Optimal Trajectory Control For Dynamic Performance Of LLC Resonant Converters

The control of the LLC resonant converter is much more difficult than PWM converter because of the fast-dynamic characteristic of the resonant tank.The natural frequency of the resonant tank is very close to the switch frequency of the converter,Since the average modeling method will eliminate the information of the switching frequency,it cannot predict the dynamic performance of the resonant tank.In order to get better dynamic characteristic and limit the resonant circulating energy,the state plane trajectory analysis mothod is adopted to optimize the start and load transient condition,the following work has been done:The method,principle of state trajectory and steady state trajectory derivation of half bridge LLC resonant converter are analyzed,the mathematical model from the equivalent circuit in different states and the expression of circuit equation are established,the trajectory when the half bridge LLC resonant converter starts,and the dynamic response trajectory when the load is step up or step down under the traditional linear controller are analyzed.In terms of soft start,the soft start scheme of half bridge LLC resonant converter is studied,and the mathematical model of each soft start process is established one by one,and the current limiting mechanism with the peak value of full load current as the maximum value is established,so that the maximum peak value of resonant current before the completion of soft start does not exceed this limiting current under any load condition.In terms of load change,the trajectory changes during load trasient is analyzed and the response mechanism of the controller at the time of load trasient.The specific mathematical expression of the increase or decrease of the pulse width during load trasient is derived,and the optimal trajectory control strategy is obtained.Based on the above research,the MATLAB / Simulink simulation model of half bridge LLC converter system is built,and the correctness of its circuit topology and control strategy are verified by simulation.The simulation model of the optimal trajectory control soft start is established.and also compared with the traditional frequency reduction control.The simulation verification of the dynamic response of the load transient is completed.Under the load trasient condition,the simulation results are compared with traditional control methods.Finally,a half bridge LLC low power experiment platform is constructed,and the FPGA control program design based on Verilog HDL is completed.The experimental results of soft start under different load conditions and load transient show the correctness and feasibility of the circuit topology and control strategy.