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August 2006 Rev 1 1/27
AN2344
Application Note
Power MOSFET avalanche characteristics and ratings
Introduction
Back in the mid-80s, power MOSFET manufacturers started to claim a new outstanding
feature: Avalanche Ruggedness.
Suddenly, new families of devices evolved, all with this “new” feature. The implementation
was quite simple: the vertical MOSFET structure has an integral body drain diode which
cannot be eliminated. By changing some process and layout parameters, it is possible to
guarantee the use of the clamping capability of this diode for withstanding accidental
voltage/power surges beyond the nominal drain source voltage.
Rating ‘ruggedness’ in a datasheet was very difficult because of the great confusion
regarding the meaning of this feature, as well as poor theoretical knowledge of it.
Nonetheless, all of the Power MOSFET manufacturers started to produce avalanche-rated
devices and propose datasheet ratings (although imperfect), to protect themselves and the
end users from this incomplete knowledge.
Now, knowledge about a device’s behavior during avalanche conditions is greatly enhanced
by a number of application notes and papers issued, which provide different explanations of
avalanche ratings and behavior. This application note briefly reviews the MOSFET physics
on avalanche behavior and supplies designers with tools and suggestions for dealing with
avalanche issues.
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