Patent ID: 8035998

Claim:
A switching power supply, comprising an RCC that is comprised of: a transformer having a primary winding, a secondary winding and a feedback winding; an oscillation circuit that includes a switching device connected with a main DC power source through the primary winding connected in series with the switching device, and a timing circuit that supplies a control terminal of the switching device with a control signal for turning the switching device on and off based on an electric current flowing through the switching device and induction voltage across the feedback winding; and an output circuit that rectifies and smoothes voltage induced across the secondary winding to generate output voltage, wherein the switching power supply further comprises: a pull-out circuit that includes a current detector that is connected in series with the switching device and detects an electric current flowing through the switching device; and an active device of which control terminal is connected between the switching device and the current detector, said active device being capable of pulling out a part of the control signal from the control terminal of the switching device; and a control signal generation circuit that changes reference voltage to generate ON/OFF time control voltage and applies the ON/OFF time control voltage to a control terminal of the active device; wherein: the active device operates to pull out the control signal so as to decrease or increase an OFF time of the switching device while fixing the ON time of the switching device in case the ON/OFF time control voltage is decreased or increased in the active region of the active device, respectively; and an element of the timing circuit operates to pull out the control signal so as to increase or decrease an ON time of the switching device while fixing the OFF time of the switching device in case the ON/OFF time control voltage is decreased or increased in the cut-off region of the active device, respectively.