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):
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.
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.
- 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
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