Passive switching to battery when paralleled USB power is disconnected

[PP3331]

I know there are some chip-based solutions to this, but I'd prefer doing it passively if possible. The power feeding into a USB input can be supplied either by the associated USB cable from a computer, or, when that cable is disconnected, from a battery source that is connected in parallel. I would like to find a way to switch between the two without using a physical switch. That is, the battery will only feed power to the USB input if the USB cable is disconnected, and the battery will not be involved when the USB cable is connected. Thanks.

[Peabody]

You can connect each power source to the load through a Schottky diode.  Then the source with the highest voltage will power the load.  The other diode will be reverse-biased, so no current will flow through it.  This assumes the battery voltage is less than the USB 5V.

If you want to eliminate the diode voltage drop in the battery line, you can use a traditional load sharing circuit, which is one diode, a P-channel mosfet, and a resistor.  The attached circuit shows a battery charger being used, but it should work the same if there's no charger.  USB voltage, if present, turns off the mosfet, which isolates the battery.  Otherwise, the gate pull-down resistor turns on the mosfet.  The orientation of the mosfet is a bit unusual, depending on how you look at it.

[PP3331]

Thanks. I had been considering using Schottky diodes that way, but am a bit concerned about leakage current back into the (lipo) battery.

[David Hess]

The leakage current through schottky diodes is typically insignificant as far as a battery is concerned.

The common alternative where lower voltage drop and/or lower leakage is required is to use "active" rectifiers made from power MOSFETs which are switched on or off to select the source.

[PP3331]

Thanks very much.