Input power selection

Discussion in 'General Electronics Chat' started by Northy, Oct 4, 2012.

  1. Northy

    Thread Starter New Member

    Sep 6, 2012

    I'm modifying an existing design that has 3 different power inputs:

    Ext power - ~12V
    Batteries - 6 x AA
    USB - 5V

    The design currently just schottky diode 'OR's' the supplies together before the main switching regulator.
    The problem is that when there is no external power, but there are batteries and USB, the current is drawn from the batteries as they are a higher voltage. I'd really like to be able to isolate the batteries completely when on USB power.

    I've been looking at high side and low side switches etc, but have yet to come up with a good solution.
    I don't want to redesign the power supply - too much risk, and I'm limited for board space. I also don't want to introduce any significant losses that will limit run time on the batteries.

    Any good ideas?

  2. crutschow


    Mar 14, 2008
    You could use a P-MOSFET in series with the battery (souce to battery, drain to diode, gate to USB power. Add a resistor (say 1kΩ) from the gate to ground. Use a Logic-level type MOSFET with a current rating and low enough on-resistance to work in your circuit.

    With that connection the P-MOSFET will turn off when the USB is at 5V and turn on when the USB power is 0V.
    Last edited: Oct 4, 2012