Patent ID: 7212415

Claim:
A resonance type switching power source comprising a DC power source; a transformer having primary and secondary windings; at least first and second switching elements connected in series to both electrodes of said DC power source; a current resonance capacitor connected in parallel to said first switching element and in series to the primary winding of said transformer and second switching element; a rectifying and smoothing circuit connected to the second winding of said transformer; and a control circuit for producing drive pulses to said first and second switching elements to alternately turn on and off the switching elements; a closed circuit being formed by the first switching element, primary winding and current resonance capacitor when the first switching element is turned off, said control circuit comprising a drive circuit for supplying drive pulses to each control terminal of the first and second switching elements; a PWM circuit for causing the drive circuit to produce the drive pulses; an input voltage detector for detecting input voltage from the DC power source and comparing the input voltage and input reference voltage; and a frequency adjuster for adjusting the oscillation frequency of the PWM circuit in response to an output level from the input voltage detector; wherein, when the second switching element is turned on, electric current flows through the primary winding to electrically resonate the current resonance capacitor and primary winding, while DC power is supplied from the secondary winding of the transformer through the rectifying and smoothing circuit to a load.