In this paper, a control strategy to minimize the root-mean-square (rms) current and eliminate the backflow power over a wide operation range is proposed to enhance the conversion efficiency of a semi dual-bridge resonant converter (SDBRC). Steady-state analysis of the converter is performed using the fundamental harmonic approximation approach. Optimal operating states are then found with the minimum resonant current and zero backflow power for both buck and boost operation. The switching conditions of the optimal operating trajectory are also elaborated in detail. The converter is enabled to work under the optimal operating condition in a wide load range by adjusting the switch-controlled capacitor based tunable resonant tank continuously. Finally, a prototype converter rated at 500 W is built and extensive experimental tests have been done to evaluate the proposed switch-controlled capacitor based topology converter and its optimal modulation. Performance improvement brought up by the proposed converter with its modulation could be confirmed by means of comparisons with previous works.