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MOSFET Switching: Part II - Turn-ON, 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. 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.

The MOSFET switching events are analyzed for an inductive load, diode clamping circuit, the only one that applies to a switching power supply. The datasheet information or technical articles regarding resistive loads have little or no relevance to switching a MOSFET in a switch mode power supply. Also the article is considering only 500V/600V MOSFETs, most relevant and for switch mode power supplies.

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

MOSFET gate waveforms - turn-on soft switching

And now the comments:
  • Gate voltage has a sharp rise at t1s due mainly to the MOSFET starting to discharge the Coss and MOSFET source inductance .
  • At t2s the drain voltage will reach Vx, usually around 25V for 500V MOSFETS.
  • Q3, gate charge associated with drain voltage reaching Vx, is much smaller then Q3+Q4, commonly specified in a MOSFET datasheet.
  • Notice the change of events order compared with hard switching! In soft switching circuits first Vds will drop to a low level before the drain current will begin to rise, having as consequence a dramatic decrease in switching losses!
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 describing the the MOSFET soft switching turn-ON and power loss calculations.
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 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 III - Turn-OFF, Hard Switching
Part IV - Turn-OFF, Soft Switching

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