Patent ID: 7729137
Filing Date: 2010-06-01
Classification: H02M

Abstract:
1. A switching power supply comprising: a transformer having a primary winding and a secondary winding; a switching element; a control circuit controlling a switching operation of the switching element to perform switching control on a first DC voltage input to the switching element via the primary winding; and an output voltage generating section converting a voltage generated in 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 limiting circuit generating a signal turning off the switching element when a current flowing through the switching element reaches a set value; a secondary current on period detecting circuit detecting a first period during which a secondary current flows, the secondary current starting to flow through the secondary winding after the switching element is turned off; a secondary current on duty control circuit oscillating a clock signal turning on the switching element so as to maintain, at a constant value, an on duty ratio of the first period to a third period comprising the first period and a second period during which the secondary current does not flow; and a secondary current on duty modulating circuit applying a modulation component to the on duty ratio to periodically modulate the on duty ratio.