Research On Push-pull Bidirectional LLC Resonant Converter Based On GaN Device

Due to the increasingly prominent energy crisis and environmental problems,people pay more and more attention to the rational and effective development and utilization of new energy.Bidirectional DC/DC converter has been widely used in electric vehicles,new energy power generation system,uninterrupted power supply system,aviation power supply and other application fields.The high speed development of various fields requires more and more output power.In order to achieve high efficiency,miniaturization,good performance and low energy consumption of the system,it is the important means that switching power supply realizes highfrequency.compared with silicon field effect transistor(Si mosfets),cascode GaN HEMT has better performance and better high frequency application potential.But because the high frequency will lead to larger switch loss,Bidirectional DC/DC converter still need soft switch technology in high frequency operation.Therefore,in low-voltage high-power situations,the push-pull bidirectional LLC resonant topology is selected to ensure not only the high voltage rating of the high-voltage side,but also the simple structure,low cost and low device loss of the low-voltage side.However,in the LLC converter's starting procedure,the circuit will generate a large surge current in a short time,which is more than ten times or even dozens of times the resonant current under the normal working state.It is easy to cause heat damage of power devices and affect the dynamic characteristics of the system,which is an urgent problem to be solved.Therefore,this project aims to select high-frequency high-efficiency topological structure in low-voltage high-power situations,and proposes an effective soft-start hybrid control strategy for the start-up process,so that the converter can operate safely and reliably.The main work contents are as follows:First of all,this paper studies the 650 V cascode GaN HEMT's working principle and characteristics,and comparing parameters,static and dynamic characteristics with similar power levels of Si MOSFET,which shows its superiority.Secondly,this paper selects push-pull bi-directional LLC topology in the low-voltage high-power situations,namely the high side for the whole bridge structure,the low voltage side to the push-pull structure.The forward and backward working modes of push-pull LLC resonant converter are analyzed in detail,and the forward and backward gain characteristics are analyzed based on fundamental wave component method.Then the realization conditions of the forward and backward modes' soft switch and the synchronous rectification scheme applicable to the push-pull LLC resonant converter are analyzed.Again,the starting process of LLC resonant converter is analyzed and the reasons of the surge current in the process of start is discussed.The working principle and deficiencies of two traditional methods are introduced in detail.Thus,a new soft start control strategy with simple operation and obvious effect is proposed and introduced in detail.Finally,the feasibility of the soft start control strategy is validated by simulation.Finally,the 3kW LLC resonant converter simulation model and experimental platform were designed to conduct soft start experiment,forward and reverse runnning verification.