LLC諧振式轉換器具備零電壓切換的優點，相較於其他硬切式架構，在較高的操作頻率下可擁有較小的切換損失，增加電路效率，以達到市場追求的高頻化以及小型化之需求。為了符合負載調節需求，LLC諧振式轉換器會以變頻的方式來控制輸出電壓，然而在極輕載時電壓的增益曲線斜率較小，頻率控制此時往往不易於實現，因此一般會加入使用脈波寬度調變（Pulse-Width Modulation, PWM）來輔助以減少主功率級傳遞能量之區間；但此模式可能會使電路無法達到零電壓切換進而導致效率不佳。因此本文提出使用於極輕載時的閉迴路波谷切控制策略，其優勢在於：相較於傳統PWM Mode操作模式，切換頻率可再降低且開關電壓可切換於波谷點，以達效率最佳化。
本文首先會先介紹LLC諧振式轉換器的工作原理；提出LLC工作於輕載時的問題並介紹已被提出之LLC輕載控制策略，接著對不同的LLC輕載控制策略進行分析與比較，再詳細說明本文提出之閉迴路波谷切控制策略以及在此控制模式下的動作原理，最後實作出一台規格為600 W/48 V的雙模式LLC諧振式轉換器，並以數位訊號處理器(TMS320F280049) 實現數位控制，使負載大於8%時以變頻方式控制；負載小於6%時使用本文提出之閉迴路波谷切控制策略，使LLC全範圍皆可穩壓及提高輕載時之效率。

LLC resonant converter has the advantages of full-range zero-voltage switching which can reduce switching losses to increase circuit operating frequency. Compared with other architectures with hard switching, it can reduce the circuit volume because of higher operating frequency. LLC usually controlled by Frequency Modulation (FM). However, the slope of the voltage gain curve at light load is gently, output voltage is not easy to regulate only by frequency modulation. In order to solve the problem Pulse-Width Modulation (PWM) is added at light load. On the contrary Pulse-Width Modulation may not be able to ZVS which will result in poor efficiency at light load, so novel valley control strategy at light load operation is proposed in this theis.
At beginning, this theis will firstly introduce the operating principle of LLC resonant converter. Then mention the problem of LLC converter under light load condition and introduce the LLC light load control strategy that has been proposed to solve the light load problem. Next part will analyze and compare different LLC light load control strategies. After that, operating principle and details of the novel valley control stratey at light load operation will be described. Finally, a 600W / 48V LLC resonant converter is implemented, and the digital signal processor (TMS320F280049) is used to achieve digital control. When the load is greater than 8%, it is controlled by frequency mode. When the load is less than 6%, the novel valley control stratey proposed in this theis is used to satisfy voltage regulation and high efficiency.

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