Time Domain Simplified Equations and its Iterative Calculation Model for LLC Resonant Converter

LLC resonant converters can achieve soft switching and loss reduction. However, the analysis methods of wide frequency range LLC converters generally have problems with incomplete working condition analysis at non-resonant frequencies and there is always a tradeoff between the accuracy and the simplicity. These problems will affect the loss calculation, synchronous rectification and so on. The research objective of this paper focuses on the full-bridge LLC resonant converter. In this paper, three conditions are analyzed in time-domain including switching frequency lower than the resonant frequency at heavy load, switching frequency lower than the resonant frequency at light load, and switching frequency larger than the resonant frequency. Based on proper assumption and simplification, the approximate equivalent of the trigonometric function is used to simplify the complex time-domain equations. The simplified equations are obtained with the resonant capacitor voltage and resonant inductor current as the key variables. In order to make the simplified equations easy to use, an iterative calculation model is proposed with more simplicity than sophisticated software to solve nonlinear equations. The simplified equations and the model proposed in this paper are verified by comparison with the fundamental analysis methods, the simulation, and the experiments. By using the iterative calculation model, the voltage, current, time and other variables of switching points can be obtained with the relative error less than 3% to theoretic values, which can be used in loss calculation, synchronous rectification and so on. And the iterative calculation model can be realized on DSP or other processors in real-time.