VEHICLE POWER SUPPLY SYSTEM, ENGINE CONTROL DEVICE, AND POWER SUPPLY METHOD

A engine control unit comprises: a three-phase bridge circuit that has a first end connected to the first end of the first load and a second end connected to a ground, the three-phase bridge circuit performing commutation control on an alternating-current power supplied from the motor generator or driving the motor generator; a DC-DC converter connected between the first end of the three-phase bridge circuit and a second end of the control switch; a diode that has an anode connected to the battery connecting terminal and a cathode connected to the second end of the control switch; a voltage detecting circuit that detects a voltage of the battery connecting terminal; a first switch element connected between a second end of the coil and the ground; a second switch element connected between a second end of the first load and the ground; and a control circuit that is supplied with a direct-current power supplied to the first end of the control switch and controls the three-phase bridge circuit and the first and second switch elements.