Abstract: The present invention is concerned with a switching power supply that utilizes an oscillator frequency tuner to broaden the maximum duty cycle of the switching control circuit and thus enhance its load driving capability. The oscillator frequency tuner includes a plurality of capacitors connected between a reference voltage output of the switching control circuit and a ground terminal, at least one bypass switch coupled across at least one of the capacitors for controlling the charging and discharging operation of the capacitor coupled therewith, and a bypass switch controller for controlling a switching state of the bypass switch. When the switching power supply is operating under a peak load condition, the bypass switch controller receives a feedback control signal and instructs the bypass switch to bypass current, and thus reducing the equivalent timing capacitance of the switching control circuit for increasing the maximum duty cycle of the switching control circuit.