From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: MOSFET overvoltage protection
Date: Sun, 06 Oct 2002 05:36:11 -0000
Organization: Your Organization
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In article ,
>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?
Even MOVs are a lousy way to protect against such spikes. They have an
extremely soft knee. They are mainly used on AC applications since they are
polarity insensitive. Zeners (or their close relatives, Transzorbs, or TVSs)
are the way to go. They are extremely fast, and have a hard, well defined
clamp voltage. MOVs require the voltage to rise to more than double the
point where they start conducting, before they are conducting significant
The integral FET body diode is useless as a means of protecting against
positive overvoltage, and is rather slow to turn on with reverse voltage.
Use a fast diode in parallel for this. You may also be seeing Miller effects
that cause the gate to blow, when very fast drain voltage risetimes are
involved. Hang a 12V zener across the gate.