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A different s.s. simulation

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

Biding time awaiting replacement IGBT bricks, I've developed an interesting secondary-feedback simulation. I'll ask Chip to post my screen-captures, <http://www.pupman.com/current/kcherrick/2007-01-07_135435.jpg>http://www.pupman.com/current/kcherrick/2007-01-08_114712.jpg and <http://www.pupman.com/current/kcherrick/2007-01-07_153223.jpg>http://www.pupman.com/current/kcherrick/2007-01-08_115534.jpg, waveforms and schematic.

In the schematic, secondary-current feedback entrains the pilot-oscillator's frequency. Its output is amplified and gated, at 100 us-on, to drive IGBTs Q4 and Q6 phase-oppositely. Overshoot from the tapped primary is snubbed by the D6/D10/C5 circuit, and C7 resonates with TX3's primary. In an identical circuit but without C7, D7 & D8, I find the expected big inconvenience--of each transistor turning off while still drawing a substantial amount of current. But here it seems to be different...

Looking at the waveforms, notice Q6 turning on at ~85.5 us, with Q6's current following closely as would be expected. But then, at 89.4 us, Q6's current goes to zero before(!) Q6's collector voltage starts to rise. The voltage rises slightly between 89.4 and 89.6, then rises to full value between 90 and 90.3 us. The same effect obtains for Q4, as can be seen.

On the face of it, this would seem to be ideal: each transistor fully on before the current rises significantly and essentially off only after the current has diminished to zero.

Incorporating D7 & D8 greatly reduces the mains current and loading on the primary; I've since moved them over to the right of D6 & D10 for even better effect.

The circuit tempts me since a) it requires only two power transistors (although, admittedly in addition, 4 high-current diodes); b) the (2 instead of 4) drives to the transistors are ground-referred; c) there's no pesky double-resonance, with its "notches", to be concerned about; and d) it's always "in tune".

But perhaps, of course, an H-bridge would deliver more power to the secondary, more quickly. This circuit produces, from the secondary, 650 KV in 100 us. A similar half-bridge resonant-primary/primary-feedback simulation--with +/-150V mains supplies--produces 400 KV in the same 100 us, implying 800 KV from a full bridge. But the latter exhibits those dread notches--and, more complexity in power components and in the drive and the tuning. Further, the push-pull drive scheme would reach the same 800 KV only shortly after the 100 us interval. And with any s.s. drive, that doesn't deliver the mighty initial wallop to the primary of a "disruptive" drive, who cares whether it takes a few more us to reach spark-breakout?

So who will comment on this? I'll be happy to email the SiMetrix schematic-file to anyone who has that simulation program, so they can fiddle with it. And I highly recommend that freebie, by the way. It's such a deal!