High-efficiency Resonant DC-DC Converter with Switchable Constant Current/Constant Voltage Charging Mode

Despite the advantages of the widely used LLC resonant converter, its efficiency can be affected by issues such as excessive high-order harmonics and reactive power losses due to frequency offset. The present study introduces a load-independent Double-T DC-DC Converter featuring switchable constant current/constant voltage charging mode, with the objective of enhancing charging efficiency across a wide range of loads. The proposed approach integrates two T-circuits in series and switch-controlled capacitor (SCC) to establish a resonant circuit that simultaneously achieves CC/CV output modes and zero phase angle (ZPA) at a fixed frequency. Our design not only significantly reduces reactive power circulation within the resonant circuit but also effectively suppresses high-order harmonics, thereby minimizing heat loss in power switching elements and enhancing energy transfer efficiency. Through simulation analysis, it is demonstrated that the DT circuit exhibits low total harmonic distortion (THD), with values 6.16% in CC mode and 2.29% in CV mode respectively. Finally, experimental results from a 70 W prototype show stable operation under electronic loads with relative output errors below 1.1 % in CC mode and less than 5.1% in CV mode. The conversion efficiency remains over 92% throughout the charging process and reaches up to 97.5% near maximum power.