US Patent 7205808 Power supply switching circuit and method

A power supply switching circuit includes an input terminal (2) for receiving a signal from a motherboard, an output terminal (4) to output a control signal, a first Bipolar Junction Transistor (BJT) (5), and a second BJT (7). A base of the first BJT is connected to the input terminal via a base bleeder circuit. A collector of the first BJT is connected to a stand-by power supply terminal (10) via a collector resistor. A base of the second BJT is connected to the collector of the first BJT. A collector of the second BJT is connected to a system power supply terminal (12) via a collector resistor. The collector of the second BJT is connected to the output terminal. Emitters of the first and second BJTs are grounded.