A Novel Three-Level CLLC Resonant DC-DC Converter for Bidirectional EV Charger in DC Microgrids

The concept of vehicle-to-grid has received considerable attentions over the past decade. Electric vehicle (EV), a common load of dc microgrids, can serve as distributed energy storage system to improve the reliability and stability of the dc microgrid, support integration of renewable energy source, and improve the overall system efficiency. In order to achieve bidirectional power transmission between the dc microgrid and EV, this article proposes a novel three-level CLLC resonant converter for off-board EV charger. By adding resonant CLLC components and by combining the working modes of the two three-level full bridges, the proposed converter adapts to the wide voltage range of EV, from 200 to 700 V. An equivalent circuit model with first harmonic approximation approach is established to analyze the frequency characteristics of the resonant converter. Moreover, an algorithm of working mode selection is proposed on the principle of minimum transformer RMS current. Finally, a 3.5 kW hardware prototype was built to verify the feasibility and the advantage of the three-level CLLC resonant converter. The efficiency of the proposed converter changes little over a wide output voltage range, and the flying capacitor voltages of the proposed converter are balanced well.