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):
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!
- 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
- Current at the time this
article was last updated, not known articles describing
the the MOSFET soft switching turn-ON and power loss
- 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
- Last Revision: 3 Sep 2005