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 which
> 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 an=
d
> then a gate > driver. I would look at using two FETs in series, as you wi=
ll
> have the same problem with N channel FETs. Wire the two sources together
> and the two gates together, then connect one drain to the source and the
> other drain to the battery, in a similar way to what is done for AC
> switching using FETs.*
>


Two P Channel FETS are used.
These are connected in series as above.
Gate to gate. Source to source.

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.

This cam be "semi floated" by driving the gates with a resistor of suitably
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.

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 one
FET and away it goes [tm].

This series opposing dual FET arrangement works very well.

You can see this in commercial use with many LiIon protection ICs that use
external series opposed MOSFETS to provide true bidirectional battery
disconnect.



  Russell
--=20
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