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Is it feasible to miniaturize OLTCs?

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Dr. Shark

Wed Mar 15 2006, 12:24PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

All the people who have build and written about OLTCs emphasize the importance of using a large, low f_res secondary, preferrably below 100KHz. I don't understand the reason for that. Since the IGBT only needs to be switched at the beat frequency, which is much lower than the resonant frequency, it should be possible to run at much higher frequencys than e.g. a DRSSTC using the same IGBTs could. Thus the OLTC driving style would seem to be ideal for very small coils. Is there some important point I am overlooking here?

Steve Conner

Wed Mar 15 2006, 01:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The problem is the primary surge impedance. This is the square root of (L/C). As you move to higher frequencies, it goes down, and the IGBT has to switch higher currents with faster rise-times to get the same bang energy. You also run into the "less than one primary turn" problem.

Having said that, smaller coils don't need as much bang energy as big ones, and small IGBTs have faster rise times, so you may be able to find a solution that gives good performance. Maybe OLTC drivers really will be best for very small pulsed SSTCs. I just lost interest in OLTCs when it started getting obvious that DRSSTCs would pwn them, and never bothered trying.

Marko

Wed Mar 15 2006, 02:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I tried this on mini-coils, but with triac (slow) and got few mm spark, nothing interesting.
Cap has to be just uF or two, very low bang energy, cap cannot be increased because primary has only one turn.

Maybe IGBT could give me some better preformance but I doubt it could match royer/bridge SSTCs for that size.

I made some tests on flyback and roasted it quickly, it gave some nice sparks before it died.
BTA12 was hooked to GDT and signal generator, and it shorted 1uF cap trough 5 turn primary, nothing special actually.

Dr. Shark

Wed Mar 15 2006, 03:29PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Thanks for pointing that out Steve.
I think I will give this a shot if I have the time, because I really like the simplicity of the design with only one active component. And, of course, it's a trve TC like Tesla _wanted_ it to be, not just a lame power amp pumping energy into a coil.

Edit:
Found some good info on the archives:

which explains all the calcuations around surge impedance very well. Now I just need to figure out why I want the largest bang energy possible, i.e. why this gives bigger sparks than a lower band energy at a higher break rate. I suppose this is all about duty cycle and trying to pump a lot of energy into one single streamer, but in that case wouldn't a rapid succession of bangs do the trick just as well?

Some people seem to have figured out

that 18 is the magic number of bangs that go into one streamer. This is weird, should not the time interval matter much more that the exact number? I mean, at one bps, you surely won't get 18 bangs into one streamer, and at a very high rep rate with a mini coil, I imagine iit could be considerably more.

Dr. Shark

Thu Mar 23 2006, 08:21PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

OK, since my Class E SSTC is not getting anywhere, I am trying it out. 1MHz, 600V 600A 170nF in the MMC (well, its a single cap actually), and a single TO220 IGBT. I'll see how it stands up.
Incidentially, its my first PCB, so I went for a rather simple layout:

a 555 timer is providing someting like 10us pulses every 10ms or so.

I though it would be a good idea to keep the loop area between tank cap, IGBT and primary really small, since the primary is only 170nH, so I came up with this:

the IGBT case is soldered to the PCB directly, so there is pretty much _no_ loop area.

Marko

Thu Mar 23 2006, 08:24PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Did you use backward diode across IGBT, it may be needed in order not to stress internal one too much.
And cap is too small, you should use something in uF range for reasonable bang energies.
Finding esonance may be a little problem, and I dont know is that small IGBT going to live for long but iminpatient to see results

Dr. Shark

Thu Mar 23 2006, 09:12PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Gosh, you've got a comment on anything, don't you?
The reason I am using 170nF and 170nH in the primary is that this gives me the required frequency at a managable surge impedance of 1Ohm. Besides that I cannot really go below one turn in the primary, it would surely kill my IGBT to increase capacitance. I don't really understand the fuzz about bang energy anyway, whats the bang energy in a DRSSTC? I don't think its a meaningfull concept, as there are a lot of tricks like doing bursts.

Well, I'd also like to see results, but I dont even see the ringing waveform on the tank capacitor when I use 30V instead of the eventual 600V, so there must be something seriously wrong. Somehow I seem to be able to get the simplest circuits wrong

Marko

Thu Mar 23 2006, 09:24PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Sorry if you calculated it for exact resonance.
maybe you could scope and ring primary/secondary circuit independetly of IGBT and contorller to see what is happening?

Bang energy is total energy that capacitor transfers to primary in one ''bang', for OLTCs and SGTCs. For DRSSTCs its total energy transfered to secondary by the bridge during one burst.

With your cap you get only 30 milijoules at 600V, and even for such small coil its very low amount of energy.

As Steve COnner stated obivously you run into less than one turn problem.
You can help yourself using more voltage and putting large toroid to lower resonant frequency but I doubt you will get some preformance.

Maximum spark I got was 0,5cm from my mini secondary, totally untuned with 1uF cap and one turn.

Dr. Shark

Fri Mar 24 2006, 08:55AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I know what the bang energy concept is, all I am saying is that it is not to be taken as the only true wisdom. If you look at VTTCs or ISSTCs you will see that they dont really have a bang energy. So I am kind of trying to proof a principle here.
I really want to build this OLTC around a single IRG4BC20UD (or IRG4PC50UD if I have to), so I am quite sure I wont be able to go higher than 600V 600A. In fact the 600A figure is based on the assumption that the thermal limit of the IGBT is based only on the action integral (Int i^2 dt), which seems to hold true for SCRs but has not been tested for IGBTs to my knowledge (look here

). If this assumption does not hold, I'll be down to 100A. I've got more than 50 of these devices I am willing to sacrifice, so I will just experiment.

I am not going to try and lower the resonant frequency, in fact my design goal is to get this to work at 1MHz and with a single To220 IGBT. Call it ambitious, call it stupid, I don't care

I think that this _could_ work if I run bursts of shots as quickly as I can charge the tank cap again, a bit like an ISSTC. Anyway I am not looking for big sparks here, I just want a proof of concept. Steve says

Maybe OLTC drivers really will be best for very small pulsed SSTCs. I just lost interest in OLTCs when it started getting obvious that DRSSTCs would pwn them, and never bothered trying.

and I am determined to test this hypothesis. Shame the whole thing is not working at all...

vasil

Fri Mar 24 2006, 09:06AM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Do you use an IGBT for 1 MHz res freq? Replace it with a FET...
At 1 MHz, your IGBT will not have the time to recovery so quickly.

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