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As the title says, I have a power mux IC and two power inputs. I need them to be protected from reverse voltage input. The power mux I have already comes with a reverse voltage input protection but I need the second input voltage for another IC and its not always the second input is selected by the PMIC. So I must protect it beforehand.

Typically a PMOS does the trick but I do not know if it will work when there are two inputs that share the same ground. If one of the inputs is powered and if one of the other is wired correctly, what would happen?

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Yes, a PMOS can provide high-side reverse polarity protection. However, the way you have drawn your circuit is a bit backwards. You need to swap source and drain. See this question for a schematic.

Also, note that the gate-to-source voltage on the PMOS will be equal to the input voltage. So you need to select a PMOS where Vgs max is greater than Vinput max and Vfused max. Otherwise the PMOS may fail.

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  • \$\begingroup\$ Oh so i have to swap Drain and Source if i want it to use as high side reverse polarity protection. But just making sure, Even if the other rail is powered the protection on the other rail is still good right? \$\endgroup\$
    – DrakeJest
    Commented May 31, 2020 at 7:37
  • \$\begingroup\$ Yeah. I mean, Vgs = -Vin. So if you apply negative voltage to Vinput, then gate will be higher than source and FET will be off. You may need to analyze the specific scenario to see if you are shorting Vinput to ground OUTSIDE of this PCB. \$\endgroup\$
    – user57037
    Commented May 31, 2020 at 8:49
  • \$\begingroup\$ With something like this it is always advisable to test it before you rely on it. There could be things you are doing that I don't know. But the circuit does work and I have used it before as have other people. \$\endgroup\$
    – user57037
    Commented May 31, 2020 at 8:50

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