Abstract: LLC resonant converter has outstanding advantages in high power conversion efficiency and wide range adaptability of input voltage, but research on the peak gain is clearly insufficient. Therefore, the peak gain of LLC resonant converter is studied. According to the operation process of the series-parallel resonance, the converter can be divided into three resonant modes: inadequate resonant mode, complete resonant mode and over resonant mode. When the converter operates in complete resonant mode, it reaches its maximum value, and the corresponding switching frequency is named critical frequency. Finally, derivation of time-domain expression of the peak gain and the critical frequency is completed through the establishment of the converter state equations, and the accuracy of theoretical analysis is verified through simulation and experiment.

Key words: LLC resonant converter, resonant modes, critical frequency, peak gain