降壓直流轉換器現今已廣泛使用於許多場合，如通訊伺服器、個人電腦、消費性電子及電動車電池等。當其應用於高降壓比的情況時，會造成輸出電流漣波大，因此需要較大的輸出電容，且當降壓比不夠高時，開關的工作週期需操作於臨界值，會造成更高的電磁干擾。另外，當其應用於大功率的情況時，如果為單相轉換器會有嚴重的傳導損失。因此，本文提出一新型轉換器，以解決上述問題。
本論文之主要貢獻可摘要如下:第一點貢獻為提出新型低輸出電流漣波高降壓比轉換器，與現有文獻相比元件數量較少，因此具備成本低及可靠度高之優點，且具有最低輸出電流漣波，如此可減少電容值，以減少電容體積並提高功率密度，且可達到D2/2之高降壓比，以減少二極體導通時間，其中 為轉換器的責任週期。此外，本轉換器不需添加額外的均流電路，即能達到自動均流之優點。第二點貢獻為針對新型轉換器之工作原理與穩態特性進行分析，且進一步推導出直流模型與小訊號數學模型，以作為閉迴路之設計依據。第三點貢獻為實際製作一輸入400V、輸出24V、輸出功率400W雛型架構，經由模擬與實測結果比對，以驗證本論文所提新型轉換器的可行性，而在負載為125W時，最高效率可達92.6%。

Step-down DC converters are widely used in various applications such as server power、personal computer、VRMs of CPU boards and battery chargers. For higher step down ratio applications, the resulting output current of the above converters becomes rather large and often require a rather large output capacitor. Besides, when the step down ratio is too large, the corresponding switch duty cycle is required to operate at the threshold resulting in much high electric magnetic interruption. Furthermore, for higher power applications, the conduction losses of the converter will be severe if a single-phase converter is used. Hence, the goal of this thesis is aimed to developing a novel high step-down voltage ratio converter with low output ripple.
Basically, the contributions of this thesis may be summarized as follows. First, a novel high step-down voltage ratio converter topology is proposed. Compare with the existing converters, it has low component count which can result in low cost and high reliability. Also, the low output current ripple of the proposed converter can reduce the output capacitor volume and increase the converter power density. An improve voltage gain of D2/2 can be achieved where is the duty ratio of the converter, so that the diode conduction time can be reduced. In addition, the inductor currents in two phases can achieve uniform current sharing automatically. Second, derivation of both DC and AC models and mathematical analysis of the new converter are also made in the context for design of the closed-loop control. Third, a 400W prototype with a 400V input voltage and 24V output voltage is constructed to verify the feasibility of the new converter. Experimental results show that a maximum efficiency of 92.6% can be achieved when the load is 125W.

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