From: "Bruce Kingsley"
Subject: Re: MOSFET overvoltage protection
Date: Sat, 5 Oct 2002 20:22:29 -0700
Organization: MindSpring Enterprises
X-Server-Date: 6 Oct 2002 03:28:24 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2919.6600
With guidance from National Rectifier back in the early 1980's, I used a 15v
zener in a circuit that switched a 120 VAC line signal with no failures. I
would like to know what others have used.
I have found by looking at failed MOSFET, that pins with low resistances
means you over-voltaged a signal, open resistance means over current. Be
careful, even some VOM meters can fail a MOSFET.
"Bill Bowden" <**firstname.lastname@example.org*> wrote in message
> 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?