Research On Key Technology Of Photovoltaic Grid-connected Inverter Based On Full-bridge LLC Resonant Converter

Two-stage high-frequency isolated photovoltaic grid-connected inverter has certain advantages in power density and security, and has wide application in countries and regions with high system reliability requirements. In this paper, the key parameters design method of the resonant converter and the control strategy of the maximum power point tracking (MPPT) were studied emphatically in the pre-stage of two-stage photovoltaic grid-connected inverter with full-bridge LLC resonant converter used. The main contents are as follows:Firstly, the basic conditions to achieve the soft switching of the full-bridge LLC resonant converter primary side switch tube and the secondary side rectifier diode were expounded, and the voltage gain characteristics of the converter was analyzed to reveal the influence rule of the inductance factor K and the quality factor Q on the converter voltage gain. The mathematical expressions of the primary and secondary side current root mean square (RMS) and the turn-off current of the primary side switch tube were derived to get the relationship between the two and the value of transformer primary side magnetic inductance Lm. On this basis, the key points of parameter design of full-bridge LLC converter were summarized, and the correctness of the important conclusions of this paper were verified by simulation.Then, two sets of parameters design methods of transformer turns ratio N,transformer primary magnetic inductance Lm,resonant inductance Lr and resonant capacitor Cr in full-bridge LLC resonant converter were studied and summarized.Through method one, not only the voltage gain required of converter can be satisfied,but also the Zero Voltage Switching(ZVS) of primary side switch tube can be achieved. Besides, the primary side switch tube turn-off current and the primary and secondary side RMS current can also be reduced to reduce the loss of switching devices further. However, through method one, it is possible that the operating frequency range of converter will be too wide at sometimes, and in this case, the design of the pre-filter is inconvenient. In order to solve this disadvantage, method two was summarized. Through method two, the operating frequency range of the target converter can be limited further. The validity of the two sets of parameters design methods above was verified by the experimental results of 200W prototype.Finally, the disturbance observation method of the existing MPPT method was applied to the pre-stage converter,and the experimental study was carried out by the DC voltage source connected in series with the power resistor to simulate photovoltaic. The validity of control strategy when used in the grid-connected inverter of this paper was verified by the experiments.