The reversed body diode can be 'eliminated' as follows:

1. Take two mosfets (either both N or both P, N typically have lower
RDSon)

2. Connect the two sources together

3. Connect the two gates together

4. Connect a resistor (e.g. 30K) between the sources and the gates
(important)

5. Use the two drains as the two ends of the switch

6. Drive the gates with appropriate voltage (when designing the voltage
range, examine the voltage on the sources and forget to consider the
reversed diode, in this case they actually help to set the bias
voltage).

In OFF state, both mosfets will be off and one of the reversed diodes
will be off which will block the current.

In ON state, both mosfets will be on and the affective RDSon is double
the RDSon of each of the mosfets.

We have used it in several circuits and it worked well for us.

Tal


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Russell McMahon
> Sent: Thursday, February 13, 2003 2:07 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: MOSFET as a digital switch
>
>
> > I just pulled an old text from the shelf, and it clearly
> explains how
> > IN THEORY, MOSFET's, both N & P channel will work with
> either positive
> > or negative gate bias.
>
> No arguments here :-)
>
> I mentioned enhancement and depletion mode FETs and that the
> overwhelmingly most common form is enhancement mode. This
> means that they are OFF until ENHANCED on. A depletion mode
> FET is on until DEPLETED off. This will all be covered in the
> text books. If we are dealing with real world devices then we
> have to run with what they make.
>
> Also note that IF you could reasonably make an N Channel
> MOSFET which operated with either positive or negative gate
> bias relative to the source and IF it was about as good as
> existing real N Channel FETS then it would seeep away almost
> all P Channel FET applications (as it does what they do as
> well) and probably replace most N FET designs as well.  I'd
> buy them :-).
>
> > Well, Russ, I guess you didn't write all the books...
>
> Indeed. But I read all the data sheets that I can get hold of :-)
>
> What people make and sell is the ultimate arbiter (alas) of
> what you have to do to make them work. Current MOSFETS are
> sold as 2 quadrant devices at present (ie gate with one
> polarity, drain with either) and the 2nd quadrant is not
> overly useful in most cases due to the body diode.
>
>
>
>             Russell McMahon
>
> --
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