Something like the attached should work.Resistor could be 10K. Shottky diodes would work best. MOSFET is logic-level P-channel. Make sure the MOSFET and diodes can take the current you need. VSupply should be at VUSB or higher for best results. You could probably replace D1 with a wire.In fact, if you want to keep it really simple and if VSupply > VUSB you could probably loose the MOSFET and just use the 2 diodes.
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