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I am wondering why using the PMOS this way in the reverse polarity protection, where the drain is connected to the positive terminal and the source is connected to the load: enter image description here

But not in the opposite way where source is connected to the positive terminal of the power supply and the drain to the load: enter image description here

Isn't the current flowing from source to drain in the PMOS?

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The circuit is a reverse voltage protector so, flip the battery and look at this redraw of your proposed circuit (your 2nd diagram): -

enter image description here

Then, ask yourself if that idea is good or not (given the bulk diode direction inside most MOSFETs).

I am wondering why using the PMOS this way in the reverse polarity protection, where the drain is connected to the positive terminal and the source is connected to the load

Because that circuit works and your proposed circuit doesn't work.

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  • \$\begingroup\$ Ok I understand, so in most of the configurations the PMOS is inserted as to have the current flowing from drain to source, but in this special case( reverse polarity protection) we insert it in the opposite direction to play that role. Then the PMOS let the current follow in both directions but in normal switching applications it is configured so that the current flow from the drain to the source. \$\endgroup\$ Commented Sep 23, 2021 at 8:06
  • \$\begingroup\$ In normal usage a PMOS has current flowing from source to drain; an NMOS normally has current flow from drain to source. \$\endgroup\$
    – Andy aka
    Commented Sep 24, 2021 at 7:05

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