From: John Larkin
Subject: Re: Using the body diode of a MOSFET to protect against relay back EMF
Date: Tue, 21 Jan 2003 18:48:48 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Wed, 22 Jan 2003 02:33:17 GMT, Robert Baer
>Spehro Pefhany wrote:
>> On 21 Jan 2003 06:38:52 -0800, the renowned email@example.com
>> (Klaus Kragelund) wrote:
>> >According to the subject message - is this possible?
>> Not the way you're thinking of. When the transistor switches off,
>> the voltage on the drain will increase beyond the positive rail,
>> not go down below ground.
>> Best regards,
>> Spehro Pefhany
>> "it's the network..." "The Journey is the reward"
>> firstname.lastname@example.org Info for manufacturers: http://www.trexon.com
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> Yes, but the inductance will ersonate with its capacitance and that of
>the FET, so the voltage will tend to swing negative almost as much as it
>did in the positive direction.
> It is *that* swing that is being asked about.
OK, if you're not clamping the positive swing (and the fet can stand
it) then it will ring back down.
> I have found it very difficult to use a diode in parallel with the
>FET, in a manner that will shunt that current away from the FET.
Difficult? What's happening?
> One might think of a schottkey diode, since the forward drop is a lot
>lower; but the reverse voltage rating stinks (making it useless for this
Won't the substrate diode clamp the negative swing?