一般而言，應用於前端DC/DC電力轉換器，可分為PWM電力轉換器與諧振式電力轉換器。由於PWM電力轉換器開關切換屬於硬性切換，導致嚴重的切換損失，使得電力轉換效率無法提升，故發展出諧振式電力轉換器，利用諧振電路本身具有柔性切換的特性，降低開關切換損失，提升轉換器整體效率。因此，本論文係以LLC諧振式電力轉換器為研究主題，將其應用在前端DC/DC電力轉換器，使滿足高效率、高功率密度、高可靠度之設計考量。
論文中，吾人首先介紹三種傳統型諧振式電力轉換器：(1)串聯諧振式電力轉換器(SRC)；(2)並聯諧振式電力轉換器(PRC)及(3)串並聯諧振式電力轉換器(SPRC又稱LCC)。經由分析可知，此三種傳統型諧振式電力轉換器，當輸入電壓上升與負載變為輕載時，皆因提高切換頻率以維持輸出穩壓，造成轉換器具有高循環能量的缺點。
接著，吾人分析LLC諧振式電力轉換器之動作原理，並與LCC諧振式電力轉換器進行性能比較。LLC諧振式電力轉換器具有以下優點：(1)開關具有零電壓切換的特性，(2)輸出整流二極體無逆向電流，(3)負載變動(輕載)時，轉換器可操作具零電壓切換特性的範圍增大，(4)輸入電壓上升時，轉換器循環能量甚小。
最後，應用IsSpice模擬軟體與實作，驗證理論推導之正確性。由實作結果，驗證LLC諧振式電力轉換器，在輸入電壓與負載變動下，均滿足柔性切換的性能。吾人實作之LLC諧振式電力轉換器，當輸入電壓150V、輸出電壓24V及輸出功率100W時，轉換效率可達88.9%。

In general,front-end DC/DC converters are classified into two types of pulsewidth modulation converters and resonant converters. Since a PWM converter is operating in hard switching, it leads to large switching losses so that the efficiency is degraded. A resonant converter with soft switching is thereby developed to reduce the switching loss and increase the efficiency. Therefore, an LLC resonant converter is investigated in this thesis. It is employed as the front-end DC/DC converter with high efficiency, high power density and high reliability on its performances.
In the thesis, three types of traditional resonant converters are introduced. They are series resonant converter (SRC), parallel resonant converter (PRC) and series parallel resonant converter (SPRC, or LCC). The switching frequencies of these converters must increase to maintain a regulated output voltage, when the input voltage increases and the load becomes light. However, more circulating energy will be resulted form the higher switching frequencies.
To proceed, the operating principle of an LLC resonant converter is presented. Its performances are compared with those of an LCC resonant converter. The LLC resonant converter exhibits the advantages over LCC converter. The main switches achieve ZVS operation and rectifier diodes have no reverse recovery current. The ZVS operation region of the converter increases when the loads become light. The circulating energy is still small even if the input voltage increases.
Finally, the theoretical analysis is validated by IsSpice simulations and experimental results. It reveals that the LLC resonant converter achieves ZVS operation despite variations in input voltage and load resistance. A prototype of output LLC resonant converter with efficiency of is implemented in this thesis.

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