Research On Strategy Of Parallel LLC Resonant Converter Modules

LLC resonant converter,as a new soft-switching converter,can realize zero voltage switching(ZVS)on of primary power transistor and zero current switching(ZCS)off of secondary power diode in wide input and full load range.It works at a high frequency,helps to reduce the volume of the converter,and can further improve the power density by using magnetic integration technology.However,the output ripple,device stress and other factors restrict the further improvement of the power of LLC resonant converter.Parallel mode is an important means to expand power level.How to ensure current sharing among parallel modules is an important guarantee to give full play to the advantages of parallel connection.Affected by the differences of process manufacturing and environmental factors,there must be errors in parameters between parallel modules,which will lead to unbalanced currents of each module.As a resonant converter,the current sharing of LLC converter is very sensitive to the difference of resonant parameters,which will cause serious imbalance of load current.However,the traditional parallel current sharing method has some shortcomings,such as poor load adjustment rate and too wide frequency modulation range.In this paper,the working principle and characteristics of a single LLC resonant converter are described.Then,the parallel model of two LLC resonant converters is established by using First Harmonic Approximations(FHA).The unbalance degree is introduced to reflect the unbalance degree of output current of parallel LLC module,and the function of unbalance degree and related variables is deduced.On this basis,the influence of resonant parameters and their combination on the current sharing is analyzed,and the effect of parameter optimization on improving current sharing is predicted.Finally,a strategy of parallel LLC resonant converter based on virtual impedance is proposed.From the impedance point of view,the virtual impedance is used to simulate the series impedance of resonant network,and the equivalent output impedance is adjusted by the series impedance,thereby improving the current sharing characteristics of LLC converter.The design and implementation of a two-phase parallel LLC resonant converter with 400 V input and 12V/80 A output are completed.The experimental results verify the feasibility and effectiveness of the proposed control strategy.