POWER SUPPLY DEVICE

A power supply device includes a main battery, a backup battery, a first charge circuit, a second charge circuit, a first transistor, a second transistor, a switch element, and a control circuit. The first transistor has a control terminal for receiving a first control voltage, a first terminal coupled to the main battery, and a second terminal coupled to a first supply node. The second transistor has a control terminal for receiving a second control voltage, a first terminal coupled to the backup battery, and a second terminal coupled to a midway node. The switch element selectively couples the midway node to a second supply node according to a third control voltage. The control circuit monitors a first supply voltage at the first supply node, and generates the third control voltage according to the first supply voltage.