From: Spehro Pefhany
Subject: Re: Using the body diode of a MOSFET to protect against relay back EMF
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Date: Tue, 21 Jan 2003 16:37:47 GMT
NNTP-Posting-Date: Tue, 21 Jan 2003 11:37:47 EST
On Tue, 21 Jan 2003 08:29:40 -0800, the renowned John Larkin
>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
Yes. If the gate gets zapped, it's probably due to seriously bad
inductance in the source and unnecessarily fast switching. IOW,
bad layout and going a bit to spartan on the parts.
>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.
I like to do that. There are other considerations when switching a
mechanical relay that don't come up when you're just switching a
nice clean transformer coil with a known circuit on the other
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