Included are: a circuit-specific printed circuit board to be a circuit board that generates a power supply voltage; a first diode that is connected in antiparallel with the transistor and is configured so as to be capable of passing a current through a path to circumvent the transistor; a second diode that is connected in series to the transistor and prevents a current flow through a parasitic diode formed on the transistor; a protection circuit that is connected in parallel with the second diode to protect the second diode from a high voltage breakdown, and is formed on a substrate different from the circuit-specific printed circuit board; and a cooling unit that is joined to the protection circuit outside the circuit-specific printed circuit board and cools the protection circuit.