Switching power-supply device

A switching power-supply device comprising:

• a transformer that includes a primary winding, a secondary winding, and an auxiliary winding;
• a switching device that is connected in series with the primary winding;
• an oscillation circuit that causes the switching device to perform a switching operation;
• an output voltage generating circuit that generates an output voltage based on a voltage generated in the secondary winding;
• a control-circuit power-supply voltage generating circuit that generates a control-circuit power-supply voltage based on a voltage generated in the auxiliary winding;
• and
• a feedback control circuit that uses a pulse voltage generated in the auxiliary winding according to the switching operation of the switching device, as a feedback signal, to control the ON-width of the switching device,
• wherein the switching power-supply device is configured to generate a COMP-voltage by performing phase compensation on the feedback signal, the switching power-supply device further comprising:
• a COMP-voltage comparator circuit that compares the COMP-voltage obtained by performing phase compensation on the feedback signal, with a first threshold voltage which is a threshold value;
• and
• an intermittent-oscillation control circuit that, if it is detected by the COMP-voltage comparator circuit that the COMP-voltage is lower than the first threshold voltage, stops the switching operation of the switching device and performs a transition to a first intermittent oscillation operation in which the switching device performs the switching operation every predetermined first period,
• wherein if a predetermined delay period elapses in a state where the COMP-voltage is lower than the first threshold voltage, the intermittent-oscillation control circuit performs a transition to a second intermittent oscillation operation in which the switching device performs the switching operation every second period which is an integral multiple of the first period,
• wherein the COMP-voltage is proportional to a current peak of a switching current of the switching device;
• wherein, when the transition to the second intermittent oscillation operation is performed, the intermittent-oscillation control circuit switches the threshold value, which is compared with the COMP-voltage of the COMP-voltage comparator circuit, to a second threshold voltage higher than the first threshold voltage, thereby increasing a peak value of the switching current, and
• wherein if it is detected by the COMP-voltage comparator circuit that the COMP-voltage is equal to or higher than the second threshold voltage, the intermittent-oscillation control circuit performs a transition to the first intermittent oscillation operation and also switches the threshold value, which is compared with the COMP-voltage of the COMP-voltage comparator circuit, to the first threshold voltage, thereby reducing the peak value of the switching current.