應用於串列匯流排電力傳輸之變頻峰值電流控制返馳式轉換器之小訊號分析與補償器設計

Abstract

返馳式轉換器因其電路架構簡單且具有電氣隔離的特性，所以被廣泛地採用於手持式裝置的低功率適配器中。為了使轉換器在寬範圍應用中達到高轉換效率，此架構經常會採用變頻峰值電流控制。此控制方法可以根據轉換器的輸出負載量自動調整其切換頻率，進而使轉換器於重載及輕載操作時維持高轉換效率。 本論文主要是研究變頻峰值電流控制的返馳式轉換器應用於日漸重要的串列匯流排電力傳輸技術。在此應用技術中，電源轉換器具有不同的輸出電壓以提供各式外部負載所需之電能。相較於傳統的適配器應用，此技術的操作範圍是更加地寬廣，使得變頻峰值電流控制返馳式轉換器的穩定度問題變得更為嚴苛。 在本論文中，電源轉換器在整個寬範圍操作下的各種控制行為會有詳盡的敘述。依據輸出負載的大小，轉換器共有四種不同控制模式。針對這四種控制模式，本論文會回顧其小訊號模型。由於同一組補償器將使用於四種不同的控制模式中來穩定轉換器操作，所以分析最差情形之未補償迴路轉移函數是必須的。根據這些分析將提出一套針對串列匯流排電力傳輸應用的補償設計策略。模擬及硬體實測證實此補償策略之可行性。

Flyback converters have been widely adopted in low-power adapters for many portable devices, mainly because of its simplicity and electrical isolation characteristic. In order to achieve high conversion efficiency over the entire load range, the variable-frequency peak-current mode (VFPCM) control scheme is often used in this topology. With this control scheme, the converter switching frequency can be automatically adjusted according to output load level, and therefore, maintains high efficiency under both the heavy-load and the light-load conditions. In this thesis, a flyback converter with VFPCM control used for the up-and-coming universal serial bus power delivery (USB-PD) application is the focus. For such an application, the converter has to supply various output voltage levels for powering different loads. Compared with traditional adapter application, the operation range of this specification is much wider and the stability issue of VFPCM controlled flyback converter becomes more severe. In this thesis, a detailed description of the circuit control behaviors throughout a complete wide operation range is given. Depending on the output load condition, there are four control modes for this converter. The small-signal models for each of four control modes are reviewed. Since only one compensator is used for keeping the converter stable, a worst case analysis of uncompensated loop gain transfer functions is indispensable. Based on the analysis, a design strategy of compensation network for USB-PD application is proposed. Simulations are conducted and a hardware experimental circuit is built to verify the validity of the proposed strategy.