A power supply device comprises a printed circuit board for generating a power supply voltage, a first diode which is so connected antiparallel to a transistor so as to flow current through a path bypassing the transistor, a second diode connected to the transistor in series to prevent current from flowing through a parasitic diode formed in the transistor, a protective circuit connected parallel to the second diode so as to protect the second diode from high-voltage breakdown and fabricated on a board different from the printed circuit board and a cooling unit joined to the protective circuit outside the printed circuit board to cool the protective circuit.