Patent ID: 8451635

Claim:
A switching power supply device comprising: an input unit which includes a first input terminal and a second input terminal, and configured to receive an input voltage; a transformer which includes a primary winding and a secondary winding, said primary winding having one end connected to said first input terminal; an output voltage generating circuit which is connected to said secondary winding, and generates an output voltage by rectifying and smoothing an alternate-current voltage appearing on said secondary winding; an output unit configured to provide, to a load, the output voltage generated by said output voltage generating circuit; a switching element which includes a first terminal, a second terminal, and a control terminal, said first terminal being connected to an other end of said primary winding, and said second terminal being connected to said second input terminal; a control circuit which is connected to said control terminal, and controls on-off switching of said switching element; and a voltage-corresponding-to-secondary-current detecting unit configured to provide a first signal having a voltage level corresponding to a secondary current flowing into said secondary winding, wherein said control circuit includes: a secondary-side conduction time period measuring circuit which measures a secondary-side conduction time period based on the voltage level of the first signal provided by said voltage-corresponding-to-secondary-current detecting unit, the secondary-side conduction time period representing a time period in which the secondary current flows; a reference value determining circuit which determines a reference value of the secondary-side conduction time period; a frequency control circuit which provides a second signal used for changing an on-frequency of said switching element in order to narrow a difference between (i) the secondary-side conduction time period measured by said secondary-side conduction time period measuring circuit and (ii) the reference value determined by said reference value determining circuit; a primary current detecting circuit which detects a primary current flowing into said primary winding; a current threshold value determining circuit which determines a first threshold value of the primary current; a current comparing circuit which compares the primary current with the first threshold value to provide a third signal used for turning said switching element off when the primary current becomes equal to the first threshold value, the primary current being detected by said primary current detecting circuit, and the first threshold value being determined by said current threshold value determining circuit; and an oscillation control circuit which controls on-off switching of said switching element according to the second signal and the third signal, and wherein the switching power supply device has a constant voltage characteristic.