Steady-State Characterization of LLC-Based Single-Stage AC/DC Converter Based on Numerical Analysis

The LLC-based single-stage ac/dc converter can achieve zero voltage switching under full load range, which offers a very attractive solution to realize high efficiency, low cost, and extreme compactness of ac/dc power conversion. However, with the introduction of LLC resonant tank in this ac/dc conversion, it becomes very challenging to accurately analyze the steady-state characterization of the converter due to its complex resonant circuit behaviors and time-varying ac input voltage. Although some analysis methods, like first-order harmonics approximation (FHA), extended FHA, and multiharmonic approximation, etc., can be adopted to analyze this converter, they cannot obtain the accurate gain curves, especially when the switching frequency deviates far from the resonant frequency. In this situation, the numerical analysis method is offered to cope with the difficulties resulting from time-varying ac input and dc output fluctuation. A set of equations describing resonant circuit behaviors, power conservation, inductor current, and capacitor voltage are presented. Also, the numerical analysis procedure is given in detail. The numerical calculation results are found that they match with those results in PSIM simulation pretty well. A 1-kW 48-V output prototype is built to verify the effectiveness of the numerical analysis.