Parameter Design for Symmetrical CLLC-Type DC Transformer Considering Cascaded System Stability and Power Efficiency

The symmetrical CLLC-type dc transformer (DCT) has been widely adopted in dc microgrid for its high-power density and outstanding bidirectional power transfer capacity. However, the practical values of the inductors and capacitors in the circuit may fluctuate because of variations of temperature and power, which results in some challenges for the parameter design of the CLLC-type DCT to meet the following two design requirements. First, the CLLC-type DCT is expected to maintain stable when it is cascaded with a constant power load (CPL) even if its output impedance is affected by fluctuating values of inductors and capacitors. Second, the CLLC-type DCT is expected to maintain satisfactory power efficiency even if the inductors or capacitors fluctuate. To meet both design requirements, cascaded system stability and satisfactory power efficiency, this article proposes a four-stage particle swarm optimization (PSO)-aided parameter design approach for the CLLC-type DCT which fully considers the unpredictable fluctuations of practical inductors and capacitors. With this proposed approach, even if the values of inductors or capacitors fluctuate, the CLLC-type DCT can work stably when it is cascaded with CPL and meet power efficiency requirement. Finally, the effectiveness of the proposed four-stage PSO-aided parameter design approach has been experimentally verified.