Research On Wide Input Range And High Efficiency Of LLC Resonant Converter

With the energy crisis and environmental pollution becoming more and more serious,people pay more attention to make full use of renewable energy.The renewable energy power generation unit have a general characteristic of wide output voltage range,so switching power supplies with wide input range has widely used at the second stage of the generating unit.In the current study,the adopted topologies of cascade,series parallel and multi-mode structure are relatively complex,and can not satisfy the requirements of high effic iency and wide input range for the converter.In this thesis,with the goal of the converter having the adaptability of wide range input,high effic iency and high stability,a method of combining Variable Frequency Multiplier(VFM)technique and Burst mode control strategy was adopted.A LLC resonant converter with the efficiency advantages of the LLC circuit was designed,which can satisfy the wide input range and has high effic iency at any load.The main work of this thesis can be summarized as follows:(1)Working princ iple of the converter was analyzed and realization of wide input range for the converter was researched.To solve the problem of the LLC resonant converter is not satisfy the wide input range,the topological structure was studied.A Variable Frequency Multiplier(VFM)technology was adopted,which narrows the input voltage range of the resonant circuit.So the converter can achieve a wide input range,and has high effic iency at more than 20% load.Then the working principle of the converter was analyzed.Gain characteristics of the converter,the input impedance characteristics,the ZVS conditions and voltage balance characteristic of the input capacitors were studied.(2)Main circuit of the converter was designed.According to the design parameters and structure of the circuit,main circuit parameters of the converter were designed in detail.Then the components of the converter were designed and selected,mainly including the selection of power switch,the design and selection of resonant capacitor,resonant inductor and excitation inductor;the design of transformer;the selection of output filter capacitor and output rectifier diodes.(3)Control system of the converter was designed.Small s ignal model of the converter was established by using the extended describing function method and the transfer function was derived.According to the different operating modes,the corresponding controller of the converter was designed,which makes the system can satisfy the requirement of dynamic response.To solve the problem of LLC resonant converter having low effic iency at light load,the Burst control mode was adopted,and the design was carried out in detail,so that the converter can achieve high efficiency when working at light load.(4)Software and hardware of the control system were designed based on DSP.Digital control method was adopted.Hardware circuits of the control system were designed,which mainly including the design of the driving c ircuit,the output voltage sampling c ircuit,the output current protection circuit,the digital sampler and so on.Meanwhile,the main program and subroutine were designed,and the digital PI controller was completed through discretization.In this way the digital control of the converter is realized.(5)The circuit was simulated and verified.Using PSIM software simulates the circuit,the results show that the theoretical design is reasonable.A 24V/1.5KW prototype was developed.The test data show that the converter can achieve wide input range of 100 V to 400 V.Under the input,the efficiency can reach 94.8%~96.5% at the load more than 20% and achieve more than 93.5% at the load 5%~20%,while the effic iency is more than 87.8% on the condition of at the load less than5%,which can verify the feasibility of the theoretical analysis.The LLC resonant converter can satisfy the requirements of a wide input range,and can realize ZVS with high effic iency at any load,which also has the advantages of stable output voltage,good dynamic performance and high control precision,etc.It has a excellent application prospect in the renewable energy power generation.