Russel:

In one schematic, it shows an opto-isolator from gate to source, but
with no bias at all on the gate. Normally, if I was driving a Pfet, I
would have a resistor from the drain to the gate to provide the voltage
to hold the fet off, then ground the gate to turn it on.

The opto-isolator arrangement leaves the gate floating, and then shorts
it to the source to turn on. Is this OK? I've never left a  gate
floating like this.


--=20
Larry Bradley =20
Orleans (Ottawa) Canada=20

On Sun, 2014-02-09 at 17:28 +1300, RussellMc wrote:

> This is an extremely useful circuit arrangement to be aware of.
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> >
> >
> >
> >
> >
> >
> >
> > *> The P-fet switch is simple to drive, but the body diode presents a
> > problem if > the input voltage is less than the output voltage, in whic=
h
> > case the body > diode will conduct and current will flow FROM the start
> > battery. > > For this reason, I might be better of to use an N-channel =
and
> > then a gate > driver. I would look at using two FETs in series, as you =
will
> > have the same problem with N channel FETs. Wire the two sources togethe=
r
> > and the two gates together, then connect one drain to the source and th=
e
> > other drain to the battery, in a similar way to what is done for AC
> > switching using FETs.*
> >
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> Two P Channel FETS are used.
> These are connected in series as above.
> Gate to gate. Source to source.
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> The two drains then become the terminals of a 2 terminal pure resistive
> switch.
> This can be floated if a floating gate-source voltage can be provided.
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> This cam be "semi floated" by driving the gates with a resistor of suitab=
ly
> high resistance not to interfere with main signal.
> Say drive gates with 100k in a DVC on/off switching mode.
> Connect Rgates input to high / V+ to turn switch off.
> Connect Rgates input to ground to turn BOTH P FETS on.
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> With ideal components this may notionally not turn on as with two ideal
> FETS there is no return path for gate charge current to flow / for gate
> voltage to be relevant to.
> In practice there will be enough real world effects to start to turn on o=
ne
> FET and away it goes [tm].
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> This series opposing dual FET arrangement works very well.
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> You can see this in commercial use with many LiIon protection ICs that us=
e
> external series opposed MOSFETS to provide true bidirectional battery
> disconnect.
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>   Russell
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