I'm looking at a circuit and trying to understand the purpose of the dual PNP transistor.
I'm guessing the P Channel mosfet is for reverse polarity protection.

 

Looks like a low drop-out half wave rectifier circuit.

 

I agree it's likely just a reverse-polarity protection arrangement, but don't see how the dual PNP adds anything useful to the normal arrangement with just a pull-down resistor (R17).

 

Could it function as a current limiter?
As the current increases, the voltage across the MOSFET will increase and this will change the relative collector currents in the BJT and so control the MOSFET gate.

 

So where did you find this circuit?

 

Could it function as a current limiter?

Simulation suggests not.

 

It’s an ideal diode circuit.
https://www.diodes.com/assets/Datasheets/DZDH0401DW.pdf

 

So tiz.

 

It’s an ideal diode circuit.
https://www.diodes.com/assets/Datasheets/DZDH0401DW.pdf

Very helpful, many thanks

 

So where did you find this circuit?

It comes from an existing CanBus networking board that attaches to a Raspberry Pi. The 5v_GEN on the left comes from a step down power circuit and 5v output is used to power up the Pi.
https://github.com/omzlo/pimaster-hardware
I need to change a few things on the board so I'm reverse engineering each block to what they do / are they required.

 

So now that I understand what it is doing I've got a follow on question.
My normal go-to for reverse polarity protection is based on Rod Elliott's app note #13, figure 3 using a P channel mosfet with a pull down resistor on the gate and zener protection to protect the gate.
https://sound-au.com/appnotes/an013.htm


What is the advantage of the original circuit in this thread (using the ideal diode) compared to the one shown here ?
This one has a lower BOM count and cost.

 

You have Q1 drawn as a N channel.

 

You have Q1 drawn as a N channel.

How so?

 

What is the advantage of the original circuit in this thread (using the ideal diode) compared to the one shown here ?

The original circuit will block reverse current even if there is voltage on the left side.
The simple one won't.

 

My mistake.

 

The original circuit will block reverse current even if there is voltage on the left side.
The simple one won't.

Thanks - I'll stick with the design then Edit: on reflection that is important in case somebody powers up the Pi via an external power supply