具中心抽頭變壓器LLC諧振轉換器之數位磁通平衡控制

Abstract

LLC諧振轉換器由於其高效率、高功率密度且具有電氣隔離的特性，所以被廣泛採用於各類電子產品中。然而，此轉換器之變壓器會有偏磁(flux walking)的現象，此現象會造成輸出電壓之漣波增大，變壓器亦有飽和的可能。在本論文中，為了解決偏磁現象，提出一磁通平衡控制策略。首先，影響變壓器之磁通平衡的各項因素會被分析，藉由此分析，偵測磁化電流之直流成分調整開關責任週期這一磁通平衡控制策略由此提出。因為磁化電流之直流成分無法輕易被量測到，一個電流偵測方法被提出來做為電流估測器，此方法僅需一個電流偵測器用來對一次側電流取樣。最後，構建了一個模擬方案和一個硬體原型來用於驗證，其額定輸出功率為200瓦特，輸入電壓為380伏特和輸出電壓為20伏特。經由模擬和實驗之結果表明，所提出的控制策略有效地降低了在不匹配條件下的磁化電流之直流成分和輸出電壓漣波。

LLC resonant converters have been widely adopted in various types of electronic products because of their high efficiency, high power density and electrical isolation characteristic. However, these converters suffer from a flux walking issue in transformer, which causes a larger output ripple and possible transformer saturation. In this thesis, a flux-balance control strategy is proposed for resolving the flux walking issue. First, the various elements that affect the transformer flux balance are analyzed. From the analysis, the flux balance control strategy, which adjust duty cycle based on sensed DC magnetizing current, is proposed. Since the DC magnetizing current is not easily measured, a current sensing strategy with a current estimator is proposed, which only requires one current sensor sampling the primary side current. Finally, a simulation scheme and a hardware prototype with rated output power 200 W, input voltage 380 V, and output voltage 20 V is constructed for verification. The simulation and experimental results show that the proposed control strategy effectively reduces the DC magnetizing current and output voltage ripple at mismatched condition.