A high-voltage (800V) bidirectional converter is proposed for the integration of batteries at cell level in a modular multilevel converter, where high efficiency at medium-low power and low current ripple are mandatory. Triangular Current Mode (TCM) with variable switching frequency (to minimize negative inductance current) fulfills these requirements since it enhances the efficiency of the converter thanks to soft-switching operation, but it requires variable switching frequency and large current ripple through the inductor (that can be easily minimized using interleaved modules). At low power, TCM requires high-switching frequency (solved by using SiC MOSFETs), a minimum negative inductor current and a minimum dead-time to obtain ZVS. In this paper, an accurate power losses model, including dead-time losses, is used to analytically select the optimum values of dead-time and minimum inductance current for different input/output voltage ratios. The main objective is the improvement of the global efficiency, reducing dead-time losses (especially relevant at high frequencies and for devices with high reverse conduction voltage drop, such as SiC MOSFETs).