Design Of Constant Frequency LLC Resonant Converter Based On Flexible Inductor

With the continuous development of economy,the demand for energy is increasing rapidly,and the energy crisis and environmental pollution problems have become more serious.Using new energy to generate electricity is an important measure to solve these problems.Photovoltaic power generation has become one of the main power generation methods because of its safety,reliability,high efficiency and no pollution.However,the cost of traditional photovoltaic power generation system is high and it is difficult to develop on a large scale,so photovoltaic micro inverter is applied.This paper is dedicated to study the former stage DC/DC converter with boost function.Soft-switching technology reduces switching loss by adding resonant circuit to DC/DC converter.Common resonant converters include LC series resonant converter,LC parallel resonant converter and LLC resonant converter.The main control strategy is variable frequency control with high conversion efficiency.However,when the input voltage and load range are wide,the switching frequency of the converter varies widely,so it is difficult to optimize the design of magnetic components.Therefore,a constant frequency control strategy is proposed in this paper,in which a flexible inductor is used in the resonant converter,and the gain of the resonant circuit is changed by adjusting the inductance of the resonant inductor,and then the output voltage is adjusted.The control strategy keeps the switching frequency constant and provides conditions for the optimal design of magnetic components such as transformer in the converter.In the first section,the operation principle and gain characteristics of LC series resonant converter,LC parallel resonant converter and LLC resonant converter are analyzed in detail.Since LLC resonant converter combines the advantages of LC series resonant converter and LC parallel resonant converter,and can realize the soft-switching of power devices in the full input range,take it as the research object of this paper.The gain characteristics of LC series resonant converter and LC parallel resonant converter with flexible inductor are introduced.On this basis,the principle of LLC resonant converter based on flexible inductor is explained,and its gain characteristics are analyzed by the basic wave component simplification method.The gain formula and gain curve of resonant circuit with flexible inductor are deduced.The structure and design method of flexible inductor are introduced.In addition,the closed-loop control scheme of the converter designed in this paper is given,and the output constant voltage is regulated by voltage single closed-loop.Through the simulation of saber,the converter designed in this paper is proved to be feasible.Although the traditional analog control method is simple,but the circuit structure is complex,it is difficult to meet the high-precision applications.Therefore,this paper presents a digital control scheme based on DSP.The hardware parts mainly include the design of driving circuit,sampling circuit,control voltage output circuit,voltage control current source circuit and main circuit of the proposed converter.The software parts mainly include the generation of PWM drive waveform,the generation of control voltage PWM waveform and the design of PI controller,also give the flow chart of main program,interrupt program and PI algorithm subprogram.Finally,a experimental prototype of 23-35V input and 100V/200W output is made.Experimental debugging and waveform analysis of the prototype are carried out.The experimental results show that the proposed constant frequency control strategy is feasible.The designed converter can output constant voltage in a wide range of input voltage and load.