From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: MOSFET overvoltage protection
Date: Mon, 07 Oct 2002 07:05:03 GMT
NNTP-Posting-Date: Mon, 07 Oct 2002 00:05:03 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Bill Bowden wrote:
> I have a question about using a MOSFET (IRF640 or 740)
> to drive an automotive ignition coil (about 6 millihenrys,
> 1.5 ohms, and charged to 50 mJ). Worked for awhile using
> only a MOV across the drain/source terminals, however the
> device failed using an RC snubber (0.5uF, 8 ohms) in place
> of the MOV. The gate to source resistance of the failed
> device reads about 60 ohms. I'm guessing the device failed
> from a high negative voltage to the gate as there was no clamp
> diode in place to protect the gate. However there is an integral
> diode inside the device from drain to source which should have
> limited the drain voltage to less than a volt negative, so I'm not
> sure why it failed. What is the usual practice to protect a
> MOSFET from overvoltages on the gate and drain?
> If a clamp diode is used on the gate, should it be a zener diode,
> or fast acting schottky diode?
Have not seen this problem.
What you should have, is a low resistive gate drive source; lower than
Otherwise a large dV/dT on the drain can induce a voltage on the gate,
larger than the gate-source rating.
Having a crappy drive source and then trying to fix problems caused by
that source is not conducive to good engineering practice.