Patent ID: 8059431

Claim:
A switch power supply for outputting a secondary-side voltage of a voltage conversion section after rectifying and smoothing the voltage, and controlling switching on a primary side of the voltage conversion section according to a detected load current to perform overcurrent protection; the switching power supply comprising: a detection resistor for detecting a load current on a secondary side of the voltage conversion section; a first current mirror circuit made up of a first transistor of an input side and a second transistor of an output side; and a second current mirror circuit made up of a third transistor of the input side and a fourth transistor of the output side, wherein a detected voltage of the detection resistor is applied between one end of the first transistor and one end of the second transistor, the fourth transistor fed with a current set by the third transistor is interposed between the second transistor and a reference potential, and switching on the primary side of the voltage conversion section is controlled according to a potential of a junction point of the second transistor and the fourth transistor to perform overcurrent protection.