Secondary-side center-tapped transformer is a common transformer design of the high step-down forward converters owing to its effective reduction of the copper loss. Particularly, the extremely high step-down converters tend to have one-turn secondary coils to increase the winding turn ratio. However, due to the proximity effect in the terminal and the wire connecting the secondary coils and the rectifier, this design tends to suffer from significant deterioration of the copper loss reduction effect. To mitigate this problem, this paper proposes a novel transformer's secondary-side structure that integrates the one-turn secondary coils and the rectifier. The proposed transformer structure can mitigate the proximity effect in the secondary coils and the rectifier, thus suppressing the local concentration of the current distribution. Experiments revealed effective reduction of the copper loss, supporting the effectiveness of the proposed structure.