A method and apparatus for efficiently converting power from an AC line is disclosed. The power converter supplies power to the load, performing power factor correction at nominal and high loads, i.e. above a predetermined output power threshold, and operating in an on-demand burst mode synchronized to the AC line at low loads, i.e. below a predetermined output power threshold. The duration of an operating interval during which power conversion takes place may be reduced and varied at low loads as a function of the output power demands to increase overall conversion efficiency. The operating interval may be centered about a peak in the input voltage waveshape for operating intervals that are less than a full rectification period. For operating intervals that are less than a full rectification period a modified power factor correction may be used, in which the input current waveshape approximates the input voltage waveshape during the operating interval.