POWER SUPPLY

摘要

Input primary voltage from a small-capacity DC power supply is controlled so that an excesive current does not flow into the switching element of a switching converter without using a current detection element. The switching converter comprises an FET (30) for turning the input primary voltage from a DC power supply (1) on/off, and a choke coil (31) charged/discharged depending on the on/off state of the FET (30). A switching section (13) drives the FET so that an output voltage detected at an output voltage detecting section (12) converges to a target voltage. There is provided a means for calculating a current I flowing to the FET (30) from the input primary voltage, the duty ratio of the FET (30) and the inductance of the choke coil (31). The switching section (13) drives the FET (30) with a duty ratio set so that an estimated current level I does not exceed an allowable current level, or stops switching when the estimated current level I exceeds the allowable current level.