Inductive Kickback Question

Thread Starter


Joined May 19, 2008
I am running 32- 3amp DC motors on a 12v system with and alternator and using N Type MOSFETS to drive the motors. I am having problems with the MOSFETS failing...going into a closed state. I have a kickback diode between the drain and 12v. Its a 3S2D. My frequency to the MOSFETS is 400 hz. The MOSFETS fail randomly. It will run from 2 minutes to 5 hours with no problems, then randomly a MOSFET will fail. I was able to get a scope on the drains today. With the ES2D's installed I still see a 70 volt spike when the MOSFET is turned off. I ordered a FFPF20UP20DN rectifier with a much higher voltage and amperage rating.

I know the MOSFETS aren't failing due to amperage overload. I even tried using two MOSFETS in parallel and they still failed. There is no heat on the MOSFET either.

I have been reading up on using a resistor along with a rectifier to help with kickback. How do I get the right resistor?

This thing has been a <snip> to sort out with its randomness of failure. Any input is greatly appreciated.
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Joined Nov 20, 2008

Brush noise from DC motors can generate very fast rising edges, too fast for diodes to respond to. Place a 0.33uF 200V capacitor across the motor load, but near the driver. This will slow down the edges so the diodes can work. Use a good quality, low series inductance capacitor. An extended foil film type, or ceramic should work okay. You may need to experiment with the value needed.

Good Luck,
I would also decouple the power supply well too.
Could be the spikes are getting back onto the supply and causing bus pumping which will pop the mosfets.

Thread Starter


Joined May 19, 2008
Here is the kickback I am going to try tomorrow

Here is the kickback I was using

On the schematic when rotated clockwise one click on the middle right D1-4 are the kickbacks.

Here is the MOSFET

Here is a link to the schematic

This is on a 12v system, some of the references on the schematic are labeled +24v. There is an alternator on the power supply. The MOSFETS fail randomly. On my test bench they run many days straight and no problems.
Last edited:


Joined Jul 26, 2010
Well, your new snubber diode will handle more current but it's slower, 45 nS vs 25 nS on the one you had been using. Check out some of the Schottkey diodes to see if any are faster.

I'd put at least 0.47 uF 100V ceramic disc capacitors across the motor as well as across the D-S connections of the MOSFETS. Most ceramics usually have less inherent inductance so they will act upon pulses faster. The larger the better on the motor but I wouldn't go above 1 or 2 uF as those start to get inductive with a higher ESR due to the way they're constructed.