The bipolar configuration is regarded as a promising way for the integration of distributed/centralized energy resources (CER and DER) in future meshed DC grid. In order to achieve a DC-DC voltage conversion capability with bipolar integration, a monopolar-to-bipolar DC-DC converter interface is usually requested for the inner power collection network of a given CER/DER with the monopolar configuration. Three topologies of the monopolar-to-bipolar DC-DC converter interface have been proposed in this paper, termed as dual-DAB topology, three-port topology and four-port topology, respectively. A detailed topology analysis is given, and the power transmission features have been derived with the considerations of unbalanced neutral-line current. The corresponding control strategy has been proposed for the constant voltage at monopolar side and zero neutral-line current at the bipolar side. Finally, all proposed control strategies have been validated by simulation results.