Im building a bi-directional variable speed motor controller to control a 12V 8A motor. In the design im using Power Mosfets. The controller is controlled by a PIC. The H-bridge drives the motor at full speed in both the forwards and reverse direction perfect. Mosfets only get barely warm.
But when i use a PWM to vary the speed by changing the duty cycle to mosfets get very hot and burn out after less than a minute. Ive tried running at 1khz, 5kHz and 10Khz with no difference.

Does anyone know the cause and solution for this???

 

You need to post a schematic of the H-bridge so we can see what might be the problem.

 

If your using the PWM to vary the gate voltage, the gates may not be turning on enough. The gate voltage has to be correct for the proper turn on of the mosfet.