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What purpose does the mosfet and BJT network serve in the image below?

The circuit was pulled from a SBC schematic. VBUS_5V is the input power from the micro USB port. Ultimately I am attempting to figure out if there is current limiting between VBUS_5V and VDD_5V.

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The body diode of Q1 cannot be turned off so there is no chance to limit current with this circuit. The function of of this circuit is to protect against reverse voltage while not causing a diode voltage drop.

Short explaination of p-channel reverse voltage protection

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