數位式適應性輸出電流前饋機制之LLC諧振轉換器

Abstract

LLC雙迴路控制相較於傳統單迴路控制，在添加內迴路控制下可將系統轉移函數簡化為一階，除了可降低控制器設計複雜度外，電路頻寬亦可提高以改善動態響應。本文推得更為精準之雙迴路控制轉移函數，既保留了利用能量守恆方法求得轉移函數之低、中頻準確度，也保留了擴展描述函數由開關頻率及其諧波等造成之高頻極、零點資訊，使模型不論是在低、中與高頻，皆與模擬相差無幾。然而，隨著越趨嚴格的設計規格，雙迴路控制動態響應越趨不足。為了改善這個問題，可加入輸出電流前饋機制。當負載改變時，直接透過前饋路徑調整電壓命令至穩態，以大幅改善動態響應。然而，對於LLC轉換器，負載和電壓命令之關係未知。本文首先利用狀態軌跡分析當電路操作於低於、等於或高於諧振頻率之電壓命令與負載之關係。接著由此延伸，求出負載改變之電壓命令改變量，其係數即為輸出電流前饋增益。最後分析共同性，得出輸出電流前饋增益通解，使電路具適應性。另外，推導了精準輸出阻抗關係式。從輸出阻抗可以得知，加上輸出電流前饋控制可使輸出阻抗大幅降低，證實其動態響應改善。最後分析了數位延時造成之最大輸出電壓下降量，確保最糟糕情況仍滿足制定規格。 本文設計並聯相加形式之電壓迴路控制器，充分利用數位模組化益處。接著，將電壓迴路控制器與輸出電流前饋控制演算法數位(離散)化。最後架構實現並設計圖形使用者介面(graphical user interface, GUI)，除了可以靈活調整數位控制器變數以滿足各個LLC設計規格外，也可將變數傳回GUI中檢查，方便尋找電路問題處。為了驗證所提演算法與架構，進行模擬與實驗驗證。除了電路模擬與實驗數值相符外，從動態響應可以看出，不論操作條件(頻率)為何，動態響應可以在不失效率下改善動態響應約90%，大幅改善動態響應。

Compared to traditional single-loop control, LLC dual-loop control simplifies the system transfer function to the first-order model by incorporating inner-loop control. This approach not only reduces the complexity of controller design but also increases the circuit bandwidth, thereby enhancing dynamic response. This thesis derives a more accurate dual-loop control transfer function, which retains the low and mid-frequency accuracy derived through energy conservation methods and preserves high-frequency pole-zero information caused by switching frequency and its harmonics through extended description functions. However, as design specifications become more stringent, the dynamic response of dual-loop control becomes inadequate. To improve this, an output current feedforward mechanism can be introduced. When the load changes, the feedforward path directly adjusts the voltage command to steady-state, significantly enhancing dynamic response. However, the relationship between load and voltage command in LLC converters is not well understood. This thesis uses state trajectory analysis to determine the relationship between voltage command and load when the circuit operates below, at, or above the resonant frequency. From this analysis, the change in voltage command due to load variation is derived, with the coefficient representing the output current feedforward gain. Furthermore, the analysis provides a general solution for output current feedforward gain across different operating frequencies, enhancing circuit adaptability. Additionally, a precise output impedance relationship is derived, demonstrating that output current feedforward control significantly reduces output impedance, thus improving dynamic response. The thesis also analyzes the maximum output voltage drop caused by digital delay to ensure compliance with the established specifications under worst-case conditions. The thesis designs the parallel summation form voltage loop controller, having the benefits of digital modularization. The voltage loop controller and output current feedforward control algorithm are then discretized. Finally, the framework is implemented with a graphical user interface (GUI), enabling flexible adjustment of digital controller variables to meet various LLC design specifications, and allowing variables to be returned to the GUI for inspection, facilitating circuit issue identification. To validate the proposed algorithm and framework, simulations and experimental verifications were conducted. In addition to the consistency between circuit simulations and experimental results, the dynamic response shows 90% significant improvement without compromising efficiency, regardless of the operating conditions (frequency).