Research On Control Strategy And Dynamic Characteristics Of Wide-range LLC Resonant Converter

In recent years,many countries and auto companies in the world have begun to turn to the research and development of environment-friendly new energy vehicles,and the market share and ownership of electric vehicles have reached a new high.New types of high-voltage power batteries emerge in endlessly in various vehicle enterprises.While realizing the super fast charging technology of electric vehicles,it also puts forward the requirements of wide range,high frequency,high power density and flattening for the DC/DC converter in the vehicle auxiliary power supply system.The third generation wide band gap semiconductor devices are very suitable for the design of high frequency and high power density power electronic converters below 1000 V due to their low on resistance and high electron mobility.In this paper,the design,control strategy and dynamic characteristics of stacked bridge three-level LLC resonant converter are studied,and the corresponding optimal control strategy is proposed to achieve the wide range input and output index of the converter.Firstly,based on the requirements and design specifications of the whole converter,the stacked bridge three-level LLC resonant converter topology is selected for related research.Then the basic working principles of the variable-frequency mode and fixed frequency PWM mode are introduced and analyzed,based on the asymmetric driving signals between the upper and lower bridge arms,the principle of the unbalanced voltage of the divider capacitors is analyzed under the three working modes: half-bridge LLC,frequency doubling and PWM three-level mode,and the reason why the phase-shift three-level mode is not adopted is explained separately.The source of the voltage-balancing problem is pointed out,then the control law of the divider capacitors’ unbalanced voltage is summarized.On this basis,a control strategy is proposed to achieve the voltage balance of the divider capacitors by adjusting the phase difference of the driving signals of the upper and lower bridge arms’ switches.In order to further achieve the wide range input and output index of the converter,according to the principle and characteristics of each mode,it is pointed out that the topology is suitable for half-bridge LLC mode and frequency doubling mode in high and low gain region respectively,the gain of PWM three-level mode is between the other two modes,so PWM three-level mode can be used in the middle gain region.Then,the voltage gain range width WG and the cavity gain range width WM of LLC converter are defined.By dividing the voltage gain range,the wide range operation characteristics of stacked bridge three-level LLC resonant converter are analyzed,finally,the conclusion is that the width of the resonance gain range can be minimization by using a single mode or multi mode combination strategy at different voltage gain range width.On this basis,a hybrid control strategy of stacked bridge three-level LLC resonant converter is proposed.Then,the small signal model of the converter is built,and the dynamic characteristic analysis and controller design are carried out.Finally,the parameter design and devices selection of the stacked bridge three-level LLC resonant converter are carried out.An experimental prototype with400-800 V input,25-40 V output and 2k W maximum output power is built to verify the feasibility of theoretical analysis and control strategy.