Russell McMahon wrote:

[SNIP]
> All that is correct.
> Except that the MOSFET DOES turn on when the MOSFET body diode is
> forward biased.
> ie MOSFET is in quadrant 3, with DS polarity the opposite of normal
> BUT  the Vgs voltage the correct polarity for turn on ie gate more
> negative than source for a P channel device.
>
>
>   
Last time I thought about "quadrant" was in relation to triacs......

To clarify,

    * you are saying that, in my circuit, the mosfet D-S junction is on
      IN ADDITION to (in spite of?) the body diode conducting? (thus
      giving the possible advantage of lower voltage drop/power
      dissipation than the body diode alone can provide)
    * you are NOT saying that the fact of diode conduction is THE SAME
      as the D-S junction conducting?

OK, so you say the mosfet _IS_ on. I think it is too.

Olin seems to disagree with us about this...but I see his point if only 
the body diode is conducting.

Then lower voltage drop (than a schottky) is possible, thus lower power 
dissipation, given a low enough R-DS-On.

Right?

However, given change in R-DS-ON due to power dissipation/heating 
effects, it may be a wash...(or worse) I suppose. I could look for a 
mosfet with a lower RDSON. I think I'll have to actually try both and 
compare. Or maybe there won't be enough difference to matter, in which 
case the diode would certainly be simpler. Using more than one mosfet 
would negate in cost/complexity any advantage it would provide.

If (hypothetically) the MOSFET solution does result in significantly 
lower power dissipation vs a schottky, did you guys see anything wrong 
with my circuit?

Mark
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