We use three Freescale MC33982 high-side switches in parallel to drive a DC motor, and our electrical engineer suggests that a 50V, 10uF capacitor on the VPWR pin (then to ground) will guard against inductive kick back, but I cannot understand how, and I wonder whether someone here could explain how? Note, it's the location of the capacitor that confuses me, not how a capacitor could work as a shunt. (Also, this switch has an output clamp, but we are concerned it's not sufficient.)
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If there is no diode clamp on the motor to catch back emfs when the motor current is interrupted, that energy in the motor will force current to flow through the parasitic diode of the MOSFET to the VPWR rail. Adding a "soak up" capacitor on the rail ensures that the VPWR rail won't rise very much. However if you have an output clamp this will largely prevent this from happening and a much smaller capacitor can be used on VPWR.
answered Oct 16, 2014 at 17:27