Switching power supply unit

A switching power supply unit comprising:

• a first switching circuit including a first switching element, a first diode, and a first capacitor, which are connected in parallel with each other;
• a second switching circuit including a second switching element, a second diode, and a second capacitor, which are connected in parallel with each other;
• a capacitor that is connected in series with the second switching element to provide a series circuit, the series circuit being connected to one end of the first switching circuit;
• a transformer including a primary winding, a secondary winding that is provided with a rectifying/smoothing circuit, a first drive winding, and a second drive winding, wherein the first drive winding generates a voltage for bringing the first switching element into conduction and the second drive winding generates a voltage for driving the second switching element;
• a switching control circuit that alternately turns on and off the first and second switching elements with a period in which the first and second switching elements are both turned off therebetween;
• and
• an input power supply;
• wherein
• the first switching circuit, the transformer, and the input power supply are connected in series with each other;
• the switching power supply unit performs self-excitation oscillation by storing energy in the primary winding of the transformer in a period in which the first switching element is turned on and by releasing energy from the secondary winding of the transformer in a period in which the first switching element is turned off;
• the switching control circuit includes a first switching control circuit which includes a first delay circuit, a first control element, and a first impedance circuit and which is connected between the first switching element and the first drive winding, and a second switching control circuit which includes a second impedance circuit and which is connected between the second switching element and the second drive winding;
• wherein, during detection of a light load, the impedance of the second impedance circuit varies to cause the time constant thereof to change such that the ON-time of the second switching element becomes shorter than the ON-time thereof at rated load and energy release from the secondary winding is completed after the end of the ON-time of the second switching element;
• the first delay circuit determines a time from when a voltage is generated across the first drive winding until when the voltage generated across the first drive winding causes the first switching element to be turned on;
• the first control element is driven for forcibly turning off the first switching element or for preventing the first switching element from turning on;
• the impedance of the first impedance circuit varies to cause the time constant thereof to change in such a manner that, at rated or heavy load other than during detection of a light load, after a voltage generated across the first drive winding causes the first switching element to be turned on, the first control element is driven to cause the first switching element to be turned off, and in such a manner that, at light load, the impedance of the second impedance circuit in the second switching control varies to cause the time constant thereof to change, and at the same time, the first control element is driven to prevent the first switching element from being turned on before a voltage generated across the first drive winding causes the first switching element to be turned on;
• and
• during detection of a light load, the switching power supply unit performs an intermittent oscillation operation, in which an oscillation period and a non-oscillation period are alternately repeated due to a change in time constant resulting from impedance changes in the first and second impedance circuits.