Research On High Performance Control Strategy Of LLC Resonant Converter

The DC charging system is an important support system for electric vehicles,and it is a key condition for solving "difficult charging","slow charging" and realizing the further promotion and development of electric vehicles.The DC charging system is generally a two-pole structure,that is,the front-level AC/DC+the rear-level DC/DC.As the key part of the DC charging system,the rear-level DC/DC converter is directly connected to the power battery.Performance,safety,and reliability all have a major impact.LLC resonant converters are mostly chosen as the post-stage DCDC topology due to their advantages of wide soft switching range,high power density,etc,which are of great research and application value in the field of electric vehicle DC charging systems.However,the current research on LLC resonant converters for DC charging systems still faces many difficulties and challenges.For example,there are many disturbance problems in the operation of LLC resonant converters,mainly including the sudden change of output voltage of the front-stage AC/DC converter,load switching and LLC.The drift of the internal dynamic characteristics of the resonant converter will affect the working performance of the converter and even affect the stability and safety of the system,which puts forward requirements for the immunity and robustness of the system;with the continuous deepening of research on battery characteristics,the proposed charging strategy is becoming more and more complex.It imposes stricter requirements on the dynamic response capability.However,for the LLC resonant converter,the traditional linear control has limited improvement in its dynamic performance.It is also difficult to make the converter output effectively track charging command.Therefore,this paper takes the LLC resonant converter of the electric vehicle DC charging system as the research object,and aims to improve the immunity and dynamic response characteristics of the system,and focuses on the sliding mode control strategy of the LLC resonant converter,which effectively improves the Immunity performance and dynamic response capability of LLC converter.The main content and innovation of this paper:Aiming at the requirements for the immunity and dynamic performance of LLC resonant converters in the DC charging system of electric vehicles,this paper proposes a non-singular fast terminal sliding mode control of LLC resonant converters based on the reduced-order small-signal model of LLC resonant converters.The Novel stability theory verifies the stability of the system.The designed sliding mode controller has a simple structure and achieves good disturbance immunity and dynamic response performance.The voltage loop and current loop sliding mode controllers are designed respectively,and the charging strategy based on this design realizes the output following the voltage or current charging command.The platform is built for experimental verification.The experimental results show that,compared with the PI controller,the designed sliding mode controller can effectively improve the system’s anti-disturbance capability and dynamic response performance.Due to the inherent chattering problem of sliding mode control,there is a large ripple in the output of converter.The ripple entering the power battery will cause capacity attenuation and other hazards to it,and damage the power battery’s health status.Standing around problem,an anti-step sliding mode coordination chattering suppression strategy is further proposed.When the system state is judged to be moving in the sliding mode switching area,the sliding mode control is switched by the switching function In order to control the continuous backstepping control of the control signal,the reduction of the chattering amplitude value in the steady state is realized,and the experimental results also prove the effectiveness of the proposed chattering suppression control strategy.