Research On A Parallel LLC Resonant Converter With Automatic Current Sharing Characteristics

In high-power applications with low-voltage and high-current,the input and the output of the each converters are in parallel to improve the efficiency and reliability of the whole system.Since the gain of the LLC resonant converter is very sensitive to component parameters,the sharing current problem is much prominent when work in parallel.In addition,synchronous rectification technology is often used in high-current output situations to improve efficiency.However,the difficulty of synchronous rectification lies in how to minimize the on-time of the synchronous rectification parasitic diode,reduce the loss and improve the efficiency.The main researches of the above problems are described as follows:First of all,based on the introduction of the advantages and disadvantages of current sharing characteristics of various parallel LLC resonant converters,there is a parallel LLC resonant converter with automatic current sharing characteristics presented in this paper.Because of using a common resonant capacitor and coupling resonant inductor,to reduce the current sharing error sources,the better current sharing characteristics is achieved automatically.Then,the working principle,control strategy and main features of the proposed converter are analyzed in detail.Compared with the traditional parallel LLC resonant converter,the time-domain current sharing error model of the proposed converter is established and the current sharing error distribution characteristics is analyzed when the resonant inductor,resonant capacitor,and magnetizing inductor under asymmetric conditions.It is also compared with the current sharing characteristics of the traditional parallel LLC resonant converter,which are verified by simulation.Secondly,the existing synchronous rectification technology and its features are introduced,and an adaptive synchronous rectification control strategy with maintain capacitance is put forward.Based on detecting the voltage across the synchronous rectifier,the automatic turn-on and turn-off optimization of the synchronous rectifier is realized,thus reducing the on-time of the parasitic diode of the synchronous rectifier and optimizing the circuit efficiency.The working principle,the transient process and the realization process of the proposed synchronous rectification control strategy are described and simulated with the traditional voltage type synchronous rectification control strategy.Finally,a 2.9kW,48V/50A experimental converter is built to study the current sharing error characteristics of LLC resonant converter with automatic current sharing characteristics and further verify the accuracy of the proposed parallel LLC resonant converter.The experimental results are agreement with the theoretical and simulation results.Compared with the traditional parallel LLC resonant converter,the proposed parallel LLC resonant converter has a greater improvement in current sharing characteristics;In addition,a1.44kW,48V/30A LLC resonant converter is built to verify the feasibility of synchronous rectification of digital adaptive LLC resonant converter with a maintain capacitor.And there is a comparison of the efficiency between the proposed synchronous rectification strategy and the traditional synchronous rectification control strategy.The results show that under full load condition,the efficiency of the proposed synchronous rectification control strategy is 96.32%and that of the traditional synchronous rectification control strategy is 96.13%.