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Old 07-09-2010, 03:07 PM   #11 (permalink)
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...and subsequent failure

Disconnecting the TVS had no effect on gate waveform; my capacitance theory was wrong. I replaced it immediately.

I remembered I used to measure gate voltage *at the driver* rather than four inches away at the mosfet (which I know is a lot of inductance for these pulses)
That explains why I remember a much faster gate drive.

I shortened the wires to about 2 inches each, the lowest I can go in prototype form. Still very sluggish driving.
I put one scope probe on either side of my 10ohm gate resistor. 100ns on the driver side, 600+ns on the transistor side.

I shorted the gate resistor to see how the circuit would act without one. The motor changed pitch slightly and the gate drive looked fairly nice.
The ringing effect was still evident but nowhere near as prominant.

I reduced the PWM to 0%.
As I disconnected my scope leads from the resistor, the mosfet died.


Yup it's dead all right.
-It's cold even though it had been running for a few minutes before it died.
-The transistor was completely off when it died, 0% duty cycle.
-ESD? I would expect the TVS to protect from esd as well, but the coincidence of me touching the probes and the mosfet dying is compelling.

I'm sure stumped now. Thoughts?

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