Drain Source NFET capacitance

[Mosaic]

Hi all:

I was doing a sim with an NFET switch into an inductive load. I noticed that adding bypass caps across the Drain-Source can 'protect' the NFET from avalanching due to kickback.

I note that fast diodes are usually used for this purpose. But what does the effect of drain/source Capacitance (10uF?) have on the MOSFET operation, if any, and why are the diodes the preferred solution?

 

[crutschow]

A capacitor from the drain-to-source can significantly slow down the switching speed of the MOSFET, causing the MOSFET to absorb the inductive energy. Since you normally want the MOSFET to switch fast and minimize the transistor dissipation, a reverse-biased diode across the inductive load is usually the preferred solution to suppressing the inductive kick.

But if you are switching the MOSFET at a low frequency then the slow operation of the MOSFET may not be a problem.