A power supply system has a three-phase alternating current generator, a storage battery, and a three-phase rectifying device connected between the alternating current generator and the storage battery. The rectifying device includes three SiC-MOSFETS which are turned on when the corresponding alternating voltage is positive and turned off when the corresponding alternating voltage is negative. A duty control device switches each of the three SiC-MOSFETS on and off according to a selected duty ratio so that the alternating current generator can generate an optimum voltage in proportion to a rotational speed of a rotor of the alternating current generator.