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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Using the body diode of a MOSFET to protect against relay back EMF
References: <firstname.lastname@example.org> <3E2E0300.57EC698F@earthlink.net>
Date: Wed, 22 Jan 2003 03:11:53 GMT
NNTP-Posting-Date: Tue, 21 Jan 2003 19:11:53 PST
Robert Baer wrote:
> 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
> > Embedded software/hardware/analog Info for designers: http://www.speff.com
> 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.
> 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.
> 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
I use schottky diodes quite a bit to protect power MOS driving inductive
loads. The imporatance of the schottky is not the low drop but rather
the fast turn on. The diode to Vdd or a somewhat higher voltage if
higher speed is needed removes most of the energy from the inductive
load so that any negative swing does not damage the transistor. A design
I am currently using has 6 MOSFETs to implement a 3 phase H bridge motor
driver. The N channel devices are protected by schottky diodes across
the P channel devices and the P channel devices are protected by
schottky diodes across the N channel devices all in the obvious way. The
circuit is reliable even when back phasing which considerably increases
the current in the transistors. The resonance problem you speak of is
not evident in the output waveforms either differential between two
phases or single ended measuring one phase with respect to ground.
It is very important that the schottky diode protecting a device have
short leads since very high frequency oscillations can destroy power MOS
devices. That's one you usually learn the hard way.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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