SWITCHING POWER SUPPLY

PROBLEM TO BE SOLVED: To reduce power consumption by an input-side power circuit when a load is in standby state. SOLUTION: Alternating-current voltage outputted from an alternating-current power supply is rectified through an input-side rectifying circuit 4 and smoothed through an input-side smoothing circuit 6, thereby turning into input-side voltage. The input-side voltage is given to a switching element 9 that is on/off-controlled through a control circuit 10, through the primary winding 1a of a transformer 1. The voltage produced by on/off-control of the switching element 9 is given to an output rectifying means 55 through the transformer 1, and, after rectification, outputted to a load 45. The turn-on/off and stop of the switching element 9 are controlled based on the output voltage outputted to the load 45. If it is detected that the load 45 is in standby state, the smoothing capacity of an input-side smoothing circuit 7 is reduced, and the switching element 9 is on/off- controlled only when the input-side voltage falls below a specified value.