Voltage-Mode Variable-Frequency Controlled LLC Resonant Power Factor Correction Converter and Its Accurate Numerical Calculation Analysis

LLC resonant converter is a good candidate for single-stage ac–dc converter, which can realize both functions of power factor correction (PFC) and output voltage regulation, and contribute to the improvement of conversion efficiency and power density. However, voltage mode (VM) controlled single-stage half-bridge LLC PFC converter has the disadvantages of large dead-zone of input current and hard-switching operation, since there is no current control. Besides, the voltage ripple of input filter brings asymmetrical operation of the converter between the positive and the negative half-switching cycle. In order to solve these problems, this article presents a modified numerical calculation method, which can realize accurate characterization of the asymmetrical behavior of resonant current and voltage. Based on that, the switching frequency of LLC resonant converter for unity power factor (PF) is derived. A method of fitting the switching frequency is proposed to simplify the circuit implementation. A 250-W/48-V prototype has been built to verify the analysis results. Experimental results show that the proposed voltage-mode variable-frequency (VM-VF) controlled LLC resonant PFC converter can achieve high efficiency of up to 94.08% and power factor of 0.95 over the input voltage range of 180–260 VAC.