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MOSFET Switching: Part III - Turn-OFF, Hard 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, hard switching):

MOSFET gate waveforms - turn-off hard switching

And now the comments:
  • During t3r to t2r period, the Coss will charge with a current depending of the dV/dt associated with this period. This will decrease the actual current going through internal MOSFET channel, and therefore reducing the switching loss associated with this time interval.
  • Q3, gate charge associated with drain voltage reaching Vx, is much smaller then Q3+Q4, commonly specified in a MOSFET datasheet.
Other Considerations:
  • SMPS Power Supplies is using the above described correct theory regarding MOSFET switching to accurately calculate the switching losses in PFC hard switching and soft switching topologies. Combined with our accurate models for diodes (with voltage drop, reverse recovery time and reverse recovery current being functions of operating temperature, forward current, dI/dt), the design spreadsheets (ADH2450Des__.xls, ADH8100Des__.xls) are the most accurate design tools for designing and predicting the performances of a switching power supply.
  • 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 IV - Turn-OFF, Soft Switching

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