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For high input voltage (>;264 Vac) ac/dc applications, a buck power factor correction (PFC) converter is a good choice because of its low output voltage, high efficiency, lifetime improvement, and cost reduction by using a low voltage rating (<;200 V) electrolytic capacitor. However, due to the inherent dead angle of the input current, the harmonics of the buck PFC converter are high, which limits its application in lighting systems. In order to make the buck PFC converter meet the harmonics requirements (IEC61000-3-2, Class C) in lighting applications, this paper proposes a variable on-time controller for a critical conduction mode (CRM) buck PFC front-end converter for isolated high-brightness LED applications. By feedforwarding the input voltage and regulating the on-time of the switch, the high-order harmonics can be reduced to meet the lighting system limitations. Experimental results obtained on a 150-W CRM buck front-end PFC prototype show that the efficiency of buck PFC exceeds 96% during the entire line input range (250-530 Vac) at full load, and the current harmonics content can meet the harmonic requirements.