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MOSFET Switching: Part IV - Turn-OFF, Soft Switching by Constantin Darius Livescu

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, typical of a power supply topology):

MOSFET gate waveforms - turn-off soft switching

  • 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.
SMPS Intellectual Property
This article contains information for which SMPS Power Supplies and its partners may claim Copyright and/or Trademark rights and may be subject of a Patent application. Also SMPS Power Supplies and its partners may claim the status of "First to be published", relative to ideas published in this article. Any third parties may quote reasonable parts of this article without contacting us, assuming that the source is clearly identified and a link to the full article is included. If you wish to incorporate information from this article within a commercial product, you should contact us for permission.
  • 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
Part I - Turn-ON, Hard Switching
Part II - Turn-ON, Soft Switching
Part III - Turn-OFF, Hard Switching

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