Research Of The Current-Fed Half-Bridge Converter

The major problem of the current-fed half bridge (CFHB) converter is high voltage surges across the turned-off switch caused by the leakage inductor of transformer and difficult to achieve the soft switching. Based on the research work about the problems at home and aboard, the ZCS technique is adopted to make the course that the current of leakage inductance rises from zero to steady state value disappear. Thus the source of voltage surges is removed radically.A ZCS PWM CFHB Converter with improved symmetrical operation is studied firstly. By introducing an active resonant branch to the secondary of the transformer, the zero-current-switching (ZCS) and pulse-width modulation (PWM) are obtained. The circuit structure in the paper is simpler than in the literature by getting rid of a auxiliary switch and the circuit also maintains the symmetry. Operation principle and characteristic of the converter are analyzed detailedly and the design of the parameter is also included in this paper. It is verified by simulation that all the switches achieve ZCS and the rectifier diodes are softly commutation at ZVS. The high voltage surges across the turned-off switch caused by the leakage inductor of transformer are removed.Considering the shortcomings of the converter metioned above, a ZCS current-fed half-bridge converter with asymmetrical PWM operation is researched in detail. ZCS is achieved for main switches and rectifier diodes are softly commutation at ZVS only by leakage inductance of the transformer and resonant capacitor. Operation principle and characteristic of the converter are analyzed and the rectifier structure of the converter is discussed compared with the full bridge rectifier.Based on the analysis above, the voltage spike due to the backward recovery of diodes and the spike of resonant current are eliminated by the diodes in series with the main switchs. Operation principle and characteristic of the converter are analyzed. The advantages and influence of series diodes are also analyzed in detail. Then the paper gives the design of the ZCS current-fed half-bridge converter with asymmetrical PWM operation. The designs and the operation principle of the control circuit are also included in this paper.Finally, Simulation and experimental results are obtained from a protoyype. The experimental results show that ZCS is achieved for main switches and rectifier diodes are softly commutation at ZVS. High voltage surges across the turned-off switch caused by the leakage inductor of transformer are removed and the efficiency of the converter is improved. The voltages spike of the switches can be removed completely and the resonant peak current of the switches can be restrained to input current by the ZCS technique and series diodes. The ZCS PWM CFBH converter with asymmetrical PWM operation has simple structure and convenient control without adding any active auxiliary circuit .It has obvious advantage in the applications that the variety of input voltage is small.