近期小型裝置像手機、MP3撥放器和其他手持式裝置從USB獲得電源，然而較高功率的外接裝置像固態硬碟和螢幕並不能從傳統USB經由USB電源線開機，因為最大功率超過7.5 W，因此這些裝置需要另外增加獨立電源的成本。在2012年USB開發展論壇釋出USB電力傳輸規範，這個規範可以允許電力傳輸至100W；如果產品符合USB電力傳輸規範，可因此減少外置電源的需求並簡化連接方式。對於設計USB PD電源供應器，返馳式架構經常被選用，因為其設計複雜性較低且有小型化等優勢。
本文設計並研製同步整流返馳式轉換器，採用同步整流可以減少輸出端傳導損失並增加轉換效率。本文實際製作60 W實驗原型電路，以驗證所提出之系統架構。相較於傳統返馳式轉換器，本文所提出架構在5V、12V、20V平均效率可分別提升4.08%、0.8%、0.51%。

Recently, small devices such as cell phones, MP3 players and other hand-held devices obtain their power from USB ports. However, higher power peripherals such as solid-state drives and monitors cannot be powered by a USB cable under conventional USB specification since the maximum power was capped at 7.5 W. Consequently, these peripherals required independent power sources at an additional cost. In 2012, the USB Implementers Forum (USB-IF) released the USB Power Delivery (USB PD) Specification. This specification allows power transfers of up to 100 W; hence, the need for extra power supplies may decrease and the connection can also be simplified if the product is compatible with USB PD. For designing USB PD power supply, flyback architecture is often chosen due to the advantages such as low design complexity and small form factor.
In this thesis, a synchronous rectifier (SR) flyback converter is designed and implemented. Synchronous rectification is adopted to decrease the output side conduction loss and increase the conversion efficiency. To validate the correctness of the proposed system, a 60 W lab-made prototyping circuit is also constructed. Comparing with conventional Flyback converter, the averaged conversion efficiency of the proposed topology can be improved by 4.08 %, 0.8 % and 0.51 % when output voltage equals 5 V, 12 V and 20 V, respectively.

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