MOSFET keeps failing

[xavier60]

I would also like to see the Drain voltage waveform.

[danadak]

Transients on Drain get coupled back to gate via Cgd.


http://www.ti.com/lit/ml/slua618/slua618.pdf


Regards, Dana.

[IanMacdonald]

I'm just wondering if there is a problem with the peak current draw at startup, or perhaps if it's a commutator motor, with momentary shorting of the segments drawing a huge pulse per revolution.

Something of that kind could certainly explain why they work for a good while, then pop.

[Bootalito]

As a minor problem, 100R resistor is too low. Atmega2560 can output 40ma. At 5v this is 120R.
And as other people have said you cant charge and discharge the gate capacitor at those speeds. I made a spreadsheet a while back to help me quickly calculate the max frequency given a particular mosfets specs. I'll see if I can dig it up

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[Dave]

The output resistance of the pins is roughly 40 ohms, so it's fine.

[Bootalito]

The output resistance of the pins is roughly 40 ohms, so it's fine.

Oh okay I didn't know that was a thing. But yeah I looked at some data sheets and there it is!

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[tatus1969]

most probably overvoltage at the gate, try adding a 4v7 zener.

Atmega cannot output more than 5V. I think that Atmega has too weak output to charge/discharge gate capacitance fast enough. So at such PWM frequency mosfet stays in linear range big portion of time, exceeds safe operation area and fails. Use proper MOSFET driver IC or make driver from discrete components.

There are basically only two failure modes of a MOSFET, thermal overstress and Vgs overvoltage. Modern MOSFETs can handle Vds overvoltage well, they simply avalanche and act as a zener diode. However, this can again lead to thermal overstress.

Your oscilloscope shots seem to show the motor running in steady state, right? Parasitic effects will exaggerate at higher currents, I would go to the extreme there and drive a blocked motor for a short time, and check the waveforms (and the MOSFET temperature rise) then.

EDIT: how is your 12V supply bypassed right on the freewheeling diode? You need low ESR capacitors with substantial capacitance there, the freewheeling current must have a good (AC) return path to ground. I suggest to monitor the 12V on a scope with that blocked motor as well.

[Zero999]

if it's a commutator motor, with momentary shorting of the segments drawing a huge pulse per revolution.

That's possible. Check for swarf shorting the commutator contacts, although I would have thought the high current would vaporise it. Another possibility is a shorted turn, in which case the motor should be replaced, if it can't be economically rewound.

[bson]

What's the breakdown of D4?  If it's anywhere around 12V or less you could be shorting to ground when fully open.

Also, the body zener's purpose seems to be mainly to keep static Vds buildup under control, not to dump several amps of reverse current - so I'd give it a proper external protective diode.

[timdriedger]

Did you check the snubber to see if it is still alive when you changed the FET?

I have two failed boards here on the bench. After removing the bad MOSFETs, D4 snubber tests okay with the DMM.