Research Of Soft Switching Technique For High Frequency Charger In Urban Rail Transportation

The battery charger is an important component of urban rail train and its performance will influence the safety of the vehicle directly. With the development of power electronic equipments in high frequency and light weight, the switching loss issue becomes more and more serious. With high power density, the charger’s switching devices are often damaged by over temperature. The complicated snubber circuits reduce the reliability of the system and the efficiency is affected greatly. In this situation, the soft switching technology is attracting more and more attention. It can reduce the switching loss and overcome the inherent defects of the hard switching converters. So, it is necessary to have an in-depth study about the soft switching technology for high frequency battery chargers in urban rail transportation.At first, this paper studies the full-bridge DC/DC converter which is the most common topology of battery chargers and analyses the three types of soft switching topologies under phase-shifted control:ZVS, ZCS and ZVZCS. Eight different ZVZCS circuits are listed while their advantages and disadvantages are pointed out at the same time.Secondly, the topology which uses two diodes in series with the lagging leg is studied particularly. With the analysis of operation principles, the analytical model of the main circuit is presented and by further summary, the paper proposes a group of boundary inequalities and a new design method for the main parameters, including the snubber capacitors of the switches in leading leg, the blocking capacitor and the delay time between gate drive signals. By the functional simulation of Matlab program and the comparison with theoretical calculation results, the method is proved right.Then, facing the demand of high frequency soft switching charger in urban rail transportation, the digital control system based on DSP and FPGA is designed. The structures and functions of control center, AD module, pulse driving module and protection module are discussed in detail. The flow charts of programs are given. In addition, the implementation of digital filter and PI control are introduced emphatically.At last, the author builds a6kW soft battery charger prototype with20kHz switching frequency and finishes the functional experiment and temperature rise test. The waveforms are given and the efficiency at full load increases from92%to94%. Compared with the hard switching converters, IGBT temperature rise is reduced clearly.The experiment results show that the theoretical analysis of the main circuit topology in the article is correct and reasonable. The proposed design methods for soft switching parameters are quite valuable to engineering application at the same time.