Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>Here's a bit of a puzzle; perhaps someone can offer a clue toward solving it. I post
http://www.hot-streamer.com/temp/tch-drvr.jpg, the schematic of my IGBT driver. I briefly repeat how it is to work: Input excitation is present all the time, keeping C1 & C2 charged via R1, D1 & D2. Negative-polarity input signal, via D7, Q3 & R6, keeps the IGBT's gate at ~-27V (with R5 holding it there between half-cycles). Q1 is kept off between spark events by A1 being on.
During each burst of excitation to turn on the IGBT, A1 is turned off. R3 turns Q1 on during positive half-cycles of the input. D6 & Q1 pass the positive-going signal to Q2/Q3 and their output drives the IGBT, between ~+ and - 27V (perhaps ~26, with the Q2 & Q3 b-e drops).
I've incorporated D8 & D9 to soak up any voltage overshoot that might occur at the gates; one or the other is to conduct at an overshoot and clamp it to the voltage of C1 or C2. I would have expected neither D8 nor D9 normally to conduct.
Testing so far with no H.V. applied to the IGBTs, I see good IGBT-gate waveshapes: rise/fall of n.g.t. 200 ns, -27 to +27V. Smooth as can be with just the barest ringing top & bottom using a 100 MHz scope & probe. But here's the rub: With less than 10% duty cycle, on vs. off, Q2 >>and D8!<< rather quickly become hot. Why Q3 does not become at least equally as warm as Q2, and why D8 warms up >>at all<<, I cannot fathom. Anyone have a thought on it?
Ken Herrick Tesla list wrote:
Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx> Hello Steve-No, I decided to do away with that, on the basis of an additional simulation. The simulation showed that I could get a proper phase shift--yielding the correct phase for feedback--by taking feedback from a 1-turn loop around the primary rather than from a current transformer. So that's what I've implemented. I still have the phase-shift assy and if this is a total failure I will resurrect that--provided that my energy for this Tesla stuff continues to hold out to a sufficient degree.In the next day or so I'll bit the bullet & crank up the juice bit by bit. And I do have the camera handy! In fact, as I've mentioned before, I can now take scope photos with it. I just hope there'll be some good photos to take.Ken Tesla list wrote:Original poster: "Steve Ward" <mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>Hi Ken, Will you be employing your delay register on this next attempt to try and compensate for delays? I hope that works as expected. Be sure to have a camera handy to take pictures of the sparks! Good luck, let us know how it turns out. Steve On 3/24/06, Tesla list <mailto:tesla@xxxxxxxxxx><tesla@xxxxxxxxxx> wrote: > Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx> > > I see that my last posting on this topic was in December. In the > interim I've been slothful to a fault (altho happily busy for 3 of > the weeks entertaining our Most Perfect Granddaughter, 2 1/2, > visiting from Berlin). But I have now gotten to the stage where I'm > ready to put the H.V. to it once again. I've checked the gate > waveforms & all 4 appear OK. Next, it's... turn up the variac & > watch for the smoke. It's only trepidation, accumulated from years & > years of such practice, that keeps me from doing it today; So perhaps > I'll first just sit & think about it for a while... > > Someone asked, after I reported my last failure (death of an IGBT > brick), what might have caused it, and at the time I didn't > know. But while rebuilding the drivers, I discovered that I had > positioned a wire-wrap pin, in one of the gate circuits, so that, > when I fastened the board down above the mains capacitors, the pin > pressed against one of the capacitor terminals--hidden from view, of > course. I didn't locate the source of the resultant smoke until I > started rebuilding. > > As I've already reported, I utilize NPN/PNP emitter-follower > driver-pairs for each of the 4 H-bridge IGBTs, transformer-driven, > with the 4 transformer signals always applied (from my "pilot > oscillator")--and now rebuilt with opto-isolators acting to gate-on > drive to the NPNs during the spark-event times. That way, all 4 gate > voltages are kept at -28 or so between sparks by the continuous drive > from the PNPs. As before, the continuous transformer signal also > serves to keep the + and - drive-supply electrolytics charged up. > > So stay tuned, so to speak--& don't be startled by smoke seen coming > from the vicinity of California; it'll only be me once again. > > Ken Herrick > > > > >