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Re: SSTC- feedback vs. none



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

A follow-on to my earlier posting today: The delay problem in TX1 is due to slow turn-off of the forward-biased transistor base:collector junctions. As TX1's outputs reach their extremes, the respective 270 uF capacitors become recharged through those junctions (and that's near the tops of the waves so the cessation of normal emitter-follower action is immaterial since the IGBT gates have already been almost completely driven). At least in the simulation, the junctions apparently remain on while the signals are at their peak values, and then have to be turned off again before the base voltages can again begin to change.

As I mentioned, I have added schottky diodes across those junctions in the actual hardware, which will preclude forward biasing of the base:collector junctions. When next I fire it all up again, that will be one of the first things I'll check: whether those diodes are turning off fast enough to eliminate the delay.

KCH