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>  Also I think your topology would require the MOSFETs to switch current in
> both directions (they are fundamentally not AC switches), but not
> sure if I visualize your circuit correctly.

*** Useful application of MOSFETS as AC power switches ***

The statement is essentially true for most practical purposes, BUT actually
a MOSFET IS fundamentally an AC switch BUT the reverse body diode means that
you can't use it that way directly so the fact is usually not noted. The
channel of an "on" (enhanced)  MOSFET is for practical purposes largely
indistinguishable from a resistor while the FET is operated within ratings..
(Rdson will vary slightly with conducted current depending on Vgs up to a
certain current and then increase quite rapidly above this current level but
this can be ignored for most purposes).

However, there is a very useful application that may be made of this fact.
Take two N channel MOSFETS.
See diagram.
Assume
        +10v gate to source biases them fully on.
            0v gate to source turns FETs fully off.
Connect source to source.
Connect gate to gate.
Use one Drain as "input".
Use other Drain as "output"

Attempt to make an AC current flow from input to output.

First connect gates to sources. Vgs = 0
Both FETs are off.
On one polarity of applied AC signal one FET will be reverse biased - its
body diode with conduct BUT the other FET will have normal bias and overall
the assembly will NOT conduct.

Now bias the joined gates to say +10 volts relative to the joined sources.
(Vgs = +10)
BOTH FETs will now have gate polarity correctly applied to turn them both
on.
Both will appear to be a pure low value resistor between drain and source.
The applied AC current will flow through both FETS - in one it will be in
the "normal forward" direction and in the other in the "non-normal backward"
direction. In the latter FET it will forward bias the body diode somewhat
BUT as FETs are typically rated to have less voltage drop than the body
diode forward conduction voltage the FET rather than the diode will conduct.
If the voltage drop gets too high the body diode will join it "to assist ".
The affect is that the FET pair is now a bidirectional AC switch. This can
be used for heavy current AC switching (subject to the FET ratings).

The largest problem here is generating the FET gate voltage which must be
always positive relative to the sources. As the sources are swinging happily
with the AC voltage that is driving the AC current, the voltage source must
be fully floating. There are various ways of doing this which will vary with
circumstance.

A single FET connecting to the positive and negative outputs of a bridge
rectifier will produce a similar result BUT looks much  less like an ideal
resistor due to the 2 x diode drop added by the bridge rectifier at all
times.



        Russell McMahon

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