100ns transients causing thermal damage? Can you describe the situation
which caused this?!

It is quite normal for the instantaneous power dissipation of a FET while
switching to be in the range of 100x to 1000x the steady state dissipation
for about 100ns. Of course, when this is averaged out it does indeed
contribute significant heating, but if you are talking about a handful of
individual transients, it would be very difficult to destroy the FET in a
few 100s of nanoseconds. Not impossible, but it would have to be a very
impressive power supply which could deliver enough current during such a
short time.

For example, the IRFS4010-7 is a FET I'm very familiar with. The
manufacturer provides a nice detailed internal thermal model. The fastest
time constant in that model is 25 microseconds. This leads to the effective
single pulse thermal resistance junction to case being 0.0007 deg C/W for a
1 microsecond pulse (the shortest they show). For a 100ns pulse, it would
be approximately 0.00007 deg C/W since we are at a timescale way faster
than the time constant so the effect scales linearly in time duration. The
steady-state value is 0.4 deg C/W

The SOA plot stops at the Vds max of 100V and at the max peak current
rating of 740A (a crazy high value for this package - the maximum
steady-state current when fully on would be something like 70A under
typical circumstances). It isn't clear what effects are driving that
(possibly current crowding/uneven current distribution within the
die/electromigration, etc.) because even at 74kW of dissipation (100V *
740A), a 100ns pulse would raise the junction temperature by only 5 deg C.
Achieving this (going from a reasonable current of say 100A to 740A and
back down again in 100ns) would be difficult without very strong gate drive
and lots of low ESR/ESL capacitance located immediately around the part.

I can believe a FET might die for non-thermal reasons (or at least not
overall junction temperature issues) in 100ns under super extreme
circumstances but not simple heat dissipation in 100ns.

Sean




On Fri, Dec 16, 2016 at 5:27 PM, Van Horn, David <
david.vanhorn@backcountryaccess.com> wrote:

> I'd throw a scope on it and see why it's dying.
>
> Could be it's not turning on and off fast enough?  Linear region will
> cause dissipation to skyrocket.
> I saw a case  where a FET was dying because of extremely short (100nS)
> transients where the dissipation was over the limit.
>
> Could be a voltage spike from wiring inductance exceeding VDS.
>
> A Zener across VDS rated somewhat below the FET rating and above the powe=
r
> supply might help.
>
> Scope it and see.
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.