| In coal mine, there are a variety of voltage levels for power supplies and electrical equipment, which complex the whole power supply system. Nowadays, traditional power transformers are generally used in coal mine for voltage transformation and energy transfer to meet the requirements of different voltage levels. These transformers are simple in manufacturing process with high reliability. But the high price, huge volume, prodigious no-load loss and inflexible control would be the disadvantages, and these transformers are unable to maintain normal operation of power equipment confronted with voltage unbalances, harmonics, flickers and so on. So, it is urgent to solve the problem that is how to ensure the safe operation of electrical equipment, and providing a reliable and stable power supply for users. The Power Electronic Transformer(PET) came into being, as a new power transformer, not only has the functions of traditional transformers, but also has the ability to solve above problems, so it is worth in-depth study.The DC-DC converter of PET is an important part of affecting work efficiency, volume and weight of the device. This paper studys the DC-DC converter mainly, analyzes the role of DC-DC converter and the problems caused by it. Soft switching technology is tried to use to reduce the loss and the frequency of DC-DC converter in PET is increased to improve the efficiency of system, while reduceing the size and weight of the device.First, full-bridge LLC resonant converter is determined as the DC-DC converter of PET by comparing with different converters using soft switching technology. Based on the analysis of the working mode of full-bridge LLC resonant converter, the model is established by using First Harmonic Approximation(FHA). The suitable k and Q value, and the condition of realizing Zero Voltage Switching(ZVS) of primary side switch are determined by analyzing the voltage gain characteristic, the input impedance characteristic, the short circuit characteristic and the no-load characteristic. Based on the topology of full-bridge LLC resonant converter, its efficiency has been analyzed with studing its main source of losses, analyzing the various sources and the calculation method is given.Then,according to given indexes, main circuit of full-bridge LLC resonant converter is designed in detail, including the design of resonant parameters. And the magnetic integrated transformer is designed with the idea of magnetic integration, which greatly reduces the converter volume, and the selection of devices is completed on the basis of parameters design. In addition, the control circuit of full-bridge LLC resonant converter based on UCC25600 is designed. According to the given parameters, losses of each part and the efficiency are calculated. The results meet the efficiency requirements of design.At last,PEmag and Maxwell simulation software are used to design magnetic integrated transformer, and verified the magnetic integrated transformer parameters. Matlab/simulink is used to simulate and analyze the DC-DC converter performance of PET. A prototype of full-bridge LLC resonant converter is developed and system test platform is built according to the theoretical research and simulation results. The correctness and effectiveness of theoretical research and design methods of the DC-DC converter are verified by analyzing the waveforms of the test. |
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