Patent ID: 7697310

Claim:
A control apparatus for controlling a voltage transforming apparatus including a transformer, power switching elements provided in a primary side thereof and operating to invert a DC input voltage to an AC voltage, synchronous-rectifying switching elements provided in a secondary side thereof and performing synchronous-rectifying control operation in synchronization with said power switching elements in order to rectify said AC voltage transformed by said transformer into a DC output voltage, and a smoothing circuit smoothing said DC output voltage, said control apparatus comprising: a judging circuit making a judgment as to whether or not an output current of said voltage transforming apparatus is smaller than a specified current on the basis of a primary-side current flowing through a primary coil of said transformer; and an inhibition circuit inhibiting said synchronous-rectifying switching elements from performing said synchronous-rectifying control operation when said judging circuit judges that said output current is smaller than said specified current; wherein said judging circuit makes said judgment while compensating for a variation of a relationship between said primary side-current and said output current due to variation of duty ratio of said power switching elements, and variation of at least one of said DC output voltage and said DC input voltage; and said judging circuit includes a comparator comparing a detection value of said primary-side current with a threshold value corresponding to said specified current, and a compensating circuit compensating one of said detection value and said threshold value being inputted to said comparator on the basis of a compensation value by which said relationship between said primary-side current and said output current is compensated for.