Study On PFC Converter With High Power Factor And Low Output Ripple

To eliminate harmonic pollution in power grid and improve power factor, the input terminal of electronic equipments need to add active power factor correction (PFC) converter. Large second-order line frequency voltage/current ripple is resulting from unbalance instant power between input and output, the output voltage/current can not be regulated immediately with step load variation because of slow dynamic response of PFC converter, so PFC converter can not provide power supply to those equipments which need accurate power supply. To decrease the output voltage/current ripple, the switch power supply with PFC function usually uses two-stage PFC converter which combines cascaded input PFC converter and output DC-DC converter. Traditional two-stage PFC converter needs two switch converters and two controllers, and the energy of two-stage PFC converter is converted twice. So low efficiency, low power density, high cost, and complicated control limit the application of two-stage PFC converter. The simple controlled, low cost, and high efficiency PFC converter with high power factor and low output ripple will be proposed and studied in this paper.Compared with two-stage PFC converter, the single-stage single-switch PFC converters, such as boost-flyback, boost-forward, and buck-flyback, have the advantages of simple control and low cost. However, the presence of transformer in the single-stage single-switch PFC converter increases the size and cost of the converter. To decrease the cost and the size, the quadratic buck PFC converter is proposed in this paper. Based on LED load, the flicker-free transformerless LED driving circuit based on quadratic buck PFC converter is studied intensively. It combines two cascaded buck converters with single switch. The input buck converter can realize PFC, and output buck converter can regulate LED current. The result presents that quadratic buck PFC converter can not only achieve high power factor to comply with EN61000-3-2 Class D standard and low output voltage without a high step-down transformer, but also reduce second-order line frequency current ripple flowing through LED to eliminate light flicker of LED.Single-stage single-switch PFC converter realizes PFC, low output voltage/current ripple, and fast dynamic response at the same time, the PF is lower than two-stage PFC converter. Furthermore, the energy of single-stage single-switch converter still converted twice, its efficiency is not high. To improve the efficiency of PFC converter with decreasing energy conversion time, and to decrease output voltage ripple, a quasi single-stage flyback PFC converter with small output voltage ripple is proposed in this paper. The converter consists of a dual-output flyback converter and a buck converter. The output of the buck converter is placed in series with the main output of dual-output flyback converter so that the load voltage is the sum of the output of buck converter and the main output of dual-output flyback converter, and the buck converter is powered by the auxiliary output of dual-output flyback converter. The dual-output flyback converter is used to make input current sinusoid, while the buck converter with fast dynamic response is used to precisely regulate the load voltage with small voltage ripple. Based on the study about the effect of operation mode to the output voltage ripple of flyback PFC converter, the quasi single-stage flyback PFC converter with small output voltage ripple is studied intensively. The results present that the output voltage ripple of proposed quasi single-stage flyback PFC converter is lower a lot than flyback PFC converter, the efficiency of quasi single-stage flyback PFC converter is higher than two-stage PFC converter and near the flyback PFC converter, and the dynamic response of proposed converter is faster than flyback PFC converter and even similar to two-stage PFC converter.The input current of traditional constant On-Time (COT) controlled critical conduction mode (CRM) flyback PFC converter has big distortion because the input current is not ideal sinusoidal waveform, its PF is not unity, and its total harmonic distortion (THD) is high, especially at high input voltage. A variable On-Time (VOT) control strategy for CRM flyback PFC converter which can realize unity power factor (PF) is proposed in this paper. By using input voltage and the voltage across the auxiliary winding of the flyback transformer to control the turn-on time of the switch of CRM flyback PFC converter, the PF and THD of CRM flyback PFC converter can be improved. The study results show that the PF and THD of VOT controlled CRM flyback PFC converter are better than these of COT controlled CRM flyback PFC converter. At the last of this paper, the quasi single-stage flyback PFC converter with unity PF and low output voltage ripple based on VOT control strategy is proposed through combining both quasi single-stage flyback PFC converter and VOT control strategy.In order to verify the correctness of the theoretical analysis, a mass of simulation results are provided, and corresponding prototypes in lab is set up to present experimental results. Simulation and experimental results have a good agreement with the theoretical analysis presented in this dissertation.