To avoid the problems of the high switching frequency of LLC resonant converter under light load and the problem of large output voltage ripples in burst mode, this paper proposes a variable duty cycle pulse frequency modulation (PFM) control method. In this paper, the transfer function of LLC resonant converter under this control method was derived and analyzed by the fundamental harmonic approximation (FHA). An experimental prototype was built to verify the performance of the converter under variable duty cycle PFM. The experimental results show that the variable duty cycle PFM effectively reduces the switching frequency of LLC resonant converter, while the output voltage ripples in this case are smaller than those in burst mode.
