相移全橋(phase shifted full bridge, PSFB)轉換器被廣泛應用於中高功率的應用場合，如伺服器或電信電源，因為它無需輔助電路便可實現零電壓切換(zero-voltage switching, ZVS)。 PSFB轉換器的優點包括高效率、功率元件的電壓、電流應力較低以及固定頻率控制。然而，PSFB轉換器亦有落後臂不易達成ZVS以及循環電流較大等缺點。另一方面，操作於非對稱脈波寬度調變(asymmetrical pule-width modulation, APWM)的全橋(full bridge, FB)轉換器也可以實現零電壓切換。使用APWM控制可消除傳統PSFB轉換器的循環電流，不過APWM全橋變換器也有最大責任週期受限制以及在輕載時不易達成ZVS之缺點。
本文提出多模式切換控制之全橋轉換器，控制器會根據負載大小切換為不同的工作模式，本系統在空載時操作在突衝模式，輕載時為相移模式，重載時則為非對稱脈波寬度調變模式。由於相移式全橋以及非對稱脈波寬度調變全橋在固定盲時下，重載時會因本體二極體導通時間增加而造成效率降低，因此本文亦提出變動盲時時間技術，其會隨著負載增加而將盲時時間減少，此方法有助於提升重載時的效率。本文首先探討兩種控制法的電路操作原理及設計準則，再透過數位訊號處理器dsPIC33FJ16GS502實現多模式切換、非對稱脈波寬度調變控制、相移控制及變動盲時控制。本文實際製作一輸出功率為480W (24VDC/20A)之直流/直流轉換器，以驗證所提出之控制方法的正確性與可行性，根據實驗結果，本系統在不同負載下所有操作模式的平均效率比傳統相移全橋高2%~3%。

Phase shifted full bridge (PSFB) converters are widely used in medium to high power applications like server or telecom power supplies because it can achieve zero-voltage switching (ZVS) without requiring auxiliary circuits. The advantages of PSFB converter include high efficiency, low voltage and current stress on power devices, and fixed-frequency control. However, the PSFB converter suffers drawbacks such as narrow ZVS range for the lagging leg and large circulating current. On the other hand, operating a full bridge (FB) converter with asymmetrical pule-width modulation (APWM) can eliminate switching losses without increasing conduction losses. Using the APWM strategy, circulating currents that exists in a traditional PSFB converter can also eliminated. Nonetheless, APWM FB converter has some drawbacks such as limited maximum duty cycle and narrow ZVS range.
In this thesis, a novel switching control technique for FB converter is proposed. The presented system changes the operating mode of the FB converter according to load conditions. For very light load, burst mode control is employed. Next, phase shifted control is used under light-to-medium load conditions and APWM control is chosen for heavy load condition. To further improve the efficiency, adaptive dead time techniques for PSFB and APWM are also derived and applied. In this thesis, the operating principles, design guidelines will be described first. A low cost digital signal controller dsPIC33FJ16GS502 is then adopted in this thesis to realize the proposed switching control. To validate the correctness and the effectiveness of the proposed method, a 480 W, 24V to 20A prototyping circuit is implemented and tested. According to the experimental results, the proposed switching control technique can improve the efficiency by 2 % ~ 3% comparing with conventional PSFB converter.

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