降壓型 DC-DC 轉換器廣泛應用於電動汽車 (EV) 充電器、資訊交換技術設備等的。 此外，高輸出的電壓轉換為極低的輸出電壓的應用如今非常流行，例如電池充電器（400V/48V）、伺服器（48V/6V）等的。 因此，高降壓轉換器於近幾十年來受到越來越多的關注，高降壓轉換器主要有兩種特殊類型：隔離型和非隔離型高降壓DC-DC變換器。 本論文介紹在不改變開關元件電壓應力的情況下，對傳統降壓轉換器進行修改，以實現高降壓DC-DC轉換器的方式。
為了驗證此方式之可行性，有一個額外的電路作為額外的電壓源。此外，還分析了所提出之高降壓電路於 TCM 操作中的 ZVS 模型，以降低開關損耗並有更高的切換頻率。 另一方面，所提出的轉換器於穩態和動態響應方面使用耦合電感能得到改善，耦合電感由額外電路中的電感和傳統降壓電感傳導。考慮耦合係數對特性的影響及ZVS模型，所提出的高降壓轉換器選定耦合電感器的合適架構。 因此，推導設計耦合電感器以獲得更好的轉換器的最大優勢。 然而，由於開關元件和佈局上的寄生元件使開關元件無法具有完整的 ZVS。 因為這些元件使電感上的電壓降低和感值的增加，而導致減小電感電流的斜率。 因此，負電感電流無法計算滿足 ZVS 模型。於設計過程使用 Ansys SIwave 和 Ansys Q3D Extractor 軟體來估算 PCB 的阻抗。 然後，調整開關切換頻率使開關元件具有全 ZVS。
此外，還討論了所提出的高降壓轉換器的延伸版本，以實現更高的降壓轉換率。 藉由與其他高降壓轉換器的比較，所提出的轉換器在高降壓應用中有更高的轉換率和優勢。 最後實現並測試了48W樣機，得到了實驗波形。 結果得到了開關元件在高頻下全範圍負載具有全 ZVS。 在 80% 的負載條件下峰值效率為 96%。

The step-down DC-DC converters are widely exploited in many applications such as information and communication technology (ICT) equipment, electric vehicle (EV) chargers, and so on. Especially, many applications, which require very low output voltage converted from high input voltage, are very popular nowadays such as battery chargers (400V/48V), servers (48V/6V), and so on. Therefore, the converters with high step-down conversions have been paid more attention in recent decades. In this dissertation, a typical method is discussed to increase higher input voltage by modifying conventional buck converter without changing the duty cycle and voltage stress of switching devices.
To demonstrate the feasibility of this typical method, an extra circuit is proposed to integrate into conventional buck converter, and its operating principle is discussed in detail. In addition, the ZVS model in the TCM operation of the proposed circuit is also analyzed carefully to achieve low switching loss as well as obtain higher switching frequency. Furthermore, the performances of the proposed converter in steady-state and dynamic response are improved by employing coupled inductor, which is conducted from the inductors in the extra circuit and the conventional buck. The influences of the coupling coefficient on performances and the ZVS model of the proposed converter have been considered to determine the suitable structure of the coupled inductor. Therefore, the design considerations for coupled inductor in the proposed converter can be deduced to obtain maximum benefits in steady state operation and dynamic response. However, some parasitic components of the devices and the layout can affect the ZVS model of the switching devices, and these switching devices cannot achieve ZVS properly. Because these elements cause the voltage reduction in the inductor during the charging stages and the increase of inductance, which results in reducing the inductor current slope and increasing the minimum value of the inductor current. Hence, the ZVS model cannot be satisfied in these scenarios. To overcome this issue, a design process to calculate the impedances of Printed circuit board (PCB) by using Ansys SIwave and Ansys Q3D Extractor software. After that, these elements will be employed to calculate the frequency to satisfy the ZVS model.
Besides, the multi-level versions of the proposed converter are also discussed to obtain higher step-down conversions. By comparing to multi-level versions of the other high step-down topologies, the benefits of these multi-level versions are demonstrated, and the proposed converter can get high-rated for high step-down applications. Furthermore, the proposed converter’s design, effectiveness, and performance are verified using a laboratory-scale prototype of 48W. The results show the full ZVS of switching devices at all load conditions. The peak efficiency of the prototype is at 96% at 80% load condition. Moreover, a 48 W prototype of the two-level version of the proposed converter is also built and tested to show the feasibility of the multi-level versions of the proposed converter.

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