LLC谐振变换器开关损耗特性研究

发布时间：2018-08-09 12:18

【摘要】：功率开关管的高频化可以大幅度减小电容、电感及变压器的体积,不但降低了电子元器件的使用成本,还有效提高了变换器的功率密度。但是,简单地提高功率开关管的频率,开关管的开关损耗也会相应大幅增加,造成变换器的效率降低。同时,整流二极管在工作于高频时会有严重地反向恢复问题,不但会带来严重地EMI,增加设计难度,还会进一步降低变换器的效率。传统的PWM变换器受掉电保持和开关损耗的限制已很难满足人们对高频高效的要求。在此背景下,LLC谐振变换器应运而生。LLC谐振变换器不但可在全负载范围内实现开关管及二极管的软开关,还可实现高度的磁集成,具有高效率、高功率密度、低EMI噪声、宽电压输入范围等优点,已广泛应用于中小功率等级的电子产品中。LLC谐振变换器与移相全桥变换器在不同功率等级的市场获得了广泛的应用。国内外文献对两者的优劣进行了深入研究,但鲜有文献对其开关损耗进行对比分析过,又因LLC谐振变换器是PFM工作方式且关键参数较多,尚存在控制方式复杂,参数选取困难,谐振电流过载保护等诸多问题。针对这些问题本文主要开展了以下工作:首先,给出了LLC软开关分布式电源系统设计方案,介绍了EMI滤波器的原理、APFC电路的原理及基于UC3854的SABER仿真模型、掉电保持电容的计算过程,重点分析了半桥型LLC谐振变换器的稳态建模,利用ORIGIN绘制了电压增益特性曲线,在此基础上分析了关键参数对变换器电压增益的影响。其次,建立了LLC谐振变换器等效电路模型,并运用扩展描述函数法进行小信号建模,得出了变换器电压增益数学模型,证实了FHA建模法的正确性。再次,建立了LLC谐振变换器开关损耗数学模型,结合课题的要求,给出了变换器核心参数基于MATHCAD的设计流程及开关管和二极管的选取过程并完成了开关损耗功率计算。基于SABER仿真对移向全桥PWM ZVS变换器和半桥型LLC谐振变换器在同样输入、输出、开关频率下的开关损耗进行了对比研究。最后,基于SABER仿真对半桥型LLC谐振变换器基本型与改进型进行了对比实验,验证了改进型的谐振电流过载保护能力,并对改进型进行了小信号时域仿真,得出其输入频率变化到输出电压的波特图,再结合波特图分析进行闭环补偿控制器设计,结果表明LLC谐振变换器在不同带载情况下均可稳定输出。
[Abstract]:The high frequency of power switch can greatly reduce the volume of capacitance, inductance and transformer, which not only reduces the cost of electronic components, but also increases the power density of converter. However, if the frequency of the power switch is simply increased, the switching loss of the switch will increase significantly, resulting in a decrease in the efficiency of the converter. At the same time, the rectifier diode will have a serious reverse recovery problem when it works at high frequency, which will not only bring serious EMIs, increase the design difficulty, but also further reduce the efficiency of the converter. The traditional PWM converter is difficult to meet the requirements of high frequency and high efficiency due to the limitation of power off and switching loss. Under this background, the LLC resonant converter emerges as the times require. The LLC resonant converter can not only realize the soft switching of switches and diodes in the full load range, but also achieve high magnetic integration, with high efficiency, high power density and low EMI noise. The wide voltage input range has been widely used in medium and small power electronic products. LLC resonant converters and phase-shifted full-bridge converters have been widely used in the market of different power levels. The merits and demerits of the two have been deeply studied in domestic and foreign literature, but few literatures have compared and analyzed their switching losses. Because the LLC resonant converter is a PFM operation mode and has many key parameters, the control mode is complex and the parameters are difficult to select. Resonance current overload protection and so on. The main work of this paper is as follows: firstly, the design scheme of LLC soft switching distributed power supply system is given, and the principle of EMI filter and SABER simulation model based on UC3854 are introduced. In this paper, the steady-state modeling of half-bridge LLC resonant converter is analyzed, and the characteristic curve of voltage gain is drawn by ORIGIN. On the basis of this, the influence of key parameters on voltage gain is analyzed. Secondly, the equivalent circuit model of LLC resonant converter is established, and the small signal model is built by using the extended description function method. The mathematical model of voltage gain of the converter is obtained, which verifies the correctness of the FHA modeling method. Thirdly, the mathematical model of switching loss of LLC resonant converter is established. According to the requirements of the project, the design flow of the core parameters of the converter based on MATHCAD and the selection process of switch tube and diode are given, and the power calculation of switching loss is completed. Based on SABER simulation, the switching losses of full-bridge PWM ZVS converters and half-bridge LLC resonant converters at the same input, output and switching frequencies are compared. Finally, based on the SABER simulation, the comparison between the basic and the improved LLC resonant converter is carried out, which verifies the improved resonant current overload protection capability, and makes a small signal time domain simulation of the improved one. The baud diagram of the input frequency changing to the output voltage is obtained, and the closed-loop compensation controller is designed based on the Porter diagram analysis. The results show that the LLC resonant converter can output stably under different load conditions.
【学位授予单位】：东华大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TM46

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