I'm trying to power a board through both usb & a 9V battery (with the usb taking precedence) with a P channel MOSFET, a schottky diode & a pull down resistor. I'm confident this will work with just a battery, since the gate of the PMOS is pulled down and so the MOSFET will be on.
What I'm concerned about is when a USB cable is connected. Primarily because the battery is 9V and runs through a DC/DC converter which is disabled once the USB is connected. So the +5Vbat will actually be low when the MOSFET is off (gate & source will be at ~ +5V). I know having a floating source or gate can be bad/damaging; should I be concerned about any current leakage when the drain is ambiguous?