But Vasile, the diode is just part of the transistor. If the transistor is
in avalanche breakdown, that IS the body diode in breakdown, in the normal
way in which high voltage zeners work (actual Zener effect is overshadowed
by avalanche effect when the breakdown voltage is above about 5.6V).

I'm not sure what type of secondary breakdown you are referring to. The
term "second breakdown" that I know of is a term used in BJTs to indicate
unequal current distribution within the die due to the positive temperature
coefficient of conduction in a BJT. In other words, it has nothing to do
with overvoltage.

Sean


On Thu, Dec 10, 2015 at 2:06 PM, embedded systems <piclist9@gmail.com>
wrote:

> On Thu, Dec 10, 2015 at 7:41 PM, <alan.b.pearce@stfc.ac.uk> wrote:
>
> > > Hmm, the datasheet of the FET says its a fast switching diode, how ca=
n
> > act
> > > as a zener diode?
> >
> > Any diode will act as a Zener if you raise the voltage high enough. Jus=
t
> > that some will only do it once, and then either become an infinite
> voltage
> > Zener or a (close to) 0 volt Zener ... ;)
> >
>
> Come on Alan, :)
> Without a flyback diode on the coil, the drain voltage will rise above th=
e
> max drain voltage and a secondary breakdown will appear.
> That's is heating that particular 100V transistor we are talking about, n=
ot
> the diode frying like a zener...
> If the breakdown is short, MOSFET will survive, if not, will become a
> bulistor. Bulistor =3D defective transistor.
>
> Vasile
>
>
> >
> >
> >
> > --
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