[SI-LIST] Re: diode connected between source & drain - why?

Curt, yes in a power MOSFET the body diode is a consequence of 
construction.  Further it isn't really a diode, it is an SCR that can be 
triggered by excess dv/dt.  Triggering that SCR can spell death for a half 
or full bridge.  In a four quadrant application, it is common practice to 
insert a series Schottky diode between a power MOSFET and the reactive 
load, and to shunt that combination with a high speed or Schottky 
diode.  As someone mentioned, International Rectifier offers a wealth of 
valuable information on power MOSFETs, as does Intersil, and Vishay/Siliconix.


Steve
At 09:19 AM 4/27/2005 -0500, Curt McNamara wrote:
>My experience with the body diode (as I understand it, a by-product of
>device construction) is that you *should not* depend on it to clamp
>an inductive kick-back. There is simply too much energy for it to=20
>dissipate, and you will destroy the FET. Use an external diode
>clamp which is rated for the energy -- these should only cost a few=20
>cents.
>
>                                       Curt
>
>Curt McNamara. P.E.=20
>Senior Electrical Engineer=20
>Logic Product Development=20
>411 Washington Ave. N Suite 101=20
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>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Christopher.Jakubiec@xxxxxxxxxxxx
>Sent: Wednesday, April 27, 2005 7:44 AM
>To: roymesi@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diode connected between source & drain - why?
>
>
>Hi Roy,
>
>When current is switching through an inductive load, a reverse EMF is
>generated when the current is turned off.  This diode allows a path for
>this current in this situation.  This body diode comes built into the
>MOSFET for free, and is not an extra device.
>
>A diode connected MOSFET behaves like a diode, while a standard
>connected MOSFET behaves like a switch with a diode in the body.
>
>-Chris
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Roy M.
>Sent: Wednesday, April 27, 2005 9:21 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] diode connected between source & drain - why?
>
>
>Hi,
>Can someone explain to me why are they Transistors (mosfets), that has a
>diode connected between the source and the drain? I tried to find
>something on the internet, but nothing... i don't understand what's the
>difference between a regular mosfet and a
>diode-connected-source-drain-mosfet.
>Thanks!
>Roy
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