> You are looking at avalanche operation, and there has
> been a fair amount of study paid to it with regard to
> mosfets. Some manufacturers go so far as to have an
> entire line of them touted as "avalanche mosfets".

The reason this is done is that special design care is taken 
to ensure they work properly in this mode.

> But there's no reason why any mosfet can't be operated
> in avalanche mode as long as there are two parameters
> that are not violated. One is to make sure that the
> drain-gate voltage rating is equal to or higher than
> the drain-source rating. As far as I know that is
> always the case with modern parts. The other is to
> insure that you don't exceed the safe operating area
> of the device.

The trick with avalanche rating a part is to distribute the 
dissipated energy evenly enough to prevent localised energy 
concentration causing punch through. You know that they have 
failed in your application when Rdson is unchanging and is 
ohms to a few hundred ohms when there is no power on the 
FET. Sometimes there is a small hole in the front of the 
case as an added giveaway.

MOSFETs are usually deemed to be immune to the second 
breakdown effects that cause problems in bipolar devices 
(although this is not universally the case). While there is 
some black magic involved, second breakdown is largely held 
to be a localised thermal effect where current concentration 
cause increased temperatures which cause current 
concentration which cause ... oops.

However, when operated in avalanche mode MOSFETs that are 
not designed to rigorously distribute the current evenly 
over their multiple subdevices tend to concentrate current 
at selected sites with results that are not after the event 
especially well distinguishable from having been 
second-breakdowned. As SOA is not liable to be well defined 
or defined at all in avalanche mode in a non avalanche 
specified part YMMV and probably will if operating an 
unrated part this way.



            Russell

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