A Variable Duty Cycle PFM Control Method for LLC Resonant Converter

LLC resonant converter is a very advanced converter, but it has the problem of excessive switching frequency under light loads. The typical solution is using burst mode, but it will significantly increase the output voltage ripple. In this paper, a variable duty cycle pulse frequency modulation (PFM) control method is proposed to address this problem. The transfer function of LLC resonant converter under this control method is derived by using fundamental harmonic approximation (FHA), and the transmission characteristics are analyzed. 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.