Research On Three Phase Single Stage Full Bridge Active Power Factor Correction Technique

Active power factor correction (APFC) technique is an effective method for reducing harmonic current and increase power factor. In medium and high power applicaions, three-phase APFC technique is usually applied. Comparing with additional three-phase two-stage APFC, three-phase single-stage APFC is an important researching orientation in power electronics field, which has the advantages such as simple structure, high power density and high efficency. At present, the study about three-phase single-stage APFC has been developed in low power applicaions. However, in medium or high power applications, it is still in the primary stage. In high power applications, full-bridge structure is wildly used. A three-phase single-stage APFC technique based on isolated full-bridge boost topology is the subject of this dissertation, several problems such as topology, voltage spike, harmonics and dead zone of input current, starting and stopping magnetic reset are addressed, and some novel methods and ideas are proposed.Two three-phase single-stage APFC converters are proposed, which are based on isolated full-bridge boost topology. The converters work in discontinuous conduction mode (DCM), and the input current peak follows the input voltage automatically. The electrical isolation between input and output sides and output DC voltage grade regulation could be achieved as well as power factor correction. Using the control strategy of phase-shifted full-bridge circuit, the converters can achieve zero-current-switching (ZCS) for the up arm switches and zero-voltage-switching (ZVS) for the low arm switches without adding any auxiliary device. Problems of converters such as mechanism of power factor correction, conditions for boost inductors working in DCM, working modes and mechanism of the voltage spike across primary side of transformer are analyzed in details, which create a foundation for the following problems.Since the existing of voltage spike, voltage stress of each switch increases and reliability of converters decraeses, effective methods must be introduced to suppress it. According to the mechanism of voltage spike and characteristics of the converters, two voltage spike suppression methods based on passive snubber are proposed (parallel and series passive snubbers), which are composed of capacitors, inductors and diodes. The capacitors in primary side are used to suppress the voltage spike and the energy of capacitors can be transferred to the load during one switching cycle through the resonance of inductors and capacitors. The specific working process of two snubbers are discussed in details, influence rules of each parameter on voltage and current stresses of switches are concluded, and designing methods of each parameter are given. Experimental results show that the voltage spike in primary side is suppressed efficiently through adoption of passive snubbers.Quality of input current waveform is important for the result of power factor correction. Input current expression of the converters is derived, and the parameter which can determine the input harmonic current is analyzed. Finally, the method of sixth order harmonic injection is adopted to suppress input harmonic current after some methods have been analyzed. Analysis of the equivalent circuits in both ideal and nonideal conditions shows that, due to the on-state voltage drop of each switch and diode in the main circuit, there will be input current dead zone when its voltage is lower than the voltage drop of main circuit, and the more input voltage decreases the larger dead zone expands. According to working principle of the circuits, a dead zone compensation strategy is proposed, with which the input current dead zone is suppressed effectively.Starting and stopping magnetic reset is important to the reliability of isolated full-bridge boost topology. The starting and stopping magnetic reset process of the converters and mechism of over voltage and over current are analyzed in details under the condition of constant duty cycle. To realize starting-up according to characteristics of the converters, a method is proposed and designed in which two resistors are connected in series with the output filter capacitor to increase the virtual value of output voltage. The boost inductors are replaced by central-tapped inductors with flyback windings, and a starting and stopping magnetic reset strategy is proposed which is based on flyback mode. As for parallel and series passive snubbers, two lossless methods of starting and stopping magnetic reset are designed respectively. Finally, constrained conditions of key parameters are given through comparing with the characteristics of steady state,and the starting-up and stopping magnetic reset are realized in experiment.