With an n- and p-channel MOSFET, you can easily implement a single-pole double-throw (SPDT) switch to isolate part of a circuit and power it from a secondary supply for standby operation while the rest of the circuit is off (Figure 1). By using a complementary pair, you can use a single control input for the MOSFETs. An Si4501DY MOSFET in this topology exhibits less than a 0.1V drop at 5A for the main element and contains both MOSFETs in an SO-8 package.
When the 2N7002 or similar control MOSFET ties the gates together and pulls them to ground, the p-channel MOSFET is on. Pulling the gates above the supply rail by turning the 2N7002 off results in turning the n-channel MOSFET on. Pulling the gate of the p-channel MOSFET above the source potential has no effect; the MOSFET remains off with low leakage.
The resultant switch is a break-before-make configuration, which is necessary to ensure that the secondary, or always-on, supply never has to power the whole circuit. However, due to the fast switching of the MOSFETs, extra capacitance beyond normal design rules is probably unnecessary to maintain the operating voltage.
The arrangement of the n-channel device in the circuit ensures that the internal diode does not conduct while the subcircuit is isolated. In this direction, the n-channel MOSFET also provides a fail-safe path for the circuit's power through the diode. The forward voltage of the p-channel MOSFET's diode blocks any current from "back-feeding" the secondary supply, assuming that the two supplies are close in voltage.
An example of an application for this circuit is the Advanced Configuration and Power Interface in desktop computers providing instant-on and low power consumption in standby. The main power supply is a high-power switching power supply, and the secondary supply is a 60-Hz transformer with linear regulation. (DI #2451)
