Patent ID: 6947296

Claim:
A switching power supply circuit comprising: a direct-current-voltage generating circuit that generates an input direct-current voltage, the direct-current-voltage generating circuit including a rectifier circuit that rectifies an alternating-current power supply voltage and a smoothing capacitor that smoothes an output from the rectifier circuit; a converter transformer having primary and secondary windings, the input direct-current voltage being applied to an end of the primary winding; a switching element having a main current path and a control electrode, the main current path being connected between another end of the primary winding and a reference potential point, the switching element being turned on/off in response to a switching signal supplied to the control electrode; an output circuit which rectifies a voltage generated across the secondary winding of the converter transformer, thereby applying an output direct-current voltage to a load; and an integrated circuit that includes a switching control circuit and an oscillation control circuit, wherein (a) the switching control circuit, includes an oscillation circuit, and is configured to supply the switching element with the switching signal based on an oscillation operation of the oscillation circuit, and (b) the oscillation control circuit includes (i) a first comparator circuit that compares a first stipulated voltage with the input direct-current voltage generated at the other end of the primary winding and is applied to the switching element, the oscillation control circuit prohibiting an operation of the oscillation circuit of the switching control circuit if the input direct-current voltage is higher than the first stipulated voltage and (ii) a second comparator circuit that compares the input direct-current voltage generated at the other end of the primary winding and applied to the switching element with a second stipulated voltage that is lower than the first stipulated voltage, the oscillation control circuit prohibiting the operation of the oscillation circuit if the input direct-current voltage is lower than the second stipulated voltage, wherein the oscillation control circuit operates the oscillation circuit if input direct-current voltage falls within a range from the first stipulated voltage to the second stipulated voltage.