A flyback converter has a controller to switch a power switch so as to convert an input voltage to an output voltage for a load by monitoring an output voltage dependent signal and a current sensing signal derived from a current flowing through the power switch, a light-load efficiency improving apparatus monitors the load and a supply voltage provided for the controller to selectively clamp the output voltage dependent signal when the load is lower than a first threshold value and the supply voltage is lower than a second threshold value, so as to increase the supply voltage.