From: Jim Thompson
Subject: Re: Using the body diode of a MOSFET to protect against relay back EMF
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Date: Tue, 21 Jan 2003 16:38:24 GMT
NNTP-Posting-Date: Tue, 21 Jan 2003 11:38:24 EST
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On Tue, 21 Jan 2003 08:29:40 -0800,
John Larkin ,
In Newsgroup: sci.electronics.design,
Entitled: "Re: Using the body diode of a MOSFET to protect against
relay back EMF",
Wrote the following:
|On Tue, 21 Jan 2003 09:41:07 -0600, Crusty Curmudgeon
|>I'm not too wild about just a catch diode across the coil. The dv/dt on flyback
|>can still trash the gate.
|I've never seen this happen with a 2N7000/7002, or any other mosfet
|for that matter. The flyback turns the gate on, which limits drain
|A series gate resistor will also reduce dv/dt, which you may like to
|do for other reasons. Hmmm... with a big gate resistor, wouldn't peak
|drain voltage be nicely limited? That should work.
Since virtually everything I design is monolithic, there's no such
thing available as a catch diode without destructive parasitic
So I've become fond of turning the MOS device back on when the drain
voltage exceeds rail. It also has the advantage of taming transients
on the rail.
Of course you have to watch the SOA ;-)
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
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