USB power delivery chargers have a wide output voltage range for various charging applications. In this paper, a two-stage converter for USB-PD is implemented to meet the requirement of harmonic standard IEC61000-3-2. The front stage is a boost power factor corrector to improve the power factor, and the second stage is an asymmetrical half-bridge flyback (AHBF) converter, achieving soft-switching operation for output voltage regulation. The operating principles and control methods of AHBF for achieving primary-side zero-voltage switching and secondary-side zero-current switching are presented. The key parameters for AHBF converter are optimized, and the PFC stage is disable in light load condition to improve the system efficiency. Finally, a 240 W USB-PD 3.1 experimental prototype is built with ac input voltage of 90 Vrms ~ 264 Vrms with the output voltage/current of 5 V/ 3 A, 9 V/ 3 A, 15 V/ 3 A, 20 V/ 5 A, 28 V/ 5 A, 36 V/5 A and 48 V/ 5 A. The experimental results show that the highest efficiency of the boost PFC converter is 97.2%. The highest power factor is 0.99 at full load condition. The maximum efficiency of AHBF is 91.3%, and the overall system efficiency can reach 88.6%.
