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In high voltage AC input applications, buck PFC has the advantage of outputting a lower level voltage. This paper presents a variable on-time control buck PFC front-end for high-brightness light-emitting-diode (HB-LED) application, the function of the switch on-time in a half-line cycle is derived. Most high-order harmonics are lowered comparing to the constant on-time control in order to comply with IEC-61000-3-2 Class C standard. Experimental results obtained on a 150W prototype, including a LLC bus-converter as the 2nd stage show the overall efficiency exceeds 92.5% across the entire line input (250-530Vac), and the line current harmonics content can meet the related Class C limits.