We have a control unit used in marine applications running of 12V nominal voltage with a current consumption of about 1A.

Today we are using a P-Channel MOSFET with some added circuitry (Zener, capacitors and a resistor) as polarity protection.

We want to implement a good power ORing solution and without adding to much cost we were thinking to use the exact same circuit for the second power input.

There are much litterature on using N-channel MOSFETS instead for lower RDs on etc. But since we now have a good tested solution using P-channel we want to try and reuse this as much as possible.

Are there any reason why this circuit would not function well? Or any other aspects of it we haven’t thought of?enter image description here

See picture for schematic.

Thank you.

EDIT: After reviewing the feedback on this post we have taken the solution in a slightly different direction to a Smart Diode Controller controlling an N-Channel MOSFET. This effectively shuts down the MOSFET in a reverse voltage scenario.

We now have two of these circuits in a power ORing configuration and the TVS is now bidirectional. Power ORing scheme

  • \$\begingroup\$ Is the power supplied to P12_PLUG and ground? If so, reversing polarity will quite likely blow D7 (same for D9). \$\endgroup\$
    – Huisman
    Commented Mar 19, 2019 at 18:40
  • \$\begingroup\$ Huisman is correct, the circuit as drawn does not really need the MOSFET for polarity protection...because in front of it, you have a diode crowbar circuit. It will dump lots of current through the diode and (if the fuses are rated low enough) blow the fuses to protect the rest of the circuit. The MOSFET is doing almost nothing except adding cost to your BOM. \$\endgroup\$ Commented Mar 19, 2019 at 18:44
  • \$\begingroup\$ Are your comments on blowing D7 and D9 regardless of if both circuits are used and also in a single configuration where only one of the circuits are used? If so, how should the TVS be connected to avoid this? \$\endgroup\$
    – erikfred
    Commented Mar 20, 2019 at 6:51

2 Answers 2


PMOS-based power OR-ing solutions exist, but the biggest problem is shoot-through when the power supplies are at different voltages and both PMOS transistors are active. MOSFET transistors conduct in both directions when active. A correct solution requires that both supply voltages are sensed, and the PMOS transistors actively switched on and off to prevent shoot-through.

Here is a simplified simulation of your proposed circuit, showing that most current is flowing from the 15V supply into the 12V supply. This could cause damage or even an explosion if one of the supplies is a battery.

Simulation of simple PFET OR attempt

  • \$\begingroup\$ Thanks for giving a detailed answer with simulation to back it up. It makes very much sense. \$\endgroup\$
    – erikfred
    Commented Mar 20, 2019 at 6:55

You can use mosfets to replace diodes for polarity protection to benefit from a lower voltage drop.

However, you cannot use mosfets to replace diodes in the ORing configuration as posted.

When the polarity is correct for both circuits, the both mosfets will keep conducting both directions (in contrary to a diode). So when P12_PLUG becomes e.g 11.9V and P12_PLUG_1 is still 12.0V, current will flow back from P12V_IN into P12_PLUG.

  • \$\begingroup\$ Thanks for the detailed answer. So this is the reason why power ORing solutions with MOSFETs have dedicated controllers, to actively turn them off when a reverse voltage situation occurs. \$\endgroup\$
    – erikfred
    Commented Mar 20, 2019 at 6:52

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