本論文提出應用於數據中心48 V電壓調節模組的48 V-12 V隔離式直流-直流轉換器。本電路架構採用能夠在全負載範圍達到一次側開關零電壓切換與二次側開關零電流切換的全橋LLC串聯諧振式轉換器，藉由將開關的操作頻率固定在諧振點做為直流變壓器(DC Transformer, DCX)。考量降低啟動時的湧浪電流(Inrush Current)，以設定上、下限電流的方式使用最佳化軌跡控制法搭配狀態平面圖對軟啟動進行分析，達到保護電路以及快速建立輸出電壓等目標。而在1 MHz的開關切換頻率下，開關切換損耗大幅上升，因此採用寬能隙元件氮化鎵降低功率開關的損耗，針對變壓器一、二次側皆為低電壓大電流的電路規格下，本文採用分數圈平板變壓器結構，在同樣維持4：1電壓轉換比下，一次側、二次側繞組的圈數更少，擁有更低的直流銅線損耗優勢。並且針對變壓器繞組的並聯模式和繞組排列進行探討及優化，最終選擇交流損耗最小的情況做為變壓器繞組設計。同時將變壓器的尺寸大小利用參數化設計，在有限的電路面積下選擇最佳的鐵芯損耗與銅線損耗的平衡點。並且利用ANSYS的磁性模擬軟體Maxwell驗證四分之一圈平板變壓器的電壓轉換比與磁通密度分布是否符合設計理論，最終實現切換頻率操作於1 MHz、輸入電壓48 V、輸出電壓12 V、輸出功率為1500 W且功率密度為26W/cm3，最高效率可以達到97.6%的LLC串聯諧振式轉換器。

For data centers, this study proposes an isolated DC-DC converter with MHz-level switching frequency that can achieve zero voltage switching and zero current switching over the full load range, by setting the switching frequency above the resonant frequency to ensure the best efficiency as a DC Transformer. Considering the inrush current issue at start-up, this work uses Simplified Optimal Trajectory Control(SOTC) based on the graphic state-trajectory analysis for soft start-up to protect the circuit and quickly increase the output voltage. In the proposed LLC resonant converter, conventional silicon devices are replaced with wide band-gap gallium nitride devices to reduce switching losses in the power device. This work adopts a quarter-turn transformer structure for low voltage and high current applications to reduce the DC resistance and copper loss. The turns ratio of quarter turn transformer remains 4:1, but the path of the primary and secondary winding is equivalent to a quarter turn. The current sharing between parallel layers and winding arrangement of the transformer is analyzed to choose the lowest AC copper loss for the design. Within a limited circuit layout area, optimal points for the efficiency of core loss and copper loss were calculated. The magnetic simulation software ANSYS Maxwell was employed to verify whether the voltage ratio and the planar transformer conformed to the design theory. Finally, a resonant converter was achieved with a switching frequency operating at 1 MHz, an input voltage of 48 V, an output voltage of 12 V, an output power of 1.5 kW, the power density of 26 W/cm3, and maximum efficiency of 97.6%.

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