Patent ID: 7251146

Claim:
A direct-current converter comprising: a first serial circuit which is connected to both ends of a direct-current power supply and in which a primary winding of a transformer and a main switch are serially connected to each other; a second serial circuit which is connected to both ends of the main switch or both ends of the primary winding of the transformer and in which an auxiliary switch and a capacitor are serially connected to each other; a rectifying/smoothing circuit configured to rectify and smooth a voltage generated in a secondary winding of the transformer by energy supplied from the primary winding of the transformer when the main switch is turned on, the voltage being rectified and smoothed using a rectifying device and a smoothing device; and a control circuit configured to turn on/off the main switch and the auxiliary switch alternately using a signal with predetermined switching frequency, the control circuit having error voltage generation means configured to generate an error voltage signal based upon an error between an output voltage from the smoothing device and a reference voltage and having frequency control means configured to generate a frequency control signal which reduces the switching frequency in accordance with a value of the error voltage signal when a value of the error voltage signal generated by the error voltage generation means reaches a first threshold indicative of light load condition.