To provide a power supply enabling easy identification of a damaged component by identifying beforehand a semiconductor element that is thermally destroyed first, on the occurrence of an overload condition.

In a power supply 1 including a switching semiconductor element Q1 disposed on the primary side of a switching transformer ST and a rectifier semiconductor element D2 disposed on the secondary side of the switching transformer, when the power supply 1 falls into an overload condition, the heat radiation design of each semiconductor element Q1, D2 is performed in such a manner that the rectifier semiconductor element D2 disposed on the secondary side of the switching transformer is thermally destroyed earlier than the switching semiconductor element Q1 disposed on the primary side of the switching transformer. Thus, when the power supply 1 falls into an overload condition, the rectifier semiconductor element D2 is thermally destroyed first.