Research On Topology And Control Strategy Of Bridgeless PFC

With energy crisis and continued deterioration of global environment, energy-saving, pollutant reduction and environmental protection have become major events to maintain sustainable economic and social development. Many countries and international organizations have promulgated various energy-saving and environmental protection standards and norms for existing power system, and the Europe IEC555 regulation demands that more than 75W products need to add power factor correction (PFC) function. Using PFC technology in electric equipment can increase power factor, reduce harmonic pollution on grid, improve power utilization and make products to satisfy international and national standards.In power supply and electrical equipment, rectifier bridge is commonly used to exchange AC into DC, and then PFC stage realizes power factor correction function. With power level`s improving, the loss of bridge rectifier occupies a large proportion in the whole power stage loss, so using bridgeless PFC topology has an important meaning to improve the overall system efficiency. As a high efficiency PFC topology, bridgeless PFC attracted widespread attention. This dissertation will research on bridgeless PFC topology and control strategy, and the proposed novel bridgeless PFC topology and control strategy will be used in variable frequency air-conditioning system to verify the effect.Bridgeless PFC is firstly proposed in Boost circuit. Based on analyzing arisen bridgeless Boost PFC topologies, this dissertation designs an improved bridgeless Boost PFC with simple current sensing circuit, low conduction loss and small electromagnetic interference noise. The proposed PFC is made comparison analysis and experimental validation with average current control and one-cycle control strategies. According to the same bridgeless design concept, a novel cascade bridgeless Buck-Boost PFC topology is proposed. Comparison with conventional cascaded Buck-Boost PFC, the novel topology can improve efficiency and reduce loss. At the same time, it can solve the problem of high switching stress comparing with conventional single-switch Buck-Boost PFC.Comparison with analog control, digital control has advantages such as simple circuit, less components, none sensitive to external interference, good adaptability, easy system upgrade and etc. This dissertation studies the power stage transfer function and frequency domain design method of bridgeless Boost PFC, and deduces the generic models for circuit theory analysis and parameter design. At the same time, duty cycle constraint relationship is deduced and analyzed. Combination with motion control chip IRMCF343, a duty cycle prediction algorithm with low cost and small computation is proposed. The proposed algorithm is mainly used to improve the control performance of current loop, and it has less computation and more fast tracing speed of input voltage and current comparing with average current control.In PFC control strategies, voltage loop with low-pass filter is commonly used to stabilize output voltage. Though low-pass filter can attenuate output voltage harmonics, it reduces system dynamic response capability during load changes. In rapidly changing load application (such as compressor load in variable-frequency air- conditioning), narrow voltage loop bandwidth will lead to large fluctuations in output voltage with a long time, and it decreases the system performance and causes load damage. With control chip IRMCF343, this dissertation proposes a novel fast dynamic response algorithm based on duty cycle prediction algorithm. Without adding auxiliary circuit, the proposed algorithm can realize functions of low intput current distortion and fast dynamic response.As a high efficiency topology, bridgeless PFC causes a wide attention including air-conditioning manufacturers. But so far, air-conditioning product with bridgeless PFC technology hasn't appeared in the market. And there is no system comparative analysis for bridgeless PFC and conventional PFC in air-conditioning system. This dissertation designs two experimental platforms of outdoor units based on conventional PFC and bridgeless PFC. With the proposed novel algorithms, power factor, efficiency, harmonic currents, electromagnetic interference and cost are made comparative analysis for the two PFC topologies in variable frequency air-conditioning system.