Patent ID: 8611110

Claim:
A switching power supply apparatus comprising: a direct current power supply input unit to which a direct current input voltage is input; a transformer that includes at least a primary winding, a secondary winding, and a high-side driving winding, which are magnetically coupled; a first switching element of a low side and a second switching element of a high side, a ground level of the second switching element being different from that of the first switching element, the first switching element and the second switching element being repeatedly turned on and off in a complementary manner such that the direct current input voltage is applied to the primary winding in an on and off manner; a first switching control circuit that controls the first switching element; a second switching control circuit that controls the second switching element; the switching power supply apparatus outputting a load current and supplying an output voltage in accordance with a voltage output from the secondary winding; wherein the first switching control circuit is a circuit that outputs a predetermined on period continuance signal to a control terminal of the first switching element and that turns the first switching element off after the first switching element has been on for a predetermined on period; the second switching control circuit includes: a first capacitor; a bidirectional constant current circuit that includes a discharge current setting circuit that sets a discharge current to be discharged from the first capacitor in a negative direction at a substantially constant discharge current on the basis of a voltage generated in the high-side driving winding in the on period of the first switching element, and a charging current setting circuit that sets a charging current with which the first capacitor is to be charged in a positive direction at a substantially constant charging current on the basis of a voltage generated in the high-side driving winding in the off period of the first switching element; a transistor that is controlled by the charging voltage of the first capacitor, and when the charging voltage of the first capacitor exceeds a predetermined threshold, controls a voltage of the control terminal of the second switching element and causes the second switching element to be turned off; and a turn on signal transmitting circuit that applies a voltage generated in the high-side driving winding to the control terminal of the second switching element and causes the second switching element to be turned on; the second switching control circuit setting a charging/discharge current ratio, which is a ratio of the charging current to the discharge current; and controlling the on period of the second switching element such that an on period ratio, which is a ratio of the on period of the second switching element with respect to the on period of the first switching element, remains substantially constant with respect to changes in load current.