AC-DC converters are key equipment to interface the AC grid, DC loads, and renewable generation sources. Efficiency and power density are the main factors in the design and implementation of AC-DC converters. The two-stage power conversion has low system efficiency and high cost. A single-stage AC-DC converter achieves AC-side current regulation, DC-side voltage regulation, and high-frequency galvanic isolation simultaneously through only one stage of high-frequency power conversion, which has the potential advantages of high efficiency and power density. However, the design and implementation of single-stage AC-DC converters are difficult.

This paper presents a resonant single-stage isolated AC-DC converter based on a fixed frequency pulse width modulation strategy. When the switching frequency of the converter is set to the resonant frequency of the series-resonant tank, the impedance of the resonant tank always features zero impedance. Therefore, in steady-state, the total voltage applied on the resonant tank must also be zero, which means the fundamental voltage generated by the primary-side and secondary-side switching bridges must be equal. Following this idea, the converter can operate in both voltage step-down and step-up modes, and the equivalent voltage gain of the converter is continuously adjustable in a wide range by adjusting the pulse width of the high-frequency excitation voltages applied on the resonant tank. Hence, the voltage and current regulation requirements of the single-stage AC-DC converter can be satisfied. Voltage step-down regulation can be achieved by adjusting the primary-side duty ratio D_p, while the voltage step-up regulation can be achieved by adjusting the secondary-side duty ratio D_s.

In order to realize the soft-switching of all switches within a wide voltage range, the soft-switching characteristics of the converter are analyzed in detail. It is found that the magnetizing inductance L_m and quality factor Z_r of the resonant tank must be small enough within the entire AC voltage range, leading to much higher conduction losses. When the instantaneous AC voltage is low, the switching losses of switches are also low. Therefore, it is unnecessary to achieve soft-switching within the entire AC voltage range, and trade-offs between switching loss and conduction loss must be made to design the converter’s parameters. Therefore, an optimized parameters design method is proposed for the resonant single-stage AC-DC converter.

An experimental prototype is built and tested. The experimental results indicate that through the fixed-frequency pulse-width modulation strategy, step-up and step-down power conversions, the AC and DC side voltage and current regulation, and high power factor can be realized. With the proposed parameter optimization design method, soft switching of switches can be achieved in a wide input voltage range. The efficiency of the converter is up to 94.2%. In addition, experimental results indicate that the converter has excellent dynamic and steady-state performance.