Study And Design On Three-level LLC Resonant Converter For Fuel Cell

In recent years, the fuel cell has seen a rapid development because of the following advantages: high energy conversion efficiency, and high environmental-friendliness. Fuel cell distributed generation has become one of the most important applications of fuel cell and its energy conversion system has been paid more and more attention. However, as the load current increases, the output voltage drops rapidly, which means that the output voltage range is relatively wide. Furthermore, it is ver y difficult for fuel cell to respond quickly to load-mutation. Therefore, the research of DC-DC converter that not only meets the output voltage range of fuel cell but also has fast dynamic response characteristic has become an important issue.Focus on the applications of fuel cell power conversion system and in order to improve the adaptability for wide range of input voltage,this thesis use the full-bridge three-level LLC resonant converter topology, and study its work principle and circuit characteristic.Detail research work is as follows:First, this paper studies on common DC-DC converter topologies in fuel cell power conversion system and chooses full-bridge three-level LLC resonant converter as the main circuit topology through the analysis and comparison of these topologies.This thesis put forward an improved symmetrical double phase-shift control strategy focus on the problem that total harmonic distortion(THD) of the input voltage of resonant tank is high in traditional strategy, which is able to realize switch between 3L-mode and 2l_mode, as well as to reduce THD of input voltage of resonant tank. DC gain characteristic of LLC resonant network and soft switch characteristic is researched based on this improved control strategy. The conclusion that under control of the improve control strategy, input voltage THD of resonant tank is low, is proposed, after the way that THD varies with chopping angle in two control strategies is analyzed. Then, theoretic analysis was verified via Saber simulations.Second, according to the designing request of fuel cell power conversion system, this paper designs the parameters of LLC resonant converter in detail, including the parameters of LLC resonant tank, high-frequency transformer, resonant inductor etc, and researched the digital controller design.Furthermore, simulation of full-bridge three-level LLC resonant converter is built in Saber in order to verify the adaption of the LLC converter to wide input voltage range, ZVS, ripple wave of output voltage and response characteristic of load-mutation.Finally, based on the index of designing and analysis of simulation result, 300 W prototype of full-bridge three-level LLC resonant converter is built. The prototype realizes the improved symmetrical double phase-shift control strategy. In addition, the adaption of the wide input voltage range, ZVS and efficiency achieve the expected requirements.