Smoothing a High Current PWM
Add a Schottky diode, an inductor, and a capacitor, to create a buck-mode step-down voltage convertor like this:-



The inductor and capacitor create a smooth DC output voltage proportional to the PWM ratio. The diode recirculates current through the inductor when the FET is turned off. This is required to properly smooth the output voltage, and also prevents the inductor from producing voltage spikes that would kill the FET.