Hi Mark,

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".

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. I have not looked at the ratings of the
part you mentioned, but that's the curve I'd look at
if I needed to use a part that way. Is 500mA at 40V
within the curve?

And of course EMI is usually another reason to use a
snubber diode, but I know that's not the reason for
the question.

Cheerful regards,

Bob



--- Enki <enkitec@gmail.com> wrote:

> 
> 	I'm doing some tests with the FDN337N Fairchild
> Mosfet.
> 	It is working switching a 24V @ 500mA relay in
> series with a 1 ohm 
> resistor. The relay has the usual 1N4007 diode.
> 	If I don't use the diode, the back EMF when the
> Mosfet switches off 
> goes up to ~40V, the Mosfet breakdown voltage, for
> 10mS.
> 	Just out of curiosity, I let the Mosfet switch the
> relay each second 
> without the diode, some 100.000 times, 50% duty
> cycle.
> 	The Mosfet seems not care about the energy absorved
> during the 
> breakdown period. It warms up to some 40C.
> 
> 	My question is: 
> 
> 	Is there any expected long term problem running the
> circuit without 
> the diode?
> 	
> 	The design WILL use the diode, my question is just
> to satisfy my 
> curiosity.
> 
> 	Thanks and happy 2008!
> 	Mark Jordan - PY3SS

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