基于拓展型双曲正切函数的全桥LLC变换器大信号建模方法

Large-Signal Modeling Method of Full-Bridge LLC Converter Based on Extended Hyperbolic Tangent Function

连续模型的准确性对于电力电子变换器的特性分析与实时仿真十分重要。由于独特的谐振腔结构,LLC变换器在不同工作频率区间分别以连续电流模式和断续电流模式运行,故而传统方法很难以连续模型实现其多种运行模式的统一描述。为此,该文提出一种基于拓展型双曲正切函数的全桥LLC变换器大信号建模方法。首先,分析各工作频率区间全桥LLC变换器特性,建立一种大信号非连续模型,该模型能够实现LLC变换器多种运行模式的统一描述。然后,构建含陡度因子/脉冲系数的拓展型双曲正切函数,利用其将上述大信号模型连续化,从而建立了全桥LLC变换器的大信号连续模型。基于此模型,连续系统的方法可以被直接应用以实现对变换器特性分析、实时仿真和控制器设计。同时,相比于现有LLC变换器大信号连续模型,该模型具有较低的阶数和更高的精确度。此外,该模型在时域中能够提供次级侧功率器件高准确性的开关信息,便于作为同步整流设计的参考。最后,仿真和实验结果验证了该模型的准确性和有效性。

The accuracy of the continuous model is very important for the characteristic analysis, real time simulation and controller design of power electronic converter. LLC resonant converter operates in continuous current mode and discontinuous current mode respectively in different operating frequency intervals due to its unique cavity structure. Therefore, it is difficult to establish a continuous model which can realize the unified description of various operating modes of LLC resonant converter. Therefore, a large signal modeling method for full bridge LLC resonant converter based on extended hyperbolic tangent function is proposed to establish an accurate large signal continuous model. Firstly, the operating characteristics of full bridge LLC resonant converters in each frequency range are analyzed. Based on this analysis, a large signal discontinuity model of full bridge LLC resonant converter is established by using the symbolic function, the absolute value function and the defined operation mode switching function. The discontinuity model can realize the unified description of continuous current mode and discontinuous current mode of full bridge LLC resonant converter. Then, two kinds of extended hyperbolic tangent functions including steepness factor and pulse coefficient are constructed, and the standardized selection methods of steepness factor and pulse coefficient are provided. By changing the steepness factor and pulse coefficient, the dead time and device switching delay time of the full bridge LLC resonant converter can be approximated with high precision, thus realizing accurate large signal modeling. Based on the extended hyperbolic tangent function, the discontinuous model is continuous, and the large signal continuous model of the full bridge LLC converter is established. Based on this model, the method of continuous system can be applied directly to realize the characteristic analysis and controller design of converter. At the same time, compared with the existing LLC resonant converter large signal continuous model, the model has lower order and higher accuracy. In addition,the large signal continuous model established in this paper can provide highly accurate switching information of the secondary power components of the full bridge LLC resonant converter in the time domain, which is convenient to be used as a reference for the design of synchronous rectifier controller. Finally, the accuracy and effectiveness of the proposed large signal continuous model are verified by building the MATLAB/Simulink simulation model and the experimental prototype with rated power of 5kW. In the full range of operating frequency, the large signal continuous model is presented with high accuracy. Compared with the light load condition, the model proposed in the heavy load condition can describe the operating characteristics of the full bridge LLC resonant converter more accurately. The computing time and resource consumption are slightly higher than the existing large signal continuous model, but the proposed large signal continuous model has higher accuracy. Although the continuous model based on the extended description function method is easier to carry out linearization and small signal perturbation than the proposed large signal continuous model, the order of the model is usually higher, while the proposed large signal continuous model proposed has a lower order. In addition, the proposed large signal continuous model can accurately describe the dynamic characteristics of the full bridge LLC resonant converter.