Re: SSTC idea - DRSSTC ?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 17:21 02/04/03 -0700, you wrote:
>Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><chunkyboy86-at-yahoo-dot-com>
>
>
>Hi Stephen,
>
>In order to use one transformer for the whole bridge, I would have to add 
>resistance to damp out the ringing, to prevent the IGBTs from turning on early.

With a well designed transformer, there shouldn't _be_ any ringing. Most 
SSTCers run an entire H-bridge off a single transformer, at frequencies 
many times higher than you're proposing, with no problems. Judging from all 
the other trouble you've had with exploding driver chips and the like, your 
transformer design probably needs some work.

>The DC blocking capacitor is just one of those "good practice" things and 
>not needed if the mean voltage is zero, right?

It would not be needed if the mean voltage were _exactly_ zero. In 
practice, there will always be a small offset. The DC resistance of the 
transformer is very low, so it only takes a tiny mean voltage to saturate 
it (and/or blow your driver chips) So the capacitor (usually a combo of one 
plastic film and two back-to-back electrolytics) is essential.


>I want to keep with the microcontrollers because it is easier to do PLL. I 
>dont want my IGBTs trying to turn off at several hundred amps each cycle.

Have you tried writing the PLL code yet? It could be a bigger job than you 
think! Plus, if it crashes/glitches you'll probably blow out all your 
IGBTs. Analog chips don't usually crash! These days I believe they make 
resonant-mode SMPS controller chips that ought to pretty much plug and 
play. Or you could do like feedback SSTCers do and feed a sample of the 
secondary base current (with adjustable phase shift network) back to the 
sync pin of your TL494/whatever chip.

>The micro also allows me to do cool stuff like sweep the break rate, play 
>songs (low fidelity) and do double bursts easily. To do double bursts I 
>would have to switch phase right after the first burst, which could be 
>challenging w/out a microcontroller.

It'll be challenging even _with_ a microcontroller ;) I believe that phase 
reversal at the notch only happens if the primary is tuned at/below the 
secondary frequency.

>I could also do several bursts in a row followed by a break, like staccato 
>SSTCs and VTTCs. Didn't you mention that you were thinking about doing 
>that with your cool little OLTC?

I originally meant it to run at 400-600bps continuously, or up to 1200bps 
on half-rectified supply. However it heated less than I thought and I was 
able to run it at 1200 for 10 second bursts. That suggests that if I used a 
proper staccato with very short bursts, I could go for some ridiculous 
break rate like 6000bps and get a lot more spark. Assuming the wee stubby 
secondary can handle it without flashing over.

What's more promising are the research opportunities. With fine enough 
control, and a one-shot facility, it should be possible to grow a single 
streamer made of 1, 2, 5, 10... bursts and observe/photograph how it 
evolves. But since the control circuitry is all analog it'll be a pain in 
the butt to implement. I might leave it for the Son Of OLTC, if I ever get 
a good deal on some of those railway traction IGBTs.

Steve C.