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Mysterious FET destruction on high-power H bridge
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duak:
When I look at the datasheet for the FETs https://www.mccsemi.com/pdf/Products/MCU80N06(DPAK)-A.pdf I see a few things:

1.) the FET worst case Rds is 13 m ohm.  With a peak load current of 100 A, the peak power dissipated is 130 W.  Visually integrating the first 500 ms of the acceleration current I estimate about 80 A which gives 83.5 W.  This is close to the PDmax of 85 W for the device.

2.) the thermal resistance from junction to case is 1.76 C/W.  Depending on thermal inertia and diffusion, this implies a possible peak die temperature of 172 C at the end of the first 500 ms.

3.) figure 4 on p. 3 shows a positive temperature coefficient of Rds vs die temperature.  At 175 C, Rds is doubled, so for the same current, power dissipation is doubled. ie., as they get hotter, they become less efficient.

This is a full bridge circuit with an inductive load.  This means that load current will flow in the complentary device for some time in the switching cycle.  At any given time during the reversal or acceleration time, all devices are carrying the the load current either in the FET or the body diode prorated by the PWM duty cycle.  IOW, all the devices are dissipating some power.

I think what is happening is that the actual FET dice are heating up rapidly during the first high current time and because the heat doesn't transfer out quickly enough, the second high current time compounded by the positive temperature coefficient of Rds causes the die temperatures to rise enough to destroy the devices.

I'm discounting avalanching, but not ruling it out.  If it were avalanching, the failure would occur on the first reversal when the recovered mechanical energy from the motor pumps up the supply voltage.  That being said, high temperatures exacerbate avalanching and in combination with thermal runaway could be the killer.

I would test this by running one accel/reverse/decel cycle then an off period, to allow the temperatures to reach an equilibrium then run another cycle, then let it reach an equilibrium and so on.  As the test proceeds, the average temperature of the FETs increase; at some point the peak die temperatures will cause a failure.

BTW, the schottky diodes are probably not carrying too much load current  as they are rated for only 10 A and have a greater series resistance than the FETs.  Are these devices OK after a FET failure?

A rough calculation of the extra power dissipation due to the recovered charge from the body diode is Vbus * Qrr * Fpwm = 15 V * 47 nC * 100 kHz = 0.07 W  per device.  I'd say this can be neglected.  In my driver, the Qrr was higher and it operated at a higher voltage so it was a bigger concern.
Mechatrommer:

--- Quote from: rschlaikjer on June 21, 2019, 07:00:50 pm ---We don't have any differential probes..

--- End quote ---
that is surprising because from your capture.. your high side mosfet should not be properly ON when its Vs reaches near Vdc, and indicating charge pump is not working as well...



--- Quote from: rschlaikjer on June 21, 2019, 08:50:58 pm ---the current limiting has been mentioned by quite a few people now.

--- End quote ---
if you rate your mosfets properly, current limit will not be necessary, it will complicates your circuit further but feel free to follow anyone you like. imo your 80A MCU80N06 is not up for the job of 100A+ stall current worst case, if its me, i will look into 150 - 200Adc (continuous) rated mosfets, or put 2 x parallel MCU80N06 on each H-bridge arm (ie 8 x mosfets all) and do not stress driver IC too much (1.2A instantaneous current in your OP's circuit). but then, there is problem on your gate driving voltage and clamping/freewheel/flyback diodes function from your screenshot...


hence i'm not sure beefing up mosfets (or even implement current limit) will fix it until you clarify the problems i mentioned earlier. ymmv.
cur8xgo:
BTW a nice tool to have in situations like this is a resistive load to compare/contrast. I've recently made multi-kilowatt resistors for testing an full bridge at 125A / ~20V. Take steel wire from the hardware store, calculate how much you need for your resistance, wrap it around a plastic former, and put it in a plastic tank of water. If you just leave it in the air it glows red (or vaporizes/explodes) almost instantly and resistance rises very rapidly. But in the tank of water I found it very stable and tame.

If your circuit waveforms (or component lifetimes) have anomalies with a resistive load, it can be easier to analyze and resolve them without the added complexity of inductance and mechanical generator things.
cur8xgo:

--- Quote from: Mechatrommer on June 21, 2019, 09:27:45 pm ---if you rate your mosfets properly, current limit will not be necessary, it will complicates your circuit further

--- End quote ---

Yes I agree with this. A current limit shouldnt be used to protect the fets. Should not be a reason you can't spec the fets to where they are robust enough to handle any situation the motor might throw at them. Any sort of fet protection should be long-time-scale thermal protection, if you need instantaneous current limiting, the circuit isn't very robust. Just my instinct here maybe I'm wrong
rschlaikjer:

--- Quote ---that is surprising because from your capture.. your high side mosfet should not be properly ON when its Vs reaches near Vdc, and indicating charge pump is not working as well...
--- End quote ---

The voltages on that timing trace should be taken with a grain of salt; I was using some lower quality probes that have convenient micrograbbers but are otherwise not the best.

Here is a capture with the actual Rigol probes on Q1 (low side, teal) and Q2 (high side, yellow) gates when driving in one direction with low PWM. The ~23V reading here is a lot more reasonable; I believe that the charge pump is working fine (input voltage rail is 12V)
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