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Mysterious FET destruction on high-power H bridge
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rschlaikjer:
I have a custom motor driver board design that is failing with shorted drive MOSFETs under certain conditions, but am stumped as to what might be the precise cause of failure. The FET destruction occurs when the control system attempts to fast-stop the motor by backdriving it, which works fine for power supply currents up to 40A or so, but when connected to a 100A supply (as we'd like to in order to achieve the full drive capacity of the motor), after one or two back-drive events the bridge fails completely short.

For context, here is the schematic for the H-bridge and associated gate driver - I'm using the MC33883 gate driver in a configuration identical to the reference implementation in the [datasheet](https://www.nxp.com/docs/en/data-sheet/MC33883.pdf) except that to get the gate switching time down I am using 10R series
resistors instead of 50R. The DRIVE+/- output signals from this sheet go through a fail-safe relay and then directly to the motor.



The sub-sheet to the right is just the 74LVC mux logic for the control signals -> gate control signals; the logic is set up such that the mosfets are driven based on the MOTOR_A/B signals like so:
Q2 = A and not B
Q1 = B
Q7 = B and not A
Q8 = A

Dead time is inserted by the device controlling this board, and a 200ns value is used at a PWM carrier frequency of 100kHz. Here's a trace of the control signal completely switching direction to verify that there's no shoot through - purple/blue traces are the MOTOR_A/MOTOR_B lines, yellow and teal are the low-side / high side gates of a single half-bridge (TP5/6 in schematic above). Some of the ringing is likely my very how-you-doing scope probes, and not actual system noise.



The MOSTFETs fail perfectly short - they are able to short a tens of amps to ground without any power dissipation at all, which flummoxed my "which part has become hot" debugging strategy for a while.
Initially, I thought the problem might be that the back-emf of the motor was, when polarity was reversed, spiking the rail enough to reach the breakdown voltage of these mosfets, but the part I'm using is the MCU80N06-TP, which has a Vdss of 60V and should be safe. To verify, I took a trace of the motor controller reversing direction suddenly at speed:



Teal / Yellow are the motor terminal voltages, and the pink trace is the supply rail. As you can see, it does spike, but I wouldn't think it does so enough to cause damage. Just in case, I added a snubber circuit to the voltage rail like so (J8 here connects to a chassis mount power resistor), replaced the burned out FETs, and tried again.



This time, with the snubbing, you can see that the peak voltage caps out a lot lower, and again ought to be well within the safe range for these FETs:



But alas, even with this the FETs are destroyed.

Does anyone have any ideas as to what might be going on here, or other things that I should try taking traces of to shed more light? Been banging my head against this.

Thanks.
Siwastaja:
I fail to find the current shunt resistors or hall sensors from the schematic. You do have pulse-by-pulse current limit, don't you, right? It's completely necessary in such circuit, otherwise blown FETs guaranteed.

A non-related comment, 100kHz is over the sane range in most high power motor controllers. Probably not the cause of your issues, but definitely making things harder, and increasing losses for no reason. Your deadtime alone is a significant portion of your switching cycle. Typically motor drivers this size run at around 10-40 kHz.
rschlaikjer:
We do have a ACS781LL ±100A hall effect sensor in the motor path as well.
However, the data from that is not currently used as a break input, just as feedback to the control system - these FETs are rated at 80A continuous / 200A pulsed, which I had assumed would mean we could get away with switching a load from a 100A supply. Is pulsed current failure consistent with the resulting shorted state of the FET? Notably, the FET (package, at any rate) is not hot after failure - thermal camera reports at is only ~10-20°C above ambient.
If you do think that this is the problem, we can restructure the board such that if the current goes above (say) 90A the comparator output from that inhibits the A/B motor signals.

Thanks for the advice about the PWM frequency - we had been using 20kHz in a previous system, but had received advice from the only person we actually know in real life with motor driving experience, and they had suggested a much higher frequency. We'll revert that back down.
cur8xgo:

--- Quote from: rschlaikjer on June 21, 2019, 04:07:53 pm ---
Does anyone have any ideas as to what might be going on here, or other things that I should try taking traces of to shed more light? Been banging my head against this.

Thanks.

--- End quote ---

Hmm. So you switch to a 100A supply and they die. Question: is the actual motor current different when you switch the supply? Or are you doing the 40A test the same was as the 100A test? (I dunno..no load just spinning free?).

Can you kill the fets by doing anything else? Like starting, coasting to a stop, then starting again, on the 100A supply etc..

When you have this motor running and you suddenly back drive it..where is that motor current going to go and how high is the voltage going to get when it goes there? Check your Vds and Vgs pathways. Just because you have a "snubber" doesn't mean its working or can do its job. Your PCB layout and parasitics can matter alot at 100A. So can diodes that don't switch fast enough.

I'd do some forensics:

Failure signature of electrical overstress on power MOSFETs
https://assets.nexperia.com/documents/application-note/AN11243.pdf

If you think about it..failing short is a very specific failure mode. Fets not really intended for avalanche (even though they may have a rating) can fail short from it. Also excess Vgs can fail short as well. (I think..that paper probably talks about it.)

Have you been carefully probing (with super short ground lead) directly to the fets themselves to monitor voltages? I would do that. You should be able to catch things in the act. Maybe you can do it on the 40A supply so you dont kill the fet, but see a transient which is borderline or suspicious.

I vote Vgs and/or Vgs being exceeded.


rschlaikjer:

--- Quote ---Question: is the actual motor current different when you switch the supply?
--- End quote ---

Yes - the motor does have some load on it, but it is also just a motor with a very large stall current (134A), so on the 40A supply the acceleration is limited by the lower available power.

Here is a trace of the data captured by the control system (not scope) during a destructive test: purple is the PWM (±255), light pinkish is the motor current measured through the hall sensor (±100A range) and red is the motor velocity in arbitrary units. The motor driver survived two oscillations before the FETs croaked and the supply went into overcurrent lockout mode, and you can see the chart trail off to the right.



Running in the same direction constantly doesn't seem to cause problems; it seems to be the direction changing that kills it. I'll replace these FETs and give it another test though.
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