Self-oscillation type switching power supply unit

A self-oscillation type switching power supply unit comprising:

• a transformer having a primary winding, a secondary winding, and a feedback winding;
• a switching transistor which self-oscillates by receiving a feedback signal from the feedback winding and which switches flow of current in the primary winding between an OFF period and an ON period;
• a rectifying and smoothing circuit connected to the secondary winding;
• and
• an oscillation frequency control circuit for extending an OFF period of the switching transistor by preventing the switching transistor from turning on for a predetermined period after the switching transistor has been turned off such that a control signal to be input to the switching transistor is controlled;
• wherein, at a rated load exceeding a predetermined current, the oscillation frequency control circuit is set in a first operation mode in which a normal ringing choke converter operation is performed to lower the oscillation frequency as the loading becomes heavier;
• wherein, at light loading in which the load current is the predetermined current or less, the oscillation frequency control circuit is set in a second operation mode in which the OFF period is extended so that the oscillation frequency is constant or slowly decreases as the load current is reduced;
• and
• wherein the oscillation frequency control circuit includes a switch for setting in a third operation mode in which, when the switch is in a fixed state, the oscillation frequency is lowered further than in the second operation mode.