Bode plot and impedance asymptotes for light-load regulation of LLC series resonant converter

In general, the LLC series resonant converter (LLC SRC) is an attractive topology for the applications which require the wide input variation and high efficiency especially at light load condition. This is because of its wide gain capability with pulse-frequency modulation and zero-voltage-switching over the entire load condition. However, there is a regulation problem that the output voltage increases as the output current decreases. In this paper, Bode plot and impedance asymptotes analyses have been conducted to obtain an intuitive sense for the regulation characteristic of LLC SRC under light-load condition. Moreover, in order to improve the regulation characteristic, a new resonant tank with an additional capacitor is proposed and its design guideline is also provided by Bode plot and impedance asymptotes analyses. Therefore, the proposed LLC SRC achieves the very light-load regulation while it can maintain the advantages of typical LLC SRC.