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The FET has to be able to handle the full stalled current of the motor. if it's On-resistance is too high, then the switch fet will heat up and fail.
circuit shows an estimate of a PWM controlled motor running at 1500 rpm with the Current source acting as the spinning motor.
when controlling the motor with PWM, the average current draw shows the PWM is working, and the average voltage of the drain terminal also shows that PWM is working. But leave it running and the fet starts to heat up and eventually the motor stops?
using the scope to look at the drain terminal shows that the motor's commutation is visible. whenever the motor armature commutates a rotation, it's resistance drops dramatically; it's stalled momentarily. this resistance is lower than the FETs on resistance, sending the Vds very high, turning the fet into a heater.
over time the heat increases the on-resistance further, exacerbating the problem, until eventually the fet can't switch on enough to pass any stall current, and the motor halts altogether.
this needs a bigger fet for PWM to work reliably.
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