Switching power supply device

A switching power supply device, comprising:

• an inductor connected to a rectifier circuit that rectifies an input alternating current voltage;
• a switching element that forms a current path that passes from the rectifier circuit to the switching element through said inductor when the switching element is ON;
• an output capacitor connected in parallel to an output node;
• a diode that forms a current path between the inductor and the output capacitor when the switching element is OFF, thereby establishing an output direct current voltage to the output capacitor;
• and
• a control circuit that controls current that flows to the inductor by turning the switching element ON and OFF,
• wherein said control circuit comprises:
• an ON width control part that controls an ON width of the switching element in accordance with an error signal in which a difference between a reference voltage and said output direct current voltage has been amplified;
• a load state detection part that detects a magnitude of a load to which the output direct current voltage is provided;
• a zero-current detection and frequency reduction part that detects a timing of detecting zero-current that is defined as a timing at which current flowing to the inductor becomes lower than a prescribed zero-current detection value while the switching element is OFF and then causes the switching element to turn ON at said detected timing, wherein when the magnitude of the load detected by the load state detection part is determined to be relatively small, the zero-current detection and frequency reduction part reduces a switching frequency of the switching element by detecting a second or subsequent event of detecting the zero-current detection as said timing that triggers the turning-ON of the switching element;
• a restart timer that, when the zero-current detection and frequency reduction part fails to detect said timing that triggers the turning-ON of the switching element before the restart timer counts up a prescribed time, causes the switching element to turn ON at a timing of said count-up, the restart timer including a timer adjustment part that lengthens said prescribed time when the magnitude of the load detected by the load state detection part is determined to be relatively small.