Why P-FET does no block reverse current

[Alderelrob]

Hi all,

I am trying to understand why a P-FET is suitable for reverse polarity protection but not for reverse current blocking. See the attached image.

When we reverse the polarity of the input, the Gate-Source of the P-FET are tied together to the positive side. Vin becomes GND, and the FET remains open, with no current flowing through the FET or the body diode. Reason is Vgs=0V

However, if the input voltage is, for example, 10V, and the load is a motor that generates a back-EMF higher than Vin, how is it possible for the current to flow through the FET? the gate is always tied to the GND, the Source voltage vary making the Vgs always <0 so the PFET can conduct in both ways?

Thank you.

[wraper]

Because as long as there is positive voltage on the output, FET will be in ON state. And in ON state it conducts current regardless of polarity. You need more complicated gate control circuit to block reverse current.

[langwadt]

once the fet is on it conducts in both directions

[David Hess]

Only the gate to source voltage matters for conduction in *either* direction.  When the MOSFET is on, it will happily conduct in both directions.

[DarkMode]

Only the gate to source voltage matters for conduction in *either* direction.  When the MOSFET is on, it will happily conduct in both directions.

Does that you can use it as a Switch, regardless of direction? 

[xavier60]

Only the gate to source voltage matters for conduction in *either* direction.  When the MOSFET is on, it will happily conduct in both directions.

Does that you can use it as a Switch, regardless of direction? 

No, because the MOSFET's intrinsic body diode will always conduct in one direction.
For both directions, 2 MOSFETs are needed or one MOSFET and a series diode.
I often see this arrangement in laptop computers, https://electronics.stackexchange.com/questions/617878/why-does-mosfet-n-back-to-back-switch-work

[David Hess]

Only the gate to source voltage matters for conduction in *either* direction.  When the MOSFET is on, it will happily conduct in both directions.

Does that you can use it as a Switch, regardless of direction? 

No, because the MOSFET's intrinsic body diode will always conduct in one direction.
For both directions, 2 MOSFETs are needed or one MOSFET and a series diode.
I often see this arrangement in laptop computers, https://electronics.stackexchange.com/questions/617878/why-does-mosfet-n-back-to-back-switch-work

Exactly, a second MOSFET in anti-series is required to provide isolation from the body diodes.  If the sources are connected together, then the same bias supply can drive both gates in parallel as shown here:

https://www.bristolwatch.com/ele3/2c.htm

There are power MOSFETs which bring the substrate connection out isolating the body diode, however they are limited to lower currents and voltages:

https://www.microchip.com/en-us/product/MIC94030