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In contrast to the conventional boost PFC converter, the buck AC-DC converter can achieve high efficiency in the entire universal input voltage range. A critical conduction mode (CRM) ZVS buck converter with constant on-time control is proposed in this paper. The design methodology and criteria for high efficiency and low harmonics of the CRM buck AC-DC converter are presented. A 100-W prototype designed according to the proposed design criteria shows that the input current harmonics meet the IEC61000-3-2 Class D standard and the efficiency is higher than 0.965 during the universal input range.