Design of Symmetrical CLLC-Resonant DC Transformer Considering Voltage Transfer Ratio and Cascaded System Stability

Due to the galvanic isolation and excellent bidirectional power-transfer capacity, the symmetrical CLLC-resonant dc transformer (DCT) has received increasing attention and has been popularly adopted in dc microgrid. However, for good comprehensive performance, there are some concerns in the design of CLLC-resonant DCT. First, when it operates individually, its voltage transfer ratio (VTR) is hard to maintain within an acceptable range when fluctuations of inductance or capacitance exist in practice. Second, when the CLLC-resonant DCT operates in a cascaded system with a constant power load, impedance interactions may result in instability phenomenon. The fluctuations of inductance or capacitance will also affect the stability of this cascaded system. In order to deal with these concerns, a parameter design approach for the CLLC-resonant DCT is put forward in this article. A particle swarm optimization algorithm is specially applied to realize thorough considerations of unpredictable parameter fluctuations. Thus, the designed CLLC-resonant DCT is able to maintain robust VTR and system stability when circuit parameter fluctuations exist. In the end, a 1-kW prototype has proved the validity of this proposed design approach.