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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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!
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.
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.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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.
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.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
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).
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
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.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
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.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
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...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
bwang wrote ...
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 +...
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