cool the protection circuit 1 and the like in the present embodiment a printed circuit board intended for the diode 2 and a printed circuit board intended for the mosfet 3 are also attached to water cooled fins for cooling along with the printed circuit board intended for the protection circuit 1 fig4 is a diagram showing an example of the configuration of the water cooled fin fig4 shows a case where the protection circuit 1 is attached to a cooling fin 10 the cooling fin cooling unit 10 has a rectangular solid shape for example the column is cylindrically hollowed in the center specifically the cooling fin 10 is cylindrically hollowed through from the top side to the bottom side of the rectangular solid the protection circuit 1 printed circuit board intended for the protection circuit has a generally plate like shape and is attached with its main surface in contact with a side surface of the cooling fin 10 cooling water is fed into the cylindrical hole in the cooling fin 10 whereby the protection circuit 1 is cooled in the present embodiment some of the circuits that constitute the power supply apparatus 100 circuits other than the semiconductor switches tx are formed on a printed circuit board intended for the power supply apparatus main printed circuit board p to be described later the cooling fin 10 is arranged near the main printed circuit board circuit specific printed circuit board p the power feed from the capacitor shown in.