DC transformers based on power electronics are key
items of equipment for medium voltage dc (MVDC) distribution
systems. Both high and low step-ratio dc-dc conversions are
required to interface dc links at different voltages to form an
integrated dc distribution system. The transformer-coupled
resonant modular multilevel dc-dc converter (RMMC) is wellsuited to high step-ratio connection between a MVDC network and
a low voltage dc (LVDC) network but it is not suitable in low stepratio conversion for linking two MVDC networks with similar but
not identical voltages. This paper presents a circuit evolution of the
high step-ratio transformer-coupled RMMC into its low step-ratio
transformer-less RMMC counterpart. These two RMMCsshare the
same structure and the same resonant process within the resonant
SM stack (RSS) giving rise to the same operational advantages. Also,
the circuit design experience can be readily transferred from one to
the other. From these two base RMMC circuits, a family of RMMCs
with further configurations is elaborated that provide a wider variety
of connection optionsfor MVDC distribution systems. In this circuit
family, each high step-ratio transformer-coupled RMMC has a low
step-ratio transformer-less RMMC counterpart and one can be
transformed into the other via the circuit evolution process
presented. The theoretical analysis for both high and low stepratio RMMCs has been verified through full-scale simulations of
medium voltage examples and further verified through downscaled experiments on laboratory prototypes.