Bi-directional power supply

In the bidirectional power supply, DC-DC converter is connected as a voltage of about on-time of the first switch is longer second positive terminal and a second negative terminal becomes higher. Control circuitry, when stopping the supply of electric power from the first positive terminal and a first negative terminal to the second positive terminal and a second negative terminal, after turning off the third switch, the ON time of the first switch is the maximum operating a switching signal generating circuit so as to. When supplying power from the second positive terminal and the second negative terminal to the first positive terminal and a first negative terminal, such that the third switch is ON time of the first switch becomes maximum in a state of OFF at start, operating a switching signal generating circuit.