Patent ID: 7974110

Claim:
A switching power supply unit comprising: a rectifier circuit for rectifying an alternating current from an AC power source into a direct current; a switching circuit for switching the current rectified by the rectifier circuit using a switching device; a pulse oscillator circuit for outputting a switching signal to the switching device; a transformer circuit for stepping a voltage up or down using the current switched by the switching circuit; and a frequency switching unit for detecting a pulse output of the switching circuit of a primary side of the transformer circuit and changing a resistance using a resistor based on a state of the pulse output, thereby switching a frequency of the switching signal in the pulse oscillator circuit, wherein the frequency switching unit includes: an output detection circuit for detecting the pulse output of the switching circuit; a DC signal level conversion circuit for converting the pulse output detected by the output detection circuit into a DC signal level; a comparison circuit for comparing the DC signal level converted by the DC signal level conversion circuit with a reference voltage; and a resistance changing circuit for starting the frequency of the switching signal in the pulse oscillator circuit with low frequency by changing the resistance if the DC signal level does not reach the reference voltage when starting the pulse output to oscillate, thereafter changing the resistance if the DC signal level reaches the reference voltage, thereby switching the frequency of the switching signal in the pulse oscillator circuit to high frequency.