Switching power source device

A switching power source device which carries out self-excitation oscillation comprising:

• a transformer including a primary winding and a secondary winding;
• a first series circuit including the primary winding of the transformer and an inductor;
• a first switching circuit, a current detection device, and an input power source connected in series with the first series circuit;
• a second series circuit including a second switching circuit and a capacitor of which one end is connected to a node of the first series circuit;
• the first switching circuit including a parallel connection circuit including a first switching element, a first diode, and a first capacitor, the second switching circuit including a parallel connection circuit including a second switching element, a second diode, and a second capacitor, the transformer having a first drive winding for generating a voltage which causes the first switching element to conduct, and a second drive winding for generating a voltage which causes the second switching element to conduct a rectification-smoothing circuit provided for the secondary winding of the transformer;
• a switching control circuit for turning on/off the first and second switching elements while a period in which both switching elements are off is interposed between the on/off operations, the switching control circuit being controllable so as to turn on a first switching device connected to a control terminal of the first switching element and thereby turn off the first switching element a predetermined time period after the first switching element is turned on caused by a voltage developed in the first drive winding, the predetermined period being determined by a time constant circuit;
• and
• an overcurrent protection circuit including an on-period limiting circuit, and a peak current limiting circuit, the on-period limiting circuit being operable to set a maximum on-period of the first switching element with the time constant circuit, the peak current limiting circuit including the current detection device for detecting the peak current flowing through the first switching element, a second switching device which is turned on when the current detected by the current detection device becomes a predetermined peak current, and a third switching device which is turned on by the turning on of the second switching device, the third switching device being connected to the control terminal of the first switching device whereby the first switching element is turned off by turning on of the third switching device.