Current detection circuit and switching power supply

A switching power supply comprising a switching device, an inductor connected to said switching device, a rectifier circuit for rectifying the output of said inductor, smoothing means for smoothing the current flowing in said rectifier circuit, a current detection circuit for detecting the current of said switching device, and a control circuit, to which the current detection signal from said current detection circuit and the output voltage output from said smoothing means are input, for turning ON/OFF said switching device to control said output voltage, wherein

• said current detection circuit comprises:
• an auxiliary switching device being provided in parallel with said switching device and being ON at least when said switching device is ON,
• an offset voltage source for generating an offset voltage, and a compensation circuit for adjusting the output current of said auxiliary switching device so that the potential obtained by adding said offset voltage to the output potential of said switching device becomes equal to the output potential of said auxiliary switching device, wherein
• said offset voltage source comprises an offset resistor device, and a current source circuit for passing a current in said offset resistor device, and
• said offset resistor device is an FET having the same channel type as that of said switching device.