Analysis and design of switching converters for power factor correction applications

Abstract

Switching converters for power factor correction (PFC) applications generally adopted a two-stage approach consisting of two individual power stages - PFC stage and DC/DC regulation stage. Depending on the output power level, the two stages are put together into either serially-connected individually-controlled or serially-connected integrated-control configuration. This thesis presents the analysis and design of some converter topologies and control circuits to improve the performance of the switching converters for both configurations. For high power level, a PFC pre-regulation converter is connected in series with a DC/DC regulation converter and each power stage has its own control circuitry. Due to the continuous-conduction-mode (CCM) operation of the input inductor, the rectifier reverse-recovery transition causes switching loss and electromagnetic interference (EMI) problems. In this thesis, a CCM boost converter suitable for PFC application is studied. A passive low-loss snubber using a pair of coupled inductors is proposed so that all the rectifiers turn off softly. The power switch is also operating in low-voltage turn-on condition, further reducing the switching loss. For low to medium power levels, the PFC pre-regulation converter and the DC/DC regulation converter are basically serially-connected but they share the common switch(es) to simplify the power stages and control circuitry (integrated-control). We refer to this arrangement as single-stage PFC approach. However, this configuration causes substantial voltage and current stresses on the passive and active components of the converter and results in low conversion efficiency. An improved converter topology is then presented using the direct power transfer approach to reduce the stresses on the components. We refer to this arrangement as parallel-connected integrated-control configuration. Experimental results show that the conversion efficiency is improved. The approach is then extended to derive a flyback-topology based single-switch single-stage PFC converter to loosely regulate the storage capacitor voltage using the regulated output voltage. The range of change of storage capacitor voltage against change of line voltage is significantly reduced. In an attempt to provide more rigorous regulation of the storage capacitor voltage and to provide stand-by mode operation, an auxiliary bi-directional switch is added to the single-stage PEC converter. This auxiliary switch is operated in zero-voltage-switching (ZVS) condition and is connected in series with the storage capacitor to control the storage capacitor voltage. Besides, this switch helps to create a direct power transfer path for input power to output load and reduce the storage capacitor voltage stress at light load condition. A simple single-loop control method is also proposed to achieve power factor correction, output voltage regulation and control of storage capacitor voltage simultaneously. Experimental results to verify the operation and analysis of the proposed circuit topologies are given. Comparative results (based on calculation or experiment) are also given to prove the proposed approaches effective.