Photovoltaic Systems Research On LLC Resonant Converter

With the wider apply of secondary power as supercapacitor in areas such as photovoltaic power generation, high power microwave system, uninterruptible power supply, electric vehicles, communications power supply and aviation, bi-directional DC-DC converter emerged and has quickly become the focus. High efficiency and high power density is modern power electronics technology development trend. The soft-switching technology is the key to improve the power and efficiency of the operating frequency, and it can greatly reduce the size and weight of the power to further increase power density. Resonant converter itself has the advantages of achieving soft-switching and has become a hotpot studied by researchers in recent years. This thesis is based on LLC resonant converter and then presents a symmetrical bidirectional LLC Resonant DC-DC converter. It verifies symmetry of energy flow in both directions, and puts emphasis on the inverter control method when the load is capacitive. This paper also optimizes FM control area and finally achieves soft-switching switch.This thesis firstly describes the meaning of bidirectional DC-DC converter existence, has a detailed introduction of its practical application based on its application in PV system secondary power as supercapacitor, and summarize of its research status and classifications. Then it clarifies the soft-switch technology and the advantages of resonant converter. It compares the advantages and disadvantages of three conventional resonant converters and makes a summary. And then the paper turns to LLC resonant converter (LLC Resonant Converter, LLC-RC), It detailedly introduces its three operating modes based on unidirectional LLC-RC, and gives the typical operating waveforms.The paper builds the steady-state model on LLC-RC, gives the voltage gain curve, the current value of the curve and ZVS achieving condition using Fundamental Harmonic Approximation, FHA.Adding additional inductance in bi-LLC-RC, using FHA model design circuit parameters, it builds bi-LLC-RC simulation models to verify the symmetry of two-way flow of energy and soft switching characteristics, for the subsequent provide the basis on the capacitive load charging power application. It expounds LLC-RC control method with capacitive load, establishes the simulation model, determines the frequency modulation control mode, and executes error analysis for FHA model and all these steps provide a theoretical basis for selecting FM working area. And by simulation, it verifies the validity of nonlinear analysis. To solve the problem of excessive inrush current at the start charging, the thesis improves control method using a two-phase PWM control plus FM. It then designs to establish full-bridge LLC-RC simulation model of the capacitive load, which realizes a rated output of 60V within the input voltage ranging from 600V to 300V and the basically stable output current. And the input side of the LLC-RC switch basically achieves ZVS during the whole process. In the working phase when switching frequency is lower than the resonant frequency, no rectifier diode has the problem of reverse recovery current.Finally, based on TI’s TMS320F28027 control procedures have been written. And the paper designs the circuit parameter through practical calculation, and builds experimental platform to verify LLC-RC with capacitive load. The input voltage is within the range of 200V-100V, output reached a maximum voltage 160V, and the gain adjustment range is 0.8-1.6. The experiment has verified the reasonableness of selected working area and the correctness of plus FM PWM control mode, and has also verified the correctness of realizing soft-switching.