Patent ID: 8749996

Claim:
A switching power supply apparatus comprising: a direct-current power-supply input unit arranged to receive a direct-current input voltage; a transformer including at least a primary winding and a secondary winding magnetically coupled to each other; a first series circuit connected to both ends of the direct-current power-supply input unit and in which the primary winding, an inductor, and a first switching element are connected in series; a second series circuit connected to both ends of the first switching element or both ends of a series circuit including the primary winding and the inductor and in which a second switching element and a capacitor are connected in series; a switching control circuit configured to put the second switching element into an on-state during a predetermined time after having put the first switching element into an on-state during a predetermined time; and a first rectification smoothing circuit configured to rectify and smooth an alternating-current voltage output from the secondary winding and output an output voltage to a load; wherein the switching power supply apparatus includes a light load detection mechanism configured to directly or indirectly detect a load current flowing through the load and detect whether or not the load is in a light load state; the switching control circuit includes: a first switching control circuit configured to control an on-time of the first switching element; and a second switching control circuit configured to control an on-time of the second switching element so that an on-time ratio serving as a ratio of the on-time of the second switching element to the on-time of the first switching element becomes almost constant with respect to a change in the load current; wherein in a normal load state, an operation is performed in a current-continuous mode where the first switching element and the second switching element are complementarily and repeatedly put into on-states and off-states in a relationship in which a switching frequency decreases as the load current increases; and when the light load detection mechanism has detected the light load state, an operation is performed in a current-discontinuous mode where a current halt time period in which no current flows through the transformer exists between turnoff of the second switching element and turnon of the first switching element, and a blank time is inserted between a time when the second switching element is turned off and a time when the first switching element is turned on next such that the switching frequency is decreased in the light load state.