Dale Botkin wrote...

>On Tue, 19 Feb 2002, Vasile Surducan wrote:
>
>>   eh, eh you have too rested minds guys !
>>   This will never work, take a look better ...
>
>Vasile,
>
>Can you tell me why you think it won't work?  It looks to me like each
>MOSFET's body diode will conduct "in reverse" during the half-cycle when
>Vds < 0, and the opposing MOSFET will conduct if the gate is driven
>high...  I do see one thing less than ideal, though, which appears to be=
 a
>loss of the Vf of the body diode.  For an IRLZ44 this looks like 2.5V =
max,
>or around 1.7V for the 2SK2614.

The circuit DOES work, and the MOSFET body diodes do NOT conduct. =20

I have two IRF512's connected to a 12V transformer and a light bulb
running on the workbench right now; the circuit gives full-wave ON/OFF
control (I can turn it on and off just by touching the gates with my
fingertip... neat!), with no current other than leakage flowing when OFF
and no voltage drop when on, other than that due to Rds(on).

When the MOSFET gates are biased positive with respect to their sources,
the MOSFETs conduct regardless of applied drain-source polarity.
They're field-effect devices, remember: the field under the gate
modulates the channel resistance, and neither the gate nor the channel
gives a fat rat's patootie which direction the channel current is
flowing.  Think about it: if it were otherwise, how would MOS analog
switches ever work?

As to the FET whose body diode would have been conducting because of the
"backward" current flow had the FET channel not been turned on, consider
it to be acting as a FAD*.

>Sure wouldn't
>have to worry about condensation in the box!

Well, you'll have to; your MOSFETs aren't going to be much help in
keeping it warm in there for you.  :)

>The relay is looking better now, but I don't see where the dual-MOSFET
>idea wouldn't work.  But of course IANAEE...  I may be all wet on part
>or all of this, someone please tell me if I am.

IAAEE; I do circuits for food.  This one works as advertised.

Cheers,

Dave

* FAD =3D Field-Assisted Diode  :)

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