# What could cause the mosfet to burn out in my motor driver?

Here's the schematic: I've tried to make this motor driver as tough as possible, but I'm still seeing occasional failures where the MOSFET and the diode D1 are both failed short.

Any ideas what could be causing this? Also is there anything I can do to make this driver even tougher?

Additional info: The motor is speed controlled by a microcontroller. The PWM duty cycle is capped around 60%. The motor can draw about 5A or more depending on the load, but this whole thing is powered by a switching power supply rated for 3.75A with a (fast) over-current shutoff, so that should limit the current in cases with excessive load. PWM frequency is approx 700hz.

I don't know what's burning out first, the mosfet or the diode. I tried replacing the diode (before I knew that the mosfet was also burned), and it burned out again after I toggled the interlock a few times.

• Check if your interlock switch is of the intermittent-open type. There are switches out there which are both com-nc and com-no during switching. – Janka Nov 1 '18 at 23:31
• Probably under certain conditions your motor produces very high voltage spike which punch thru those failed components. – soosai steven Nov 2 '18 at 1:26
• I'm pretty sure the switch is non-shorting; but I'll check that. – Drew Nov 2 '18 at 3:52

Maybe your circuit stresses the MOSFET during switching. I could see two possible problems.

First, you placed a 100nF capacitor across the freewheeling diode. When the MOS is turned on, it has to dissipate an amount of energy equal to $$\frac{1}{2} C_5 (24)^2$$ plus the energy due to the recovery charge of the diode. To avoid this, you should have inserted an inductor in series between the diode and C5.

Then, the inductance of the loop constituted by the MOS, D1 and the decoupling capacitor C6 should be kept as low as possible, to limit overvoltages across the MOS during turn off. I'm afraid it is not the case in your design, because this loop also includes a switch, and possibly the wires between this switch and the PCB. I would have put the interlock AFTER diode D1.

Edit: these are the changes I would advise :

simulate this circuit – Schematic created using CircuitLab

• My suggestion: another flywheel diode after switch, without L1 and without C5. – Marko Buršič Nov 2 '18 at 8:29
• Thanks for the advice! Quick question, what is the purpose of 1uF C6? I assume it's to provide a low impedance sink for the flyback current, but I just want to make sure. – Drew Nov 9 '18 at 23:45
• C6 (could/should be less han 1µF : between tens and hundreds of nF) avoids voltage peaks when the MOS turns off dur to any stray inductances in wires coming from the power supply . It is basically a decoupling capacitor. – Charles JOUBERT Nov 10 '18 at 12:04