A control circuit 8 of a switching power source comprises a voltage detector 31 for detecting a voltage V.sub.DC of a DC power supply 1 to produce a detection voltage V.sub.DT; a comparator 41 for producing an output signal V.sub.CP when detection voltage V.sub.DT from voltage detector 31 exceeds a reference voltage V.sub.R2; a bottom voltage detector 51 for detecting a bottom point of voltage V.sub.DS across a MOS-FET 3 after energy has been discharged from transformer 2; and a switching controller 61 for selectively turning MOS-FET 3 on depending on existence or absence of the output signal V.sub.CP from comparator 41. When the input voltage V.sub.DC from DC power supply 1 so rises that detection voltage V.sub.DT of voltage detector 31 is above the reference voltage V.sub.R2, switching controller 61 serves to late turn MOS-FET 3 on at the time bottom voltage detector 51 detects the second or later bottom point of the voltage V.sub.DS across MOS-FET 3, extending the off period of MOS-FET 3 to reduce switching frequency of MOS-FET 3.
CROSS-REFERENCE TO RELATED APPLICATION
The instant application is a continuation-in-part of U.S. application Ser. No. 10/525,894 filed Feb. 25, 2005 and which issued on Apr. 25, 2006 as U.S. Pat. No. 7,035,119.
A switching power supply has a switching device connected via a primary winding of a transformer to DC input voltage; a circuit for rectifying voltage at a secondary winding of the transformer, and outputting an output signal; and a control circuit for controlling on/off states of the switching device. The control circuit detects a current signal generated based on a forward voltage generated at a drive winding of the transformer while the switching device is on; generates an input correction signal using the current signal, the level of the input correction signal varying in accordance with the DC input voltage; detects a signal of current flowing through the switching device; and compares the input correction signal with the signal of the current flowing through the switching device, and limits the maximum value of this current in accordance with the DC input voltage.
A comparing circuit and a control loop are used to maintain the peak level of current flowing through an inductor of a flyback converter. An inductor switch control signal controls an inductor switch through which the inductor current flows. The inductor current increases at a ramp-up rate during a ramp time and stops increasing at the end of the ramp time. The comparing circuit generates a timing signal that indicates a target time at which the inductor current would reach a predetermined current limit if the inductor current continued to increase at the ramp-up rate. The control loop then receives the timing signal and compares the target time to the end of the ramp time. The pulse width of the inductor switch control signal is increased when the target time occurs after the end of the ramp time. Adjusting the pulse width controls the peak of the inductor current.
