Research On Key Technology Of LLC Resonant DC Power Supply

With the rapid development of switching power supply technology,the traditional circuit topology has been unable to meet the requirements of high efficiency and high power density.Since LLC resonant converters can not only realize the zero-voltage-switching of the primary side switch tube,but also realize the zero-current-switching of the downstream rectifier diode,it is widely used in various fields.This dissertation takes the half-bridge LLC resonant converter as the object to carry out research.First,the working principle of LLC resonant converter is introduced,and six working modes under the under-resonance state are analyzed in detail.The voltage gain relation formula of LLC resonant converter is obtained by first-harmonic-approximation analysis,and the parameters of the resonant cavity are analyzed and calculated.At the same time,the validity of the parameters is verified in MATLAB.Secondly,the small-signal model of LLC resonant converter is derived by using the extended description function method to obtain the control output transfer function of the system and analyze the stability of the system.Then the PI controller,the fuzzy PI controller and the sliding mode controller were designed respectively,and the MATLAB simulation model was built to compare the results of the subsequent load mutation and the previous voltage mutation,and finally the sliding mode control with good control effect and simple structure was selected.As the control strategy of this article.When the resonant converters are used in parallel,the parameters of the resonant cavity are different due to the process,resulting in uneven distribution of the output power of each module.Therefore,a phase shift control strategy is introduced,which changes the voltage gain of each module by increasing the duty cycle to make the subsequent power distribution evenly.In addition,a parallel model of dual-module resonant converters was built in MATLAB to verify the feasibility of phase-shifting control for current sharing.Finally,an experiment platform for LLC resonant converter was built based on theoretical analysis,and a detailed introduction to the selection of main circuit components and main circuit diagrams.On this experimental platform,the soft-start of the single-module LLC resonant converter,the resonant current and the rectifier diode current in the three resonant states,the load experiment and the efficiency test are carried out.Experiments show that the LLC resonant converter can realize the ZVS of the transformer primary MOS tube in the full working range,and the ZCS of the diode in the working area of f_m＜f_s≤f_r;In the case of sudden increase and decrease of load resistance,the output voltage can reach stability after slight fluctuations.The two resonant converters are tested in parallel,and the current waveforms of the rectifier diodes are compared when add phase shifting-control and no phase-shifting control.The results prove that the current imbalance can be reduced to less than 5%when the phase shifting-control is added,which can reach the engineering design requirements.