Abstract：The three-port CLLC solid-state transformer (TP-CLLC-SST) consists of input and output capacitors, a full-bridge circuit, a distributed resonant tank, and a three-winding high-frequency transformer. The converter can be operated open-loop at a fixed switching frequency and close to 50% duty cycle. When the switching frequency is close to the resonant frequency, the LLC series resonant structure is commonly used in three-port solid-state transformers due to its many advantages such as soft switching over the full load range, inherent load regulation characteristics, high power density, high efficiency, and the ability to achieve open-loop operation. The TP-CLLC-SST can be applied in a DC microgrid system containing multiple renewable energy generation systems (RESs), energy storage systems (ESSs), and AC or DC loads to achieve efficient integration and control of RESs, ESSs, loads and the grid. The existing literature on the design method of resonant parameters for distributed resonant tanks of the TP-CLLC-SST is complex and unclear, the impedance matching degree of distributed resonant tanks is not high, and the dead time and magnetizing inductance of the converter in various operating modes has not been optimized. In order to solve the problems of determining the resonant parameters, the impedance matching of distributed resonant tanks and the optimization of dead time and magnetizing inductance, this paper proposes an optimized design method for the TP-CLLC-SST applied in a DC microgrid system. Firstly, the impedance matching and resonant parameter determination method of distributed resonant tanks is proposed in this paper. First, establish the equivalent characteristic circuit of the distributed resonant tanks of TP-CLLC-SST. Then, sequentially calculate the characteristic impedance of each port to one port, and calculate the equivalent characteristic impedance of each port. Finally, make the equivalent characteristic impedance of each port equal to obtain the characteristic impedance and resonant inductance relationship of each port. This method simplifies the impedance matching and resonant parameter design process. The maximum port voltage offset of the converter was reduced from 5.6% to 1.02%, thus enhancing the load regulation characteristics of the converter and reducing the port voltage change rate. Secondly, the RMS of the distributed resonant tank input and output currents of the TP-CLLC-SST under different operating modes and load conditions are derived. Moreover, the optimized selection method of dead time and magnetizing inductance of TP-CLLC-SST is given by combining the input current RMS equations of resonant tank, i.e., to find out the range of dead time corresponding to smaller RMS input current of resonant tank in each operation mode, and take the intersection of these optimized dead time ranges to get the optimal dead time and magnetizing inductance of the converter in three operation modes. In the experimental prototype constructed with three port voltages of 100 V, 400 V and 600 V and a power of 1 kW, the optimal dead time determined by this way enables the converter to achieve an average efficiency of 98.15% at full load and a peak efficiency of 98.3%, which improves the operating efficiency of the converter in the three operation modes.
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