Control circuit of switching power supply device

Abstract

In a control circuit of a switching power supply device, a switching cycle generating unit controls a switching cycle in keeping with a signal of an error voltage detected on a secondary side to stabilize an output voltage at a target value. A load current for overload protection is estimated based on a value produced by integrating a resonant current at an integrator and averaging at an averaging calculation unit. Here, although the switching cycle is correlated to the ratio of input/output voltages, the switching cycle is negatively correlated to the input voltage because the output voltage is controlled so as to be constant. A correction calculating unit uses a signal that decides the switching cycle as information corresponding to the input voltage, which eliminates the need to detect the input voltage.