Scheme Research And Circuit Design Of BUCK-Type PFC For Power Factor Improvement

Power Factor(PF) is defined as the proportion of active power in the apparent power. As we know, the more current harmonic content, the lower the power factor. By using active power factor correction technology, the waveform of input current is nearly sinusoidal and in phase with the input voltage. The basic topologies of power factor correction circuit are Buck, Boost, and Buck-Boost. The current structures are based on these three basic topologies with corresponding changes. All studies in this paper focus on the Buck topology and the control circuit. Compared with the other two types of converter, Buck converter has its irreplaceable advantages. However, the power factor of Buck converter cannot meet a variety of industrial standards because of the cross-over distortion of input current, which limits the application of this topology.Two improved structures are proposed in this paper, these two structures can complement the input current distortion in the cross-over time to achieve quasi sinusoidal input current, which can overcome the inherent drawback by maintaining the advantages of traditional buck converter. On this basis, two control schemes are proposed. An additional MOS switch is added to the output point compare to basic structure. By controlling the two MOS switches, the converter turns to buck-boost topology in cross-over distortion time. As a result, current distortion can be overcome. By using an additional comparator of the control circuit to detect the time node when cross-over distortion occurs, the topology of converter can be controlled.In this paper, the converters are single-stage converter in boundary conduction mode. The PAB(Pulse Assisting Buck) converter and MTB(Multiple Topology Buck) converter can eliminate the cross-over distortion and improve the performance. The PAB converter can achieve PF of 98.74% and efficiency of 97.21% in 220 V AC input voltage. The MTB converter can achieve PF from 96.74% to 99.70% at universal AC input voltage. Results show that the improved converters can significantly improve power factor and suppress harmonics to meet international standards.