**6. Conclusions**

In this paper, the LCC-S topology combined with a buck converter at the secondary side was presented. The buck converter was controlled by DFTSMC to regulate the system's output current and power. Using two-port network theory, the system was analyzed, and optimal efficiency equations were derived. Based on the analysis, the proposed system operated at maximum efficiency, when connected with optimal load, or generated optimal power. Using DFTSMC, the duty cycle of the buck converter was generated to track the required optimal power point. The simulation results verified that during charging of the ultra-capacitor, under the constant power stage, the system operated at the maximum efficiency point. The comparison with PID and SMC showed that the proposed controller overall performed better in tracking the desired current and power. Experiments were performed to validate the effectiveness of the proposed controller. The experimental results coincided with the simulation and theoretical analysis. The experimental data verified that under the abrupt perturbation in the load resistance, the controller performed well and tracked the required current and power. The future aim of this study is to model the losses incurred in the inverter, rectifier, and buck converter to analyze the effects on the system efficiency.

**Author Contributions:** Conceptualization, N.A. and Z.L.; formal analysis, N.A. and Z.L.; funding acquisition, Z.L. and Y.H.; investigation, Z.L. and X.W.; methodology, N.A., X.W. and H.A.; project administration, Z.L. and Y.H.; resources, Z.L. and Y.H.; software, N.A., Y.H. and H.A.; supervision, Z.L.; validation, N.A., Z.L., X.W., H.A. and A.A.; writing, original draft, N.A.; writing, review and editing, N.A., H.A. and A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Science and Technology Research Project of Shandong Province under Grant No. 2019GGX104080.

**Conflicts of Interest:** The authors declare no conflict of interest.
