Abstract: A power supply device capable of switching over semiconductor switches of a synchronous rectification means so as to efficiently charge up a battery in response to its charging status is provided. The device includes a switch control means for controlling the semiconductor switches in response to the battery charging status; a negative voltage detection means for detecting a negative voltage at each phase of three-phase AC voltages; and a voltage detection means for detecting the battery voltage being higher than a predetermined voltage, wherein when the negative voltage or the battery voltage higher than the predetermined voltage is detected, the switch control means controls the semiconductor switches.