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Magnetic-less multilevel dc-dc converters attract much attention in automotive industry due to their small size, high efficiency, and high temperature operation features. A multilevel-modular capacitor-clamped dc-dc converter (MMCCC) is one of the most promising topologies among them with simple control and reduced switch current stress. This paper presents the optimal design considerations for an MMCCC to achieve the highest efficiency and the smallest size. In order to design the converter with the highest efficiency, the analytical power loss equation of an MMCCC should be derived. By considering the stray inductance in the circuit, the optimal design approach should be divided into two cases, overdamped case and underdamped case. The converter can be designed to achieve high efficiency in both cases by varying circuit parameters. If the circuit is designed in overdamped case, huge electrolytic capacitor bank has to be used to achieve high efficiency, which will increase the total converter size. If the circuit is designed in underdamped case, small-size multilayer ceramic capacitor can be utilized due to the low capacitance requirement. Although relatively high switching frequency is required in underdamped case due to practical considerations, zero-current switching (ZCS) can be achieved for all the switching devices which minimize the converter switching loss. Therefore, the optimal design point of an MMCCC with the smallest size and the highest efficiency should be selected at the underdamped case with ZCS for all the switches. Simulation results are provided to validate the design. A prototype of an MMCCC by using the proposed optimal design procedure is built. Experimental results of this optimal designed MMCCC prototype are provided to demonstrate the validity of the proposed method.