High Speed Design ----> Knee in the falling edge?

I worked on this problem a couple of years ago and never did understand what the deep down root of the problem was beyond the vendor shrunk the die on an optocoupler we used to drive an IGBT and we needed to slow down the propagation times.

The weird thing was the manifestaion of the problem:

A kink or knee in the middle of the falling edge would turn into a "ring". Big time! The IGBT would then be driven in the linear region for a a half second or so. After this happens umpty ump times the IGBT would become an open.

So what is with the knee thing in the middle of the falling edge? I have seen and have an intuitive feel for ringing at the bottom or top of a square wave near the level transitions. But in the middle?

I was reminded of this by the falling edge (of all things) a PID temperature controller. Going from 100C to -40C there it was a knee right in the middle.

It has got to have something to do with some sort of level transition for the temp controller case I would imagine SW. The opto coupler though. . .

Sorry if I got rambling here.

Thanks, Ed V.

PS - yeah, I know I should say the part numbers and edge rates but hey it was two years ago. : )

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That's Miller effect--when the collector starts to move, a current opposing the base drive flows through the collector-base (in this case collector-gate) capacitance. That causes the "knee" in the drive waveform--the transistor's fighting back.

Since the transistor's collector has started to move, the thing is linear, and prone to oscillate.

An opto-coupler driving an IGBT is kinda wimpy--that's not what we'd usually call high-speed design!

A heftier driver (i.e., spend less time in the danger zone), a damping resistor / ferrite bead at the gate, or some combination thereof might be the cure.

Best, James Arthur

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James Arthur

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