Here the schematic of a very simple circuit enter image description here I know I don't have the flyback diode on the motor. And the MIC4422 is flipped in the schematic.

I want to control the high power motor with this high power mosfet while using PWM.

To drive the mosfet(AUIRFS8409-7P), I got a MIC4422 to increase the mosfet switching speed, but I still get some extreme heat after only a few second of running and there is no load on the motor. For sure no heatsink can dissipate this heat. I didn't put any resistor at the gate of the mosfet to increase the switching speed.

I try lowering down the pwm frequency. At about 50 hz, the heat is OK with heat sink, but the frequency is too low for the motor to run smoothly and it's vibrating a lot. Maybe I could find a equilibrium point between the ''vibration'' and the heat output.

Do you know anything to reduce the switching time even more, when the motor will have load, it will be even worst. Is my circuit ok?

BTW, when the mosfet is not under pwm, the mosfet is not even a little bit warm. The problem is really with the switching time.

Thanks a lot, Bernard

  • 1
    \$\begingroup\$ What current does the motor draw? And what do you mean " I got a MIC4422 to reduce the mosfet switching speed"? \$\endgroup\$
    – stefandz
    Aug 13, 2015 at 17:36
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    \$\begingroup\$ Perhaps the problem is not having a small resistor in series with the gate causing ringing see electronics.stackexchange.com/questions/75228/… \$\endgroup\$ Aug 13, 2015 at 17:39
  • \$\begingroup\$ small resistor to the gate, large (10k) resistor to ground on the gate for the purposes of reset/power on/off state transistions.. Add those, in addition to the flyback diode \$\endgroup\$
    – KyranF
    Aug 13, 2015 at 18:47

1 Answer 1


If you don't have a fast flyback diode on the motor, the MOSFET will avalanche and it will overheat.


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