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Class E revisited

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Steve Ward

Sat Aug 02 2008, 04:56AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

So the last day or so i started to play around with the Class E circuit again, using a 1200V IGBT this time (TO-247 package). To my surprise i could push a nice 16" spark from the thing.

Right now its powered up from a 120V to 240V step up transformer (which is sagging under the load, btw). The thing has already blown a 15A house fuse, so i moved it to my 40A circuit just to be safe

. The heatsink gets fairly warm after just a few seconds of running, i havent done much endurance testing as im still a bit surprised the thing works this well at all!

Im still working out all the details on this circuit, but its pretty much the same thing as my other class E setup, just more power and a better setup. The IGBT is a 18N120BND from fairchild. The tank capacitor is about 27nF, and the primary is 2 turns on a 6" form. The secondary Fo is 300khz with no spark loading, and drops to about 290khz with sparks going. I also put an 800V string of TVS diodes across the IGBT for some safety, it should at least give me indication that the voltage is too high if they heat up, so far they havent.

I added a series inductor (ferrite cored) with the primary with the hopes that it might tame things, which it does, but now im considering removing it, which will require retuning everything... maybe for the better. The ferrite doesn't get warm, so i dont think its doing much anyway.

Ok, well im gonna get back to playing with this thing

Arcstarter

Sat Aug 02 2008, 05:15AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Wow, nice man! I am glad to see that you are getting back to doing more work :P.

Dr. SSTC

Sat Aug 02 2008, 05:23AM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222

holy cow :o

i would have never thought a Class E could produce those kind of arcs

well try audio modulation before the IGBT blows or somthing, i should be extra loud

are you using an interuppter or running CW

how much energy do you think it is using

don't forget to draw a schematic when everything is tuned

Steve Ward

Sat Aug 02 2008, 06:41AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Well, a little more tinkering and ive gotten rid of my series inductance with the primary. Essentially i kept the total L about the same, and lowered the coupling between primary and secondary. The IGBT C-E waveform goes from overdamped (non ZVS switching) at the start of the 60hz half-wave, to being a bit underdamped throughout most of the burst. This is of course a tuning compromise.

I also removed the 240V transformer from the circuit, and to my surprise, the coil worked just as good with a sturdy 120V input over that crappy transformer (which is bigger than a MOT, so id expect it to handle some power, but apparently not much).

And then of course, the IGBT exploded

. From the best i could tell, the Vce peaks at about 500-600V, so it had to be overheating from either too much current or switching losses. I could see either one being the problem since 1) its a lot of power to begin with, and 2) it doesnt achieve ZVS for part of the envelope.

KresoLiubov

Sat Aug 02 2008, 06:46AM

Registered Member #1153 Joined: Mon Dec 03 2007, 07:27PM
Location: Croatia
Posts: 213

I got similar results from class E SSTC with a 1200V IGBT-s. However they didn't live long

Steve Ward

Sat Aug 02 2008, 06:56AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Yeah, this topology is just so finicky... not sure if i want to continue messing with it or not since i dont have so much free time as before to play with stuff.

Firnagzen

Sat Aug 02 2008, 09:42AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Amazing sparks! If you don't mind my saying so, the whole set up looks rather... eh... 'ghetto'? Perhaps a short appeared somewhere and killed the IGBT?

Also, are you running on 1/2 wave rectified?

Steve Conner

Sat Aug 02 2008, 11:27AM

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

Amazing, SSTC development has come full circle back to the single transistor flyback driver.

Mates

Sat Aug 02 2008, 12:26PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Welcome in the club of the single switch high power coilers

Remeber that the biggest advantage of single switch topology is that you can parallel the transitors very easily. Next time use two of them and your coil will survive much more ;) It is also good idea to use more TVS in parallel...

Since I'm operating quite powerfull single switch SSTC in a kind of pseudo-classE, based on BUP314 IGBT transitor I can share some experience. I'm using tl494 driver which seems to be poor choice for ClassE since there are very limited feedback possibilities. However, I can drive the system in 1/2 of the resonant frequency which lowers the demands on transitor switching speed. I can run the coil for minutes (no interrupter) and the power meter says 2.5KW

I know your toplogy is problably much different, but maybe you could find some inspiration...

The coil details:

MRacerxdl

Sat Aug 02 2008, 04:05PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Nice two coils! I need to try to make one of that too!
But I never think that was possible with big IGBT's.

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