This paper proposes an LLC-DAB integrated high-frequency isolated DC-DC converter topology structure to reduce the loss and cost of the DC power distribution system. Furthermore, the complexity and efficiency of the system also can be improved. The proposed converter consists of a DAB converter and two input-parallel-output-parallel half-bridge LLC resonant converters. It provides two DC voltage level buses, namely the HVDC bus and the LVDC bus. Compared with the typical converter topology of the DC power distribution system, the number of power semiconductors of the proposed converter topology is reduced by sharing the power semiconductors of the DAB converter and LLC resonant converter. As a result, with the number of power semiconductors decreasing, the cost and loss of the system are reduced. Moreover, the proposed converter can control the flow direction of power conversion and supply power to the loads. The single-phase-shift modulation method is adopted for the DAB converter, and two LLC resonant converters operate as the DC transformer under resonant frequency with open-loop control. The operation principles and characteristics of the proposed converter are discussed. Finally, the simulation is carried out and an experimental prototype is developed to verify the effectiveness of the proposed topology.

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In order to solve the problems of poor thermal stability and high frequency noise of transformer in traditional DC-DC converter, the medium frequency transformer in isolated DC-DC converter is designed and studied, which adopts 0.15 mm ultra-thin silicon steel sheet. Firstly, the magnetism of ultra-thin silicon steel is measured by using singlepiece silicon steel sheet measuring device, and the B-H curves and B-P curves at different frequencies are given. According to the magnetic measurement results, the electromagnetic scheme of three port medium frequency transformer is designed by AP method and applied the particle swarm algorithm to further design under the constraint of loss minimization. Secondly the three-winding transformer of DC-DC converter is optimized and simulated. The magnetic flux density, loss, temperature field distribution, output voltage and current waveform of the transformer under different working conditions are given. Finally, the 20 kW DC-DC converter was manufactured and the magnetic characteristics of the medium frequency transformer were experimented to verify the correctness of the proposed scheme.

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