Research On High Power LLC Resonant Converter Based On SiC MOSFET

In the photovoltaic grid-connected system based on medium and high voltage power grid,the high-power DC/DC converter is the key part,which has attracted the attention of many scholars.As a member of high-power DC/DC converter,LLC resonant converter has received a lot of attention and applications,because of its small electrical stress of power devices,wide range of soft switching and high efficiency of the whole machine.LLC resonant converter can work in two working states.When it works in LLC DC transformer(LLC DCT)state,it needs to maintain the constant gain ratio of input and output voltage,which has different requirements for the selection of working area,parameter design principle and control strategy of the converter.In the application of high power LLC resonant converter,the problem of eddy current loss caused by high frequency transformer is aggravated,which affects the operation efficiency of the converter to a great extent.The design of high frequency transformer using Litz line can effectively reduce the high frequency loss caused by skin effect and proximity effect,and then improve the efficiency of the converter.Therefore,the selection of Litz line is the key to the design of high frequency transformer.Thanks to the development of wide band gap semiconductor devices,the efficiency of LLC resonant converter can be further improved,but there are still some problems to be solved in the application of wide band gap semiconductor devices.In the research of high power LLC resonant converter based on SiC MOSFET,because of its high current density and short short circuit withstand time,if the system has a short circuit fault,the SiC MOSFET can not protect off quickly,it will be damaged instantly,reduce the reliability of the system,and cause economic losses at the same time.This paper takes the high power LLC resonant converter based on SiC MOSFET as the research object,and conducts the following research:(1)According to the characteristics of high power level of the converter in the photovoltaic grid-connected system,the one-to-four LLC resonant converter topology is proposed,and the multi-output design method is adopted to reduce the number of modules in the photovoltaic grid-connected system and reduce the cost of the system.(2)In view of the design problem of LLC resonant converter working in LLC DCT state,the working principle,gain characteristics,working area,parameter design and control strategy of LLC resonant converter are analyzed and explained in detail,and the simulation and experimental verification are also carried out.(3)Aiming at the problem of eddy current loss of high frequency transformer in high power LLC resonant converter,the Dowell model was used to analyze the eddy current loss of transformer and determine the optimal number of shares of transformer winding Leeds line.With the help of 100 KW LLC resonant converter platform,the efficiency test of different transformers is carried out to verify the correctness and effectiveness of the selection method of strands.(4)Aiming at the short-circuit protection problem of SiC MOSFET in LLC resonant converter,the protection effect of two short-circuit protection methods is compared through experiments.Aiming at the problem that the SiC MOSFET in the three-level topology is damaged by overvoltage and overcurrent due to short circuit fault,a large capacitance flyover capacitor is introduced into the NPC topology to avoid the risk of overvoltage damage while realizing the short-circuit protection of all switches,and the short-circuit test is carried out to verify the problem.