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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.

Load Sharing Circuit

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?

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  • \$\begingroup\$ "the +5Vbat will actually be low or floating" Will it be low, or will it be floating? \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 27 '16 at 22:31
  • \$\begingroup\$ @IgnacioVazquez-Abrams Low, sorry to be confusing \$\endgroup\$ – Colin Jul 27 '16 at 23:28
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Your circuit does not work as you are expecting. In case you unplug 5Vusb: you cannot rely that source of mosfet is connected to 5V. MOSFET "has" diode between drain and source, and that is what plays role here. This is scheme of standard P-MOSFET: enter image description here

So, when I redraw your scheme, this is what is going on in your circuit in reality: enter image description here

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  • \$\begingroup\$ Thanks for the lesson on the body diode, I didn't know about that behaviour between the drain and source. \$\endgroup\$ – Colin Jul 28 '16 at 18:23
  • \$\begingroup\$ I couldn't recall the name of that diode. So I thank you to remind me, that it is body diode ;) \$\endgroup\$ – Chupacabras Jul 28 '16 at 18:54

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