The Study On Novel Soft-Switching PWM Buck Converters

Soft-switching technology is one of the hot spots in the research area of the power electronics. With the use of it, the switching loss can be reduced, and the electromagnetic interference (EMI) also can be restrained, therefore it contributes to increase the switching frequency, which helps the power supplies to minimize their size and weight, to increase their efficiency, and to enhance their power density. The developmental course of the soft-switching is reviewed in this thesis, and the characteristics of several representative resonant converters are introduced. On the base of the analyses of the basic Buck converter, the operation principle and the circuit features of the traditional ZVT PWM Buck converter are analyzed theoretically, and the converter is simulated with computer. To solve its problems, an improved ZVT PWM Buck converter is presented. Moreover, based on the study above, a novel ZCS PWM Buck converter is proposed.The improved ZVT PWM Buck converter adds a snubber capacitor and a diode being in series with the resonant inductor, to overcome the problems of the hard switching of auxiliary switch and the internal circulating current that exist in the traditional ZVT PWM Buck converter. The topology, operation principle and main characteristics of the improved ZVT PWM Buck converter are analyzed. Based on the circuit topology, a project of control and drive is designed, which chooses UC3525A as PWM controller. The results of simulation and experiment show that the improved circuit is feasible and valid, and the performance of the ZVT PWM Buck converter is enhanced.The auxiliary resonant cell of the novel ZCS PWM Buck converter offers the zero-current-switching condition for the main switch without additional current stress and conduction loss. Meanwhile, its auxiliary switch also achieves the soft turn-on and turn-off. The topology, operation principle and circuit features are described in detail. Then the calculation method of the resonant parameters and the design of control and drive circuits are presented. The simulation results verify that all of the semiconductor devices in the converter are turned on and turned off under exact or near soft-switching, and there is almost no additional current stress in it. In addition, it can solve IGBT's "tail currents" at the turn-off which causes high turn-off switching loss significantly. Moreover, the proposed converter has a simple structure and a low cost, and is ease to control. So the converter is deemed suitable for the applications where the minority-carrier semiconductor devices, such as IGBT and MCT, are predominantly used as the power switches.