Research On The Control Strategy Of LLC-buck Converter With Series Input Parallel Output

Modern electric vehicles vary their electronic equipment voltages from 48 V to 1.8V,which requires on-board DC/DC converters to be significantly reduced voltage and excellent dynamic response.Single-stage Input-Series Output-Parallel(ISOP)converters can achieve large voltage drop and large current output,but the structure of the converter has different power distribution ratios,slow transient response and complex controller design.In order to solve the above problems,this paper takes the ISOP LLC-Buck converter as the research object,and studies and designs its topology and control strategy.Firstly,on the basis of the overall introduction of the topology converter,the working principle and modal analysis of the two parts of the converter are carried out in detail respectively.The gain,frequency and impedance characteristics of the LLC converter are analyzed.According to the conclusion,the LLC resonant converter should work in the switching frequency is less than and close to the resonant frequency.This frequency range can better complete the zero-voltage conduction of the primary side switch tube and the zero-current turn-off of the secondary side rectifier tube.In addition,several basic control modes of Buck converter are introduced and analyzed.Based on the analysis of each part of the converter,a master-slave control mode for the whole converter is proposed.LLC resonant converter as the main module,working in open loop state processing most of the power;Buck converter is a slave module,responsible for voltage regulation of the converter.Soft start and synchronous rectification control strategies are proposed for the main module.Aiming at the slow response of the traditional voltage feedback controller to the load transformation,the output voltage and inductance current ripple of the slave module are proposed as the fixed conduction time control strategy of the modulation signal,and the stability of the controller is analyzed by using the Bode diagram of the parameters.MATLAB/Simulink software is used to simulate the stability and dynamic of the converter system,and the results show that the control strategy is doable.Finally,the main components and circuits of the combined converters were selected and designed,and an experimental prototype with 48 V input and 1.8V/24 A output was made.Experimental results show that the control strategy proposed in this paper can achieve stable operation of the system and improve the transient response ability of the system,which is feasible.