A switching power supply is provided to supply an AC input voltage. The supply includes a control circuit configured to detect a voltage of the AC power source in a voltage waveform, and switch elements in a synchronous rectification switching mode in synchronization with polarities of the voltage waveform when the AC input voltage is equal to or greater than a predetermined voltage value. The control unit also operates the switching without synchronization with the polarities when the AC input voltage is smaller than the predetermined voltage value.