In article ,
J M Noeding wrote:

Another trick I've seen is to use a hefty power MOSFET (one with a low
Rds[on]), with the source and drain leads reversed, in series with the
power. Depending on the voltages involved, you can either connect the
gate directly to the opposite side of the power supply, or (if the
supply voltage may exceed the Vgs limit) to a high-Z resistive divider.

Nice one Dave

indeed, if you could tell me about a MOSFET without protexion diode

That's why I mentioned hooking it up with the source and drain leads
in the reverse of the usual orientation.

In the normal hookup situation, the intrinsic/substrate/protection
diode would prevent the circuit from working as one would want (as you
clearly understand). However, by hooking up the MOSFET "backwards"
(e.g. an NMOSFET with the drain on the negative side), the orientation
of the intrinsic diode is reversed. It'll be forward-biased when the
power supply is hooked up the way you want it, and would be
conductive. It probably won't have a chance to conduct, though, since
you'll be biasing the gate up several volts above either the source or
drain pins, turning on the MOSFET, and the relatively low Rds[on] of
the MOSFET will keep the voltage drop across the MOSFET down below the
substrate diode's voltage drop.

If the supply connections are reversed, the MOSFET itself will be
biased off, and the intrinsic protection diode will be reverse-biased
and will not conduct.

It's an odd trick. Many (most?) JFETs have a highly symmetrical
internal construction - the distinction between source and drain is a
matter of convention rather than construction. A typical MOSFET's
construction is not entirely symmetrical... but, according to the
description I've read of this trick in Pease's book, it _is_
symmetrical enough to allow it to be used with source and drain
reversed, in this particular application. By flipping 'em around, you
make the intrinsic diode work for you rather than against you.

--
Dave Platt AE6EO
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