The voltage ratings for dc transmission and distribution are not currently standardized because cable technology is advancing rapidly and therefore low step-ratio dc-dc converters at multi-megawatt scale are crucial technology for combining point-to-point dc links into multi-terminal networks. This paper presents a modular multilevel dc-dc converter configuration with only a single stack of half-bridge sub-module (SM) and a resonant tank. The power processed through the stack is a small fraction of the throughput power if the step-ratio is small. Detailed analysis shows the required semiconductor ratings are much smaller compared to the front-to-front MMC and modular converters derived from standard dc-dc topologies. Further, the requirement for SM capacitor energy storage is also significantly less than those converters. The theoretical analysis and the operating principles have been verified by full-scale simulation examples and experiments with down-scaled prototype. The SMs are operated with an adjustable phase-shift to create a high frequency excitation across the resonant tank for further volume reduction. The SM capacitors self-balance without the need for switching pattern adjustments and the resonant operation provides soft-switching for all power switches to increase efficiency.