A switching power supply device, comprising:
- a rectifier circuit for obtaining a rectified voltage by full-wave rectification of a sine wave voltage from an AC source;
- a first smoothing capacitor for obtaining a ripple current output voltage by smoothing the rectified voltage, the first smoothing capacitor having a first terminal connected with a first output terminal of the rectifier circuit and a second terminal connected with a second output terminal of the rectifier circuit;
- a first series circuit constituted by connecting in series (i) an inductor including a plurality of windings, (ii) a switching element, and (iii) a current sensing resistor, the inductor having a first terminal connected with the first terminal of the first smoothing capacitor and a second terminal connected with a drain of the switching element, the switching element having a source connected with a first terminal of the current sensing resistor, the current sensing resistor having a second terminal connected with the second terminal of the first smoothing capacitor, the inductor receiving the ripple current output voltage via its first terminal;
- a second series circuit constituted by connecting in series a rectifier diode and a second smoothing capacitor, the rectifier diode having an anode connected with the drain of the switching element and a cathode connected with a first terminal of the second smoothing capacitor, the second smoothing capacitor having a second terminal connected with the source of the switching element;
- a fundamental wave component extraction circuit for extracting a fundamental wave component of a voltage induced across any one of the plurality of windings;
- an oscillation circuit for generating a clock signal having an oscillation frequency that changes according to a change in the fundamental wave component;
- and
- a control circuit for (a) generating a control signal for controlling the switching element to be in an ON state or OFF state, the control signal having a duty that changes according to a change in the oscillation frequency of the clock signal or a change in a voltage of the second smoothing capacitor and (b) supplying the control signal to a gate of the switching element.