Gene Heskett wrote: > I'll have to go look at the outputs of my xylotex again when I get a chance, > but ISTR it was quite a bit faster than 200 ns. The undershoot I could see, > but it was IIRC in the 8 or 9 ns region and about the limit of my scope to > define (its only a 100mhz scope, with usable response to around 200), and > only went down 1 Vbe below ground. The clipping caused by substrate diode > conduction was pretty obvious to the trained eye, but looked to be > relatively 'clean'. Oh, just to comment on the undershoot thing. My original PWM amp had no freewheel diodes, other than the body diodes of the FETs. When that side of the bridge had been sourcing 20 A to the load, and then switched off, the current flowing in the output filter inductor had to come from somewhere. With just the body diode, it would reach -12 V and blow the driver chip. This voltage could last for microseconds! I put several fast diodes across the low-side transistor before finding one that could turn on FAST! By slowing the high-side transistor turn-off a bit, the diode could come on in time to catch the inductive undershoot and hold it to less than -4 V, which the driver withstood fine. As you turn the DC supply voltage up, the output rise/fall times can become quite a bit shorter than the gate drive rise/fall times. I had expected with the Miller effect, etc. that the output rise/fall times would be somewhat slewrate limited, but not so, the slew rate speeds UP.
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