High Efficiency Power Factor Correction Techniques Based On Buck Converter

With the development of power electronics technology, many power converters which use capacitor-input filter have resulted in severe harmonic contamination. In order to reduce the harmonic current, many countries have forced the AC/DC converters to meet the limits imposed by IEC 61000-3-2. At present, the boost PFC is a good solution to provide an almost sinusoidal current absorption and improve the power factor to close to unity. However, in universal input condition, the boost PFC can't achieve high efficiency at low line because it works with large duty cycle in order to get high voltage conversion gain with 90Vac input and 400Vdc output. Therefore, it is hard to increases the power density of boost PFC converter attributes to the thermal problem at low line, especially for adapter application.In contrast to the conventional boost Power Factor Correction converter, the buck PFC converter can achieve high efficiency in the entire universal input voltage range. A critical conduction mode (CRM) soft switched buck PFC converter with constant on-time (COT) control is proposed in this paper. The detailed operating principle and the optimized design methodology for high efficiency and low harmonics of the COT Buck PFC converter are presented. A 100 W prototype designed according to the proposed design criteria shows that the input current harmonics can meet the IEC61000-3-2 Class D standard and the efficiency is higher than 0.965 during the universal input range.This paper also presents a detailed performance comparison (including efficiency, EMC performance and component electrical stress) between boost and buck type PFC under COT control. In universal input (90-265Vac) applications, the COT buck PFC has around 1% higher efficiency compared to its counterpart at low-line (90Vac) condition. Due to the low voltage swing of switch, buck PFC has a better CM EMI performance than boost PFC. It seems that the buck PFC is more attractive in low power applications which only need to meet the IEC61000-3-2 Class D standard based on the comparison.In North America high voltage AC input applications, buck PFC has the advantage of outputting a lower level voltage. A variable on-time control buck PFC front-end for high-brightness light-emitting-diode (HB-LED) application is present in the paper, the function of the switch on-time in a half-line cycle is derived for the optimized design. 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.