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Mysterious FET destruction on high-power H bridge

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cur8xgo:
Hmm those plots seem okay..Vgs near 15V but thats still below 20V.

Questions:

-Were you doing the "backdriving" thing during these plots?
-Is the scope capable of measuring very short transients? We are talking less than a couple uS..possibly much less.

For instance running the numbers...lets say the fet is avalanching at 70V and 100A for 1uS. Thats 7mw every time. If you are switching at 20khz, thats 140W dissipation.

If your scope at 2ms timebase wont show us transients that short, then the information to rule out or detect Vgs/Vds death isn't there.

You could possible put a very small diode with a low (but higher than supply voltage) reverse voltage across Vds and see if it blows.

Or possibly a diode + resistor + cap to integrate short transients over time and let you see them with your scope.

cur8xgo:
Or you could just set the timebase to 1uS and trigger on voltages higher than Vds...

Your scope should show something..at some point if you need huge scope bandwidth to see transients..they are so short it doesnt matter

MagicSmoker:

--- Quote from: rschlaikjer on June 21, 2019, 07:00:50 pm ---We don't have any differential probes, if that's what you mean - open to suggestions for good ones though! Motor power supply output is floating with respect to scope earth, so no problems there.
...

--- End quote ---

Pintek makes decent differential probes that are inexpensive and rumor has it that they are the ones that everyone else copies.

Oh, and Siwastaja's comments have been spot-on; you'd do well to pay attention to everything he wrote.

rschlaikjer:

--- Quote ---Were you doing the "backdriving" thing during these plots?
--- End quote ---

Yes, during these tests we are doing the same oscillatory driving behaviour.


--- Quote ---Or you could just set the timebase to 1uS and trigger on voltages higher than Vds...
--- End quote ---

Tried this - with the 40A supply and a timebase of 1uS; the scope did not trigger on Vgs with a trigger level of 15V, and did not trigger for Vds with trigger level of 25V.

jmelson:
You have no current limit!  You MUST limit the current, with a very fast control path, to prevent failure.

But, there may also be another way for failure to occur, depending on the design of the FET gate driver.
When one of the half-bridges is sourcing current to the motor (high-side transistor on) and then the high-side is turned off, the motor inductance will try to pull current from that side.  This will drive the junction of the high/low transistors below ground.  If a fast, low-resistance path is not available, this node can go MANY volts below ground, and that may destroy the gate driver chip.  The body diode in the FETs are not designed to handle this current.

I had to put in an ultra-fast diode across the low-side transistor to handle this current in servo drives I make.  I measured the motor terminal node going to -12V for aver one us without the body diode turning on at all!  In your case, these are D8 and D15.  How fast are they to turn on?  You need something that can turn on in 10s of ns.  I use the ES3D, but in your application, you probably need one with more forward current rating.

Jon

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