This article is trying to make sense out of confusing information regarding the behavior of a MOSFET during switching sequences, in numerous technical articles.

We are not attempting to explain the physics behind a MOSFET structure. For those interested to find more about a MOSFET structure, we recommend the SGS-Thomson technical articles mentioned in the references. The purpose of the article is to present a power supply design engineer with facts that will help design a MOSFET driving circuit, calculate the estimated losses for critical events, predict the efficiency of a power supply, estimate the junction temperature for critical components and various stresses, and ultimately, helping make decision to optimize a design.

Below are the waveforms, mostly self-explanatory related with a MOSFET switching off (inductive load, diode clamping, soft switching):

References:

• Current at the time this article was last updated, not known articles specifically describing the MOSFET turn-OFF and power loss calculations. Most would incorrectly consider the turn-OFF as a mirror of turn-ON.

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• First LCD Consulting internal document: 19 Oct 1991
• Major update, SMPS Power Supplies internal document: 10 Feb 1998
• Web first published: 3 Aug 2002
• Last Revision: 3 Sep 2005