Robust and low-cost design of LLC-type DC transformer in hybrid AC/DC nano-grids to improve energy conversion ability

In the hybrid AC/DC nano-grid, the LLC-type DC transformer (DCT) is widely utilized in the DC and AC bus connection system (BCS) with high frequency to improve the system power density. However, it always employs the simple open-loop scheme for not increasing the control complexity of the nano-grid system, which creates a big challenge on the inherent features of the DCT circuit for the energy efficiency and conversion ability. Therefore, a robust and low-cost design approach of the LLC-type DCT is put forward in this paper based on the specific requirements of hybrid AC/DC nano-grid. First of all, the design challenges of the LLC-type DCT are discussed by analyzing the functions of BCS in nano-grid application. Then, the detailed parameter design approach is presented step-by-step to meet the energy efficiency and conversion requirements in the full power range. Finally, a 6 kW nano-grid prototype is established to verify the effectiveness of the proposed design method. The results show that the designed DCT can enjoy good energy efficiency and conversion ability with simple control and hence accommodate the high-frequency nano-grid application.