A switching power supply device in which the increase in the switching frequency when the load is light is restrained, whereby the switching loss is reduced, intermittent oscillating operation is prevented, and heat generation in the main switching device at the time of short-circuiting is restrained. In a switching power supply device, the charging/discharging time of a capacitor of a switch circuit comprising a time-constant circuit is controlled so as to make the OFF-period of a main switching element a predetermined period that continues after the completion of the discharge of excitation energy from the secondary coil of a transformer of the power supply device.
A push-pull switching power converter includes, between high and low output terminals of a DC power source, a series circuit having a first switching device, a primary winding of a first transformer, a primary winding of a second transformer, and a second switching device in this order, and a series circuit composed of two capacitors, as well as a partial path for current resonance. The partial path connects middle points of the two series circuits. Voltage resonance causes the waveform of a switching voltage, which corresponds to the difference in voltages between high potential terminal of a primary of the first transformer and low potential terminal of a primary of the second transformer, to be a sine wave. Current resonance causes the waveform of a switching current, which corresponds to a combined current of primary winding currents of the first transformer and the second transformer, to be a sine wave.
A switching power supply unit includes a transformer including a primary winding, a secondary winding, and a feedback winding; an input power supply and a first switching element that are connected in series with the primary winding; a control circuit provided between one end of the feedback winding and a control terminal of the first switching element; a rectification circuit connected to the secondary winding; and an output voltage detection circuit for detecting output voltage output from the rectification circuit and for sending a feedback signal to the control circuit. The control circuit includes an on-period control circuit for stabilizing the output voltage by turning off the first switching element in an on-state in accordance with the feedback signal. The control circuit also includes an off-period control circuit for stabilizing the output voltage by delaying turning on of the first switching element in accordance with the feedback signal.
A pulse width modulation voltage regulator comprises a pulse width modulation circuit and a control circuit. The control circuit is operable to reduce a pulse modulation frequency of the pulse width modulation circuit when a load current increases and to increase the pulse modulation frequency of the pulse width modulation circuit when the load current decreases.
