This project proposes a modified asymmetrical half-bridge fly back converter (AHBFC) to fulfill the single-stage step-down ac-dc universal input voltage application. This topology is obtained from the integration of a buck type PFC, an auxiliary energy buffer, and a conventional AHBFC. By sharing common switches and utilizing transformer’s leakage inductor, the single stage circuit can achieve both high PF and low THD with the simple constant on-time control while regulating the dc output voltage. In addition, it is able to operate in universal input voltage with low dc link voltage and low output voltage. Moreover, the zero voltage switching (ZVS) and the zero current switching (ZCS) features of the power switches can also be achieved. Detailed analyses and design procedures of the proposed converter are given and verified by the experimental results. Finally, the comparison with reported circuit topologies and power loss analyses verified by experimental results are presented to provide the insight of practical design.