High-Precision Time-Domain Analysis Method Based on the Superposition Principle for CLLC Converter in Above-Resonant-Frequency Mode

The CLLC converter is widely used in applications requiring highly efficient bidirectional power flows. However, multiple resonant components make its modeling and analysis quite challenging, hence, hard to reach a higher operating efficiency. In this article, the method based on the superposition principle is proposed to provide a more accurate analysis for CLLC converters in the above-resonant-frequency-mode (ARFM) and simplified time-domain analysis is employed for below-resonant-frequency mode. First, the principle of the novel method is analyzed within the introduction of the operating modes for CLLC converter. Then, the time-domain expressions regarding the voltage-gain, resonant currents, and resonant voltages are derived by using the proposed method in ARFM. Afterward, the soft-switching conditions are then redefined in a more accurate way. More detailed synchronous-rectification (SR) timings are derived, which is followed by the optimal hardware parameter design. The proposed concept is finally verified through a 1.5-kW resonant CLLC prototype. The experiment has confirmed that the voltage gain and peak values are consistent with the theoretical calculations. And the operating efficiency is improved by 1.8% with the accurate soft-switching and SR compared with the uncontrolled rectification as confirmed by experimental results. Furthermore, the proposed method surpasses the time-domain analysis method to implement SR considering the parasitic capacitance of switches.