliability, you need to stick within the datasheet ratings. To be sure that an IGBT was safe for higher ratings, you would need to recharacterise it using a similar procedure to what the manufacturer does, which means a lot of computer modelling and trashing hundreds of devices.

No hobbyist (that I know of) has the resources to do that. Or the knowledge: the things that Powerex know about IGBT failure modes that we don't would fill several phone books. And besides, the manufacturer probably set the rating based on what his computer models predicted would give acceptable reliability, as opposed to, say, 10% below the point of instant destruction.

So, if you run IGBTs over their ratings, you should probably think of it in the same way as building a race engine for a car, or cranking up the boost on a turbocharged engine over stock. In other words, the warranty no longer applies and you're on your own. The only difference is that IGBTs don't have anything like knock, leaking head gaskets or little flakes of metal in the oil, to give you advance warning of a failure.

One final thing: Look at the guts of that brick, ponder the longness and thinness of the internal busbars, and try to picture how even (or otherwise) the current distribution will be at high frequencies. Is it just coincidence that the dies nearest the power rail inputs (and hence to the filter capacitors) are all intact, but the ones at the other end (possibly a few hundred volts of L*di/dt worse off) are fried?

Do you think you could make something that shared current better with a bucket of TO-247s and a bunch of sheet copper? I do.

Re: 20KW DRSSTC
Marko, Wed Apr 11 2007, 12:06PM

Oh, god.

2400A isn't 'all that much' considering what people do to little 40N60 and live on.

Also, hvguy, you said that failure happened at only 400V into bridge? I find it difficult to believe that it's just overcurrent.

Was your OCD actually tripping for 2400A?

It's just a 16y old teenager's opinion, but simple problems like iGBT blowing off before actually being over SOA usually have simple causes.

I would heavily inspect that thing for simpler errors before attributing the failure to overcurrent or bus voltage spikes again...

Do you think you could make something that shared current better with a bucket of TO-247s and a bunch of sheet copper? I do.

Hi steve

It is generally difficult to paralell a number of physically large dies and keep good current sharing just with them alone.

I think your idea is not bad at all; using a large bunch of small IGBT's (30N60's or etc) wich would still be cheaper than these bricks, and parallel them along with a split MMC bank, wich would really eliminate current sharing problems!

Re: 20KW DRSSTC
thedatastream, Wed Apr 11 2007, 02:34PM

Is it possible that opening the top has introduced some contaminants that have caused or accelerated this faliure mode?

This app note from Microsemi / APT implies that Non-Punch Through IGBTs can be paralleled because of the positive temperature coefficient.

Re: 20KW DRSSTC
Marko, Wed Apr 11 2007, 02:38PM

At high pulse currents like this most IGBT's will go PTC because of stray ohmic resistances.
Stll, splitting the MMC bank is much more *right* way to paralell IGBT's!

Is it possible that opening the top has introduced some contaminants that have caused or accelerated this faliure mode?

The dies look like they are sealed in some kind of resin, and if IGBT isn't hermetically sealed and is made to be unscrewed and opened I wouldn't worry about that too much.

Re: 20KW DRSSTC
Steve Ward, Wed Apr 11 2007, 09:03PM

At high pulse currents like this most IGBT's will go PTC because of stray ohmic resistances.
Stll, splitting the MMC bank is much more *right* way to paralell IGBT's!

Yep, we are just lucky enough that its possible to do that with the DRSSTC.

I suppose ideally you would go for the biggest, single-die base IGBTs you can find (like the 40N60 mini-blocks) and build up as many H-bridges as seems necessary. If you want to play within the spec sheet rating of 300A, and run them at only 350V, you would need 10 such bridges to build my DRSSTC-2. This is just not practical.

It seems like the same story as our MMC designs. Who can really afford to run within the *AC* rating of the caps? Surely the lifetime goes up to the n'th power when you do run within spec... and perhaps the same is true with the IGBTs. Depending on how long you want your IGBTs to actually last, you might just save money by going with more IGBTs to start with, but for most of us, its not worth it.

Re: 20KW DRSSTC
Marko, Wed Apr 11 2007, 10:56PM

Hi steve

Regarding paralelling, I ran around something wich is actually 'standard speed' IGBT but is still much faster than CM600's (except maybe delay time) and 12 of those would make 1350A peak SOA!
They cost about $30, expensive but still much cheaper than new bricks of same rating!

I guess it's all about what you run at on ebay.

Re: 20KW DRSSTC
Steve Ward, Wed Apr 11 2007, 11:41PM

Those are some really cool looking IGBTs... the 450Apk rating is really impressive. Too bad they dont co-package them with a diode. Where did you find them for 30 dollars? I found them at digikey for ~77 each .

Re: 20KW DRSSTC
Marko, Thu Apr 12 2007, 12:01AM

Oh, at rell, non-stock unfortunately..

There are lots of good IGBT's around, especially considering that even standard speed isn't 'all that slow' for bigger coils.

This one has a diode and 600A current rating, but is only for 600V.


Adding a diode shouldn't be too big problem (it conducts a bit only at the end of cycle) if IGBT's are worth of it...

Re: 20KW DRSSTC
hvguy, Thu Apr 12 2007, 03:21AM

I definitely think I can “make” a better high speed, high current IGBT then can be bought in a single package. I priced out a bridge made from IRGPS60B120KDP IGBTs. Since the peak current rating is what we are interested in it would take about 80 of these, 20 per leg, to make a full bridge for my current DRSSTC. This would give me a rated 4.8KA at 1.2KV. In my opinion the ultimate tesla coil switch is a parallel string of smaller IGBTs. They are relatively cheap, easier to obtain, uber fast, and can easily be replaced. The catch is still the price. At $22ea from rell the bridge would cost $1,760 not including the PCB (or what ever your connecting medium) and hardware and the time it takes to stuff the thing. This is much cheaper than a comparable bridge of bricks, say the CM600DU-24NFH which are $293.12 from rell. It would take 16 of those to match the IR parts and at $4689.92 it would be nearly three times the cost. Another nice thing about the smaller parts in what happens during a failure. Based on my experience with the bricks only a few of the parts should fail under normal circumstances. It’s a lot cheaper to replace a few $22 parts than one or two $293 parts.

The parts with out the co-pak diode are a little cheaper. I wonder if we can use external diodes and save some money…. I think I will test the idea on a relatively small scale some time, maybe with a 6” coil. This idea really needs some research.

Re: 20KW DRSSTC
sparky, Thu Apr 12 2007, 04:09AM

Very intriguing idea, Aron

Re: 20KW DRSSTC
teravolt, Thu Apr 12 2007, 04:21AM

hvguy do you have any dead time in betwean switching.

Re: 20KW DRSSTC
hvguy, Thu Apr 12 2007, 08:17AM

No dead time. This is because the gate drive transformer can not create a shoot through scenario, there for it gives me the closest possible zero current transition and no complex gate drive circuitry.

Thanks sparky, though I’m sure this isn’t the first time it’s been mentioned. My Dad will be in town this weekend so I will probably try to put something together next week for a test.

Re: 20KW DRSSTC
Marko, Thu Apr 12 2007, 08:59AM

IRGPS60B120KDP

I don't think they are actually worth $22, but this could be a baked idea.

I've been looking for microsemi IGBT's wich could match or beat these...
only 225A 1200V for $19, in stock at rell.

BUt, even with those 'faster' IGBT's delay time isn't all that much shorter, so if you want to pull out with less $$ slower IGBT's could be used.

But it's all up to you and your wallet size.

Re: 20KW DRSSTC
Steve Conner, Thu Apr 12 2007, 09:56AM

I was always worried about running the larger bricks without deadtime, when you consider that they can have a turn-off delay time of nearly a microsecond. In other words, they don't actually stop conducting until maybe 500-700ns after you remove the gate voltage! So even if your gate drive waveforms aren't such as to cause cross-conduction, the IGBTs can maybe "fix" that for you

That turn off-delay time is specced for hard switching, where the IGBT itself has to interrupt its full rated current. In our application, with near zero current switching, it should be shorter, but I still don't believe it can be neglected. Also, due to delays in the feedback loop, Aron's coil is nowhere near zero current switching!

This is another reason why I went with the PLL circuit: it's easy to cancel out delays and add deadtime. Once I get my gate drivers up and running, I can hook up a couple of bricks and try to test some of my hand waving ideas. I can use a Rogowski coil to look at shoot-through currents, diode recovery transients and the like. Unless nobody cares, in which case I'll just go ahead and try to make sparks, and scratch my head like the rest of you when things blow at random...

Steve: I think the situation with us and IGBTs is exactly the same as with Terry Fritz and MMC caps. The currents we run them at give levels of reliability that are acceptable to the hobbyist.

PS: Do you think you could modify your driver circuit to give a phase lead to the feedback signal? All you need is a high-pass RC filter somewhere after the current transformer. If you make the "R" of the RC filter adjustable, you may be able to adjust the lead to cancel out the switching delay in a similar way to what my PLL does.

Re: 20KW DRSSTC
Marko, Thu Apr 12 2007, 11:06AM

Guys, anyone noticed finn's idea from before?



A while ago I tought if a comparator with a fixed reference to generate deadtime, but this would make deadtime sine function with primary current ant it would be huge if current is too small.

finn's idea looks much better: by comparing peak and momentary primary current he could always have IGBT's switched at wanted % of primary current.

I wouldn't use any RC delays, though: I would just invert the output of comparator and use falling edge to initiate turn-off.
Since turn-on is usually much faster and we already have deadtime I would just trigger it directly from zero-crss of primary current.

This way first IGBT would start to turn off just before the current reaches zero, be off and zero and at that point another IGBT would start turning on.

This would prevent shoot trough and make us able to finely tune the coil for zero current switching.

Re: 20KW DRSSTC
Finn Hammer, Thu Apr 12 2007, 06:34PM


Steve,

I, for one, care.
The stuff that leaves your hand always has that special "hard to blow up" quality. this has enabled me to run 6 coils equipped with your controllers, without failure.
Only problem is, you put so many FF`s on the board I`m unable to understand how they work.
You´re the best, Steve, we all know it.
So what if you play a little guitar once in a while, that`s just terrific!.

Of course I wish I could persuade someone to build my zero current predicter, cause I`m probably not skilled enough to do it myself.

Cheers, Finn Hammer

Re: 20KW DRSSTC
Justin H., Fri Apr 13 2007, 04:45AM

Hi everyone. I haven't posted here in at least a couple of years. It's amazing to see how far everything has come. The last time I was here, triggered spark gaps were being developed as replacements for RSG’s in conventional coils (one of the many great contributions from Terry F.). Not too long after that, things got really high power with semiconductor, then things went pulsed-mode…(OLTC) and now look around. An SMT DRSSTC was developed not too long ago (Jim from Austin, TX) and there are now more than a handful of DRSSTC’s with truly impressive spark-lengths.

I’ve stopped by to congratulate my friend on his recent accomplishments. As many of you know, such milestones come with blood, sweat, tears, burns, cussing, explosions, dollars, and a never-ending train of decisions one after another after another. It’s not an easy path to take, but Aron has built his life around it. 18 foot arcs coming from a box that looks like it came out of a Stealth Fighter may be impressive. The hundreds of custom circuits that Aron has developed in the last year may be impressive. But I’ll tell you what’s most.

It’s that he’s been able to pull it off, whilst keeping up with life’s most important priorities.

Cheers to you man. The above is more of an accomplishment than anything. You’ve had some shitty times and some bad-ass times, but you’re making forward progress in your projects and in your life. Keep not-worrying about the near-term setbacks. Like I’ve said before, if you keep this drive and motivation up you won’t have a choice. Some day you aren’t going to have to worry about how much the latest IGBT’s cost, because your soldering-iron-sponge-wetting facility alone is going to make you enough dough.

Congratulations on the 18 feet man, that’s one hell of an arc. When I’m up there next, I want to see it go straight up into a big purple streamer (And then I want to go out and climb at New River!!).

And cheers to all of you here, pushing the limits of the unknown, and often doing it single-handedly on a personal budget. It’s a great feeling, but not better than…

Wait, we’re Engineers.

- Justin

Re: 20KW DRSSTC
Steve Conner, Fri Apr 13 2007, 09:51AM

Cool! It's great to see you back on the forum, Justin. Back in the day, Justin & Aron were the Penn & Teller of high voltage

I think what Aron has achieved is pretty awesome, and I send my congratulations too. Does he now have the world's biggest DRSSTC? I think Steve Ward managed 14ft, so it's a question of whether Chris Hooper has beaten Aron's 18ft, and I don't believe he has. Whatever, you have all stomped all over the 6'9" I got from my OLTC2

Finn: My PLL driver is complicated, but I believe every part of it has a purpose. I think it is the best possible way to drive a DRSSTC. Unfortunately I'm too busy to teach people how it works and pimp it to develop a user base. Running that kind of free technical support eats up a lot of time and energy that I would rather spend on my day job, or actually working on projects, or indeed playing a little guitar now and again :P

So, I'm happy for Steve Ward's driver to be the "industry" standard, as it saves me a lot of effort and hassles. In fact, if you include the odds of the average hobbyist being able to build it and get it to work, Steve's designs win over mine on account of being simpler. When noobs e-mail me asking for help with their *SSTCs, I tell them to go look at Steve's site. :P

As for building your zero current predictor, I submit that I already built it, just using a PLL. So instead of predicting the next current zero by linear extrapolation from the quarter-cycle before it, it predicts the next zero from an exponentially weighted average of when all the previous zeros were, combined with an initial guess. PLLs are usually analysed in the phase or frequency domain, but if you think about my circuit in the time domain, that is what it does.

If Steve Ward's driver were modified with a high-pass filter like I suggested, it would predict the next current zero using the derivative of the current waveform (assuming it to be sinusoidal)

I don't know the relative merits of these three approaches. All I know is that the PLL made a whole lot of other design aspects easier too, so once I started on it, I was hooked.

Re: 20KW DRSSTC
Justin H., Fri Apr 13 2007, 07:33PM

Thanks Steve. I'm glad to see that you personally are still rockin & rollin with this stuff, and it's great to be in comm's with everyone again. I'll probably be posting here periodically and lurking around.

Re: 20KW DRSSTC
hvguy, Thu Apr 19 2007, 07:05AM

Justin; thanks for the post man. Hopefully this current trend continues and I can spend more time/money developing cool HV things AND climbing. It couldn't get much better than that...


Ok, I have been researching this… a lot.

Steve C., I like your PLL driver and did get it to work on the bench with relative ease. My only problem with it is its relative complexity. Not so much parts count; that can be slimed down, but in understanding. Since this was, by far, not the first time I have built a PLL it came fairly easily but I’m not sure about your average noob. That being said it is technically superior to anything else out there and you did a great job developing and documenting it.

Since I decided to stick with the logic based driver I needed to optimize it to drive these bricks and my prototype parallel IGBT bridge (more on that later) with the absolute minimum switching delay. Steve W. has done a lot here but I had some ideas that needed to be tested.

First I measured the delay time of driver; this turned out to be ~150ns from CT to gate. Not bad. Next was the total turn on time of the bridge, almost exactly 1uS… Ouch. This basically means the bridge, witch CM1000s, will hard switch more than 1KA at 70KHz and 2.4KApk (Apk*sin(F*T*360) or just look at the scope shot). The fact is IGBTs this large should be driven with the PLL. Period. There is just too much delay inherent to large IGBTs to make the logic based drivers practical; unless you don’t care/have big enough parts to handle the current.

So how do we fix this problem? Well, using the PLL would work but my theory, which seems to have ever increasing validity, is that the parts will work best and last the longest (i.e. more than ~2min) when run in their SOA. This means the voltage spikes must be controlled or eliminated and current must be kept below the manufactures Icm. In order to meet these requirements I have started testing a bridge based on four parallel HGTP20N60A4s, with external freewheeling diodes, instead of working with the slower bricks. The rise, fall, and delay times on these parts are mind blowingly fast. While my current off-line gate driver (OLGD, woohoo another acronym) is fast it is not <50nS fast and is not really the best circuit for driving the only ~16nF total Cge of my parallel bridge.

At first I tried the good-ol UCC and four secondary GDT. This worked (about 200ns) but did not give the blistering fast gate waveforms I was after. At this point I though about going discrete but could not get past the idea of bouncing all those parts around at ~400V (only 600V IGBTs here). Since we all know I like GDTs I decided to try using small, very low leakage, GDTs at each gate. Each GDT would be driven by a full bridge of UCC drivers. This allows me to keep the HV well isolated from everything while keeping things as simple as possible. Here’s the schematic



This circuit works very well. Total delay at the gate is 160ns with less than 50ns rise time. Here are a few shots of the gate waveforms





Obviously you want to keep the distance between the parts as small as possible. I will probably build these drivers as surface mount boards that mount vertically right next to the IGBT stack. Here are some shots of the total 300ns delay from signal source to bridge output





So what does all this mean? A bridge of parallel parts is far superior in size, cost, and speed; provided you’re using 600V parts. At the 1200V level things become a little less clear. I think the next step for driving the 16” will be a divided tank cap and two or three bridges, perhaps two CM1000 bridges or three CM600 bridges. Either way I think it will use a combination of my new gate driver and a PLL. More on this soon (I hope). Far as small coils (<10’ arc, 400V buss) go I would highly recommend the parallel bridge over bricks. Just look at the datasheets; comparing bricks to a few parallel smaller parts is like comparing a 747 to an F16.

Re: 20KW DRSSTC
JimG, Thu Apr 19 2007, 08:30PM

Wow, Justin called me out by name.

I've been following this thread and Conner's Odin thread because I've been working on building a large DRSSTC for a few years now. I've built both the PLL and the feedback circuits and various variants of them in the past.

The nice thing about direct feedback is that the circuit is simple and there's no need to make adjustments to the circuit based on the size of the coil. If something goes wrong you don't need to know intimate knowledge about how a PLL circuit works to debug it.

What I'm intrigued with is how simple Aron's gate driver is.

Re: 20KW DRSSTC
hvguy, Thu Apr 26 2007, 08:14AM

Thanks Jim. I'm a big fan of Occam's Razor but this coil is starting to push the limits of "simple". My goal now is to keep it simple "for it's size" instead.

I am currently in the process of evaluating the PLL driver for use in the 16” coil system and I’ve got to say it has yet to give me any problems. Steve’s circuit has clearly been well thought out and tested. I have yet to see how well it makes arcs but the bench testing looks good. Although Steve, there seems to be a slight phase shift from the beginning of the burst to the end. I noticed this on your site as well; do you have any thoughts on this? Also I like the hi/lo lights; nice touch Oh ya, there's a typo on your schematic, VCOIN is labeled as pin 8 and it should be pin 9…

After tossing some ideas around far as gate drivers and over current schemes go I have settled on a preliminary design for a triple bridge of CM600s. I will post schematics once I draw them; probably in the next week.

On a different note this thing has been sucking time and money like a black hole. I’ve only killed one IGBT but all the small parts and prototypes I have gone though are starting to add up; not to mention the cost of three new full bridges and associated electronics. Then there’s the +400hrs of time to build and test it all... This coil better arc 100’ at 150% duty cycle when I’m done

Re: 20KW DRSSTC
Steve Conner, Thu Apr 26 2007, 12:32PM

Hi Aron,

Wow, somebody built my driver! I'm honoured

About the phase shift thing: As far as I could tell, no matter how you set the controls, you will always have a slight phase error temporarily at the beginning of the burst. This phase error seems to be a necessary part of the operation of a dual resonant system.

What I think happens is that the ideal drive signal, if you wanted perfect zero current switching, would start out halfway between the two pole frequencies, and then bifurcate to one or the other. But if you started exactly halfway, both modes would develop equally and the PLL wouldn't know which to lock to.

So you have to start from a little above or below the ideal trajectory depending on which pole you want to end up on, and this is what causes the phase error. I always got better results driving the higher of the two pole frequencies.

A word of warning: My mental picture of these things is based on a coil that had tight coupling (so two widely separated and easily distinguishable modes) and a high operating frequency (so a lot of cycles per burst). This gave the system plenty of time to settle to a steady state with only one mode excited.

Steve Ward always built coils with looser coupling and shorter burst length. Under those conditions, there may never be a time in the burst when you can say that you are driving just one mode. I'm not too sure how this affects the theory my circuit is based on.

Also, I've not gone through the math, but I think the Steve Ward style feedback driver can produce a drive waveform that actually contains both mode frequencies at once, whereas mine is limited to one frequency at a time. I don't know what the implication of that is, though.

Re: 20KW DRSSTC
Steve Ward, Thu Apr 26 2007, 10:03PM

Sooo... who will be the first to implement the brains of a DRSSTC driver on a micro-controller? yeah, im probably that foolish to try it . The PLL is cool, but its so many parts and wiring... using a single micro controller as the brains seems "better" but on the other hand, PLLs and D-flip flops dont accidentally lock up :P. Ive only begun to brainstorm such a controller... so ive got some ideas, but havent actually started programming it. Im guessing my 16MHz device will be limited to low frequency coils (maybe <100khz) in order to properly phase shift it.

I havent blown up any silicon in awhile... i need my fix!

Of course, i need to finish my uC based interrupter first (just about done!).

Re: 20KW DRSSTC
Steve Conner, Fri Apr 27 2007, 01:02PM

Well, I have been designing microcontroller and DSP based instruments for a day job for several years now, and I'm not about to try it. The reason being that it's a real time job that requires very fast response, whereas anything controlled by a micro has latency. (aka lag, all too familiar to any player of FPS.)

Latency is unavoidable because the micro has to execute instructions to transform sensor input into control output, and each instruction takes time. Even a 100MHz uC could only do about 10 instructions in the turn-off delay time of a TO-247 IGBT, and that doesn't count the time taken to fetch a result from an ADC, etc.

So to me, this means that any control algorithm running on a micro would have to make its decisions based on what it observed on previous cycles, not the current one. Therefore, I believe it would just end up as a fancy digital PLL.

I could be completely wrong, but even if I was right, there could be scope for hybrid controllers that used hardware for the time critical stuff. One possibility might be a Steve Ward controller preceded by a phase lead block whose amount of lead was adjustable by a microcontroller. The micro would run a kind of digital PLL algorithm that continuously tweaked the phase lead to get the best zero current switching. It could also do overcurrent detection, duty cycle limiting, and the like.

Re: 20KW DRSSTC
HV Enthusiast, Fri Apr 27 2007, 02:26PM


I've already done this quite successfully with my integrated DRSSTC commercial unit which is packaged in a small brick enclosure. In fact, i'm designing several digitally uProc controlled current mode control DC-DC converters for a client of mine as we speak. When I say digitally controlled, I mean the entire feedback system etc... is digital, no analog amplifiers or compensation or anything in the voltage or current mode loops.


Not true. In fact, a large part of the power supply industry is moving into this direction. For a client now, I'm actually doing a supply now utilizing real-time digital control using 100MHz Lattice CPLD for the controller. I can get 1Mhz bandwidth with pulse-by-pulse realtime voltage / current feedback and the CPLD can control pulsewidth with resolution less than 200ps. This all in a phase-shifted full-bridge topology. Pretty neat stuff.

Re: 20KW DRSSTC
Tom540, Fri Apr 27 2007, 04:52PM

I was thinking maybe using an programmable logic xilinx chip. Just using the schematic type programming and cramming all the logic into it. Of course you'd still need some analog so there would still be the comparator for current limit etc.

Re: 20KW DRSSTC
HV Enthusiast, Fri Apr 27 2007, 06:09PM

You can do all the current limiting, etc... digitally. The TI competitor to Lattice has onboard A/Ds. With the Lattice, you need to use an external A/D.

Re: 20KW DRSSTC
thedatastream, Fri Apr 27 2007, 08:01PM

I've been looking at the Atmel AT90PWMx series of micros





Which look interesting for this kind of application

James

Re: 20KW DRSSTC
Finn Hammer, Sat Apr 28 2007, 07:14AM

Steve,

I know you have officially declined to try to explain this any more, but when you say that a signal can contain both frequencies at one time, I have to ask a couple of questions.

If there is 2 frequencies present in the primary current, how does this affect the waveform of the signal?

What I am aiming at is this: the signal can only cross zero once per half cycle, and from that point of view it can only have one frequency at a time. But this frequency can change from cycle to cycle.

In audio, we look at a signal`s base frequency. This frequency is based on the time from zero crossing to zero crossing. If this signal contains contributions from other frequencies, this is called distortion.
The presence of odd order harmonic flatten the signal. even order making it more pointy

So is this what you mean? The current is not a perfect sine wave but distorted due to the other pole frequency mixed into it?

Isn`t it so, that it is a property inherent of 2 coupled LC circuits, that both pole frequencies are represented in the signal.

Cheers, Finn Hammer

Re: 20KW DRSSTC
Steve Conner, Sat Apr 28 2007, 01:21PM

EVR: A CPLD is neat, but it's a completely different thing to a microprocessor, so I stand by my original assertion.

Finn: The concept of "frequency" only really has a meaning as an average across several cycles. Sure, you can measure the period of one cycle, and say the frequency is the inverse of that. But that is only true if all other cycles of the waveform have the same period, and in order to find that out, you have to observe several cycles on a scope.

When there are two frequencies present at once in the primary current, you see ripples in the envelope of the waveform as they beat with each other. If they are both the same amplitude, you get notches, which are just very big ripples. You don't see them as waveform distortion, because they are not harmonically related.

The big question for me was always how to synthesize these two frequencies in a square wave drive that has a constant envelope that can't be made to ripple. I think it comes down to moving the zero crossings around in some special pattern, but what pattern? Does the output from a Steve Ward style driver follow this pattern? Always or only under certain conditions? Is it possible to synthesize two frequencies while maintaining zero current switching? Who knows?

Re: 20KW DRSSTC
HV Enthusiast, Sat Apr 28 2007, 01:53PM

Finn,

The primary current does contain the two major frequency components. Depending on how the coil is tuned will determine the magnitudes of those components. If you hook a current transformer to a spectrum analyzer, you'll see these components quite easily.

Re: 20KW DRSSTC
hvguy, Wed May 02 2007, 07:37AM

Update…

I have about 55lbs (48 bricks) of IGBTs ready to go for the new driver

I plan on using three or four stacked H-bridge modules composed of these CM600s to drive a divided tank cap then the primary of the 16”. Total tank capacitance will probably increase to .88uF for four modules or stay at .66uF for three. The DC rail voltage will be dropped to 700V to accommodate the 1200V rating of the CM600s. Each brick will have its own driver board based on the design I mentioned in my last post. Each bridge will also have is own CT and the system will have a modified OC circuit. Here is the revised schematic


The more I think about this stacked bridge concept the more I like it. It takes up quite a bit more physical space than a single bridge but the heat and high current are distributed which make the mechanical design considerably easier. I will probably machine some .5” AL water cooled heatsinks to save on space; four 2” thick finned heatsinks would make the final assembly almost 18” long. If I go water cooled I should be able to keep it under 12” and increase the duty cycle to 100% (if my neighbors can handle it)

I have some new controller ideas I am currently testing but no significant results yet. The idea of using a micro to control one of these things has been discussed for some time now. My big problem with them is reliability; I’m just not convinced they are better than an analog circuit. Although the fact that your using them commercially in a phase shifted bridge, Dan, does go a long way to convincing me. I have seen them used in buck/boost converters for some time but anything can do that. A phase shifted full bridge is a whole other thing though. More to come…

Re: 20KW DRSSTC
Steve Conner, Wed May 02 2007, 08:03AM

I was never sure about divided tank caps. Sure, they force sharing of current at the fundamental frequency, but what about the harmonics? The impedance of a capacitor falls with frequency. If one bridge switches a little earlier or later than the others, switching edges are made of such high frequencies that the tank caps might as well be short circuits.

If I were paralleling bridges, I would use air-cored ballast chokes instead. Or better still some crazy RF transformer arrangement so the outputs end up in series. I've seen this done in broadcast transmitters.

I don't know, maybe the switching edges from large IGBTs will be slow enough that it'll work fine. But it's one more thing to think about.

Re: 20KW DRSSTC
hvguy, Wed May 02 2007, 05:55PM

That’s a good point. The fix for this should be simple enough though; just set a little dead time. Of course this may not even be an issue if the bridges switch close enough to each other. I tend to think if these bricks can be paralleled and share current with out any problems they should work here. We have to keep in mind that, during switching, these parts are liner devices so even if these was a slight time difference the error current between the two bridges may be relatively small due to the bridge impedance at the instant of overlap. I am ordering the driver boards today so we may have a definitive answer to this in the not to distant future.