MOSFET started smoking at 42V. Was expected to run at least 54V

MMoha.10 · ‎2023-09-18

Good Afternoon,

Hope you are doing well. Looking for advise on how to resolve this issue.

We designed a motor controller using the STL220N6F7 (MOSFET) and L6387ED gate driver. We know the MOSFETs are rated for 60V. However, when we ran 42V the MOSFETs started to smoke up. Can you please review the attached schematic and advise if you notice anything wrong. The motor controller works great at 24-29V. Only at higher voltages we see it heating up a lot.

I included one of the three Gate-MOSFETs pair for reference. Let me know if you need anything else.

Would it because of R33 and C44 by chance? I have tried referencing some of ST's motor controllers and except for the B-G431-ESC1, none of them uses these resisters on the output. Interestingly, the B-G431-ESC1 is also rated for 24 V. Unfortunately, I'm not sure what is the purpose of R33 and C44 in the first place.

Any help is appreciated.

LCE · ‎2023-09-18

What's the maximum gate-source voltage at 42V?
The FET's maximum is the typical +-20V.

R33 + C44: think simple. It's kinda short circuit for high frequencies and should probably suppress the switching transients.
Check with a scope, compare without these.

AScha.3 · ‎2023-09-18

R33 and C44 is a snubber. this is to reduce switching transient, preventing self destruction of mosfet.

>Was expected to run at least 54V --> smoke ! surprise ?? no.

keep safety margin VCC : Vds-max at 1:2 ; so if want 50V supply, need 100V mosfet type.

otherwise...look at the smoke and think about...

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LCE · ‎2023-09-19

@AScha.3 wrote:
R33 and C44 is a snubber. this is to reduce switching transient, preventing self destruction of mosfet.

Thanks, that's the word that I was looking for! :D

Msolinas · ‎2023-09-19

There could be several reasons why it heats up.
1) check the gate capacitance of the mosfets, the driver you use may not be sufficient for the frequency you use.
2) the duty cycle you are using is wrong (check the dt), this becomes evident as the voltage increases as the reaction times of the mos change as the power involved increases.
3) the snubber does its duty in frequency, but it must be calculated according to the one you use, and not used one at random taken from a reference circuit.

You have to learn to look at all these phenomena from the oscilloscope.

MMoha.10 · ‎2024-08-01

I was able to resolve this issue. The MOSFETs had a higher Rds and hence were dissipating more heat. Once I changed them, everything worked fine.