Patent ID: 6208530
Filing Date: 2001-03-27
Classification: H02M,Y02B

Abstract:
A switching power supply device comprising:a transformer having a primary coil, a secondary coil and a feedback coil;a main switching element having a control terminal, the main switching element being connected in series with the primary coil;a control circuit connected between the control terminal of the main switching element and the feedback coil to turn the main switching element on and off alternatingly, the main switching element thereby having an ON-period and an OFF-period, the main switching element being turned on and off based on a voltage generated in the feedback coil so that an excitation energy is accumulated in the primary coil during the ON-period of the main switching element, the excitation energy accumulated in the primary coil being discharged from the secondary coil during the OFF-period of the main switching element and converted into electric energy to be supplied to a load, the load having an impedance level that can vary; anda period control circuit to make the OFF-period of the main switching element a predetermined period which continues after the completion of the discharge of the excitation energy from the secondary coil, regardless of variation of the impedance level of the load.