Understanding the use of Dual PNP in this circuit

fastbike

Joined Dec 29, 2020
120
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.

MrChips

Joined Oct 2, 2009
29,832
Looks like a low drop-out half wave rectifier circuit.

Alec_t

Joined Sep 17, 2013
14,005
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).

AlbertHall

Joined Jun 4, 2014
12,293
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.

crutschow

Joined Mar 14, 2008
33,346
So where did you find this circuit?

Alec_t

Joined Sep 17, 2013
14,005

Alec_t

Joined Sep 17, 2013
14,005
So tiz.

fastbike

Joined Dec 29, 2020
120
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.

fastbike

Joined Dec 29, 2020
120
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.

sghioto

Joined Dec 31, 2017
5,090
You have Q1 drawn as a N channel.

crutschow

Joined Mar 14, 2008
33,346

crutschow

Joined Mar 14, 2008
33,346
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.

sghioto

Joined Dec 31, 2017
5,090
My mistake.