Patent ID: 7778049
Filing Date: 2010-08-17
Classification: H02M

Abstract:
1. A switching power supply, comprising: a transformer having a primary winding and a secondary winding; a switching element connected to the primary winding; a control circuit for performing switching control on a first DC voltage inputted to the switching element through the primary winding, by controlling a switching operation of the switching element; and an output voltage generating section for converting a voltage generated on the secondary winding by the switching operation of the switching element into a second DC voltage, and supplying the second DC voltage to a load, wherein the control circuit comprises: a drain current limiter circuit for generating a signal for turning off the switching element when a current passing through the switching element reaches a set value; a secondary current on-period detection circuit for detecting, from a voltage generated on each winding of the transformer by the switching operation of the switching element, off timing of secondary current starting passing through the secondary winding when the switching element is turned off, and generating a signal indicating the detected off timing of the secondary current; a secondary-current detection delay time correction circuit for generating a signal indicating a time when a predetermined period has elapsed since the switching element is turned off; and a secondary current on-duty control circuit for oscillating a clock signal for turning on the switching element so as to keep an on duty ratio of a first period allowing passage of the secondary current at a constant value relative to a third period made up of the first period and a second period not allowing passage of the secondary current, based on the signal generated by the secondary current on-period detection circuit and the signal generated by the secondary-current detection delay time correction circuit.