Research On Soft Start-up Scheme Of Trajectory Control And Its Implementation Of LLC Resonant Converter

The spacecraft power supply system is the power source of the spacecraft.When the spacecraft completes space missions such as deep space exploration and posture adjustment,it provides energy for the system and is called the heart of the spacecraft.The isolated DC/DC converter connects the high-voltage DC bus and DC load of the spacecraft,and provides electrical isolation for the two voltage levels,which is of great significance for the safe operation of the spacecraft.LLC resonant converter belongs to the resonant converter,which has the advantages of good EMC characteristics,high degree of magnetic integration and high working efficiency.It is suitable for using as an isolated DC/DC converter in the power supply system of spacecraft.At present,the application of LLC resonant converter still has the following problems: inappropriate dead time at high frequency will affect the working condition of the circuit,and the dead time is often selected based on empirical values;the small signal models is often not accurate enough;the overshoot of the resonance tank and the output voltage during the start-up process is large,which threatens the normal operation of the circuit.In this dissertaion,the dead zone analysis,trajectorycontrol soft start-up scheme and small-signal modeling of LLC resonant converter are studied,and the completed work is as follows.The resonance frequency is taken as the point that divides the working frequency area of the converter into three parts.The typical waveform is analyzed in the time domain to illustrate the working principle of circuit in three frequency regions.Then,the resonant capacitor voltage and the resonant inductor current are selected as the state variables of the circuit,and the steady-state natural trajectories of critical conduction mode,continuous current mode and discontinues current mode are deduced,respectively.State trajectories of aforementioned modes combined with time-domain waveforms are utilized to analyze the influence of different dead time selections on the steady-state operating conditions of the converter.Based on the comparation,suggestions for designing dead time are given.Rapid transient change of the resonant tank happens during the start-up process of the converter,which leads to large voltage,current stress and output voltage overshoot.To limit the overshoot,the state trajectories under different start-up strategies are analyzed,and a soft start-up scheme based on trajectory analysis is proposed.By pre-calculating the whole soft start-up process,the sampling circuit of the inductor current can be omitted,and the calculation speed requirement of the digital controller is also reduced,so that the resonant voltage and current do not overshoot throughout the soft start-up process,and the output voltage can be established smoothly at the same time.The LLC resonant converter has a resonant tank with complex dynamic characteristics,so the modeling methods for PWM converter is not suitable for establishing its small signal model.This dissertaion investigates the existing modeling methods of resonant converters,and selects the extended description function method to model the LLC resonant converter.According to the frequency response characteristics of the circuit,linear compensator under rated operating conditions is designed to realize the accuracy requirements under steady state.In order to verify the validity of the soft start-up scheme proposed in this dissertation and the control accuracy of the designed compensation network,soft-start simulation and dynamic characteristics simulation are carried out on the PSIM platform.An experimental prototype is designed and built for scheme verification experiment.The scheme proposed is verified by the simulation and experiment results.