his paper presents a multiport galvanically isolated LLC resonant DC-DC converter suitable for DC applications. A three-port structure is analyzed, with full bidirectional power flow capabilities, simple control and behavior similar to that expected from a DC transformer. Each port is equipped with half-bridge modules accompanied with tuned resonant tank, partly realized with elements of a multi- winding high frequency transformer. With some restrictions that are explained in the paper, each port can be a sink or a source of power, depending on the presence of switching action on the half-bridge stage. To maximize the efficiency of the system, no active control is applied and the system operates at fixed switching frequency, below the resonant frequency of the system.