Research On LLC Resonant Converter Based On Three-level Full-bridge

As the country vigorously promotes energy conservation and environmental protection,higher requirements are placed on the power electronics industry.High-power DC/DC converters play a key role in power electronics technology.How to improve the efficiency of DC/DC converters and improve product performance is very important meaning.LLC resonant converter is an ideal DC/DC converter topology due to its simple circuit structure,high power density,high efficiency and easy integration.In this paper,a three-level full-bridge structure LLC resonant converter topology is used to study the improvement of the dynamic response performance of the converter and the problem of multi-module ISOP current sharing.First,this paper analyzes the basic working principle of the full-bridge three-level LLC resonant converter,and establishes the equivalent simplified circuit of the full-bridge three-level LLC resonant converter through fundamental harmonic approximation(FHA)method,and performs voltage gain characteristics on the converter Analyze the influence of quality factor Q and inductance k on gain.Secondly,the circuit is analyzed through the state plane trajectory method,and the PI control strategy with SOTC(Simplified Optimal Trajectory Control)is proposed.When the circuit has a sudden load change,only the output current needs to be sampled,the pulse width change is calculated,and the pulse width of the switch tube is adjusted.The system can quickly return to the desired state track,and then gradually eliminate the steady-state error,so that the system can quickly return to steady state,and the dynamic response speed of the traditional PI control system is significantly improved.At the same time,in view of the slow startup of LLC resonant converter and excessive startup current,based on the state plane trajectory of LLC circuit,the relationship between the switching period and the output voltage during the startup process is deduced,and the next switching period is found by the look-up table method,and the switching tube is controlled,which reduces the amount of system calculation and ensures the resonance during the circuit startup process,the resonant cavity current does not exceed the set value,and the circuit always works in the inductive region.Aiming at the difficulty of a single-structure resonant circuit to adapt to the high-power output environment,this paper proposes a full-bridge three-level LLC resonant converter input terminal series output terminal parallel(ISOP)structure,which can make each switch tube voltage stress only four One part,it is convenient to choose the switch tube.On the issue of current sharing between modules,a master-slave control strategy is cited.This current sharing uses the main module for voltage and current double closed-loop control,and the slave modules are controlled by the current loop,which can well realize the current sharing of each module,and the response speed fast.Finally,on the basis of theoretical analysis,this paper designs the component parameters of the full-bridge LLC resonant converter.builds a two-module full-bridge three-level LLC resonant converter simulation model in Matlab/Simulink,The feasibility of the PI control strategy with SOTC on a single module is verified,and the effectiveness of the master-slave current sharing strategy of the two-module ISOP type full-bridge three-level LLC resonant converter is verified.