This paper introduces a power flow controller that can be deployed to interconnect DC networks of different or similar voltage levels and manage the power flow between them. Its key features include: 1) bidirectional power flow 2) both step-up and step-down operation and 3) bidirectional fault blocking similar to a DC circuit breaker. Moreover, its modular architecture makes it aptly suited for high-voltage DC (HVDC) applications. The kernel of the power flow controller is a new class of single-stage bidirectional DC/DC converters utilizing interleaved strings of cascaded submodules. The converter operation is analyzed and a simplified model is implemented in PSCAD/EMTDC. A control strategy that ensures steady-state power balance of each submodule capacitor via circulating AC currents is introduced. Simulation results validate the converter's principle of operation and the proposed control strategy.