A power supply system includes at least a first power supply and a second power supply and a voltage output circuit. The first power supply provides a first set of signals while the second power supply provides a second set of signals. The voltage output circuit includes a first input terminal, a second input terminal, a first output terminal, a switch circuit and a control circuit. The first input terminal receives a first voltage signal of the first set of signals while the second input terminal receives a second voltage signal of the second set of signals, the switching circuit couples between the first and the second input terminal and the first output terminal, and the control circuit activates a first or second transistor according to a voltage difference between the first and second input terminal to generate a first output voltage signal on the first output terminal.