Analysis And Optimization Of Operation Dynamic For LLC Resonant Converter

With the development of communication system, computer and consumer electronics, more efficient and smaller power supplies become a widely demand of customers. LLC resonant converter is broadly studied and used for its ZVS of main switches, high efficiency, high power density as well as wide operation range.However, unlike the traditional PWM converters, the dynamic of LLC resonant converter is quite complicated, causing some problems for its dynamic analysis and controlling. The resonant frequency of LLC resonant converter is close to switching frequency and the operating characteristics vary with switching frequency and load, leading to non-linearity in the resonant tank, which is very difficult to analyze with the traditional linear analysis method. The dynamic of resonant tank is very fast, while the output dynamic is quite slow. With the large difference in the dynamic characteristics, it's easy to cause current stress in the resonant tank, threatening the safety operation during some rapid dynamic process, such as the start-up process and load transient.In order to analyze the dynamic of LLC resonant converter accurately, the operation modes in different operating frequencies are analyzed firstly. 9 different modes are developed. By analyzing the transition conditions of different modes, the numerical model of LLC resonant converter is established, which can exactly describe the changing process of resonant variables during switching period and can be used to analyze quick dynamic process of LLC resonant converter.With the numerical model, the start-up process of LLC resonant converter is analyzed and optimized. Besides, the numerical model of frequency decreasing method is established. With this model, the relationship between current spike and start-up frequency is derived, with which the appropriate start-up frequency can be chosen. In order to reduce the start-up frequency, a PWM start-up strategy is proposed, and the relationship between the start-up current spike and the start-up duty is revealed. The analysis shows that LLC resonant converter can start up at resonant frequency with this strategy. To limit the peak value of current to the setting value during the start-up process, the relationship between start-up duty and output voltage is revealed. Based on this relationship, the start-up process is optimized.To analyze dynamic of LLC resonant converter, this paper sets up a time based small signal analysis model, with which the effect of switch frequency and output parameter on the small signal characteristics is analyzed. According to the result, the design guide of single loop control is presented. During the load transient, it is a necessity to improve bandwidth of single loop control to speed up output dynamic, while large overshoot will appear in resonant current. To solve this contradiction, a peak value control strategy is proposed. By adding a peak resonant current inner loop, the peak current during the load transient can be control effectively.In conclusion, this paper analyzes and solves current spike problem during the start-up and load transient of LLC resonant converter to ensure the safety operation.