A quasi-resonant half-bridge converter includes a switch unit including first and second switches that are coupled in series, a capacitor unit coupled to the switch unit in parallel, a rectifier unit, an output capacitor, and a transformer coupled to the aforesaid components. The first and second switches are respectively controlled using first and second control signals that have a constant frequency. Duty cycles of the first and second control signals may be adjusted based upon a DC output voltage across the output capacitor for promoting conversion efficiency of the converter when operating at light load.