具有脈寬調光的LED驅動電路之回授穩定性探討

Abstract

近年來，發光二極體(LED)被廣泛的應用在高性能的數位電視背光照明上，因應大尺寸螢幕，LED為多顆串聯使用，因此通常採用昇壓型(boost)電路架構。穩定的亮度與調光功能是兩個主要的需求，本文所研究的LED驅動電路採用定頻電流模式控制，因電流模式控制能夠達成良好的調光功能，亦即能夠使LED產生穩定亮度。 業界在實現調光功能普遍採用脈衝式調光，有時亦稱為間歇模式，也就是在一個低頻週期內，導通時提供LED一個固定的電流，截止時使流經LED的電流為零。比起透過改變LED電流值來調光，但電路不間斷的運作，脈衝式調光的操作方式使驅動電路與LED有較好的轉換效率。有兩個議題因應而生，第一個：脈衝式調光時，LED的電流為方波，必須避免LED的跨壓在暫態時發生過衝的情形以免縮短LED的壽命，第二個：必須使驅動電路的回授能夠穩定。本文的重點即為設計適當的回授補償以達成上述兩個議題。 本文中，首先透過狀態空間平均法建立LED驅動電路的等效模型，接著利用電流模式控制法的小訊號方塊圖與輸出阻抗特性來說明回授穩定度與暫態電壓議題，分析結果看起來相當複雜，但探討後發現複雜的分析結果在大部分實際應用時能夠簡化成二階系統，因此回授補償的設計變得簡單許多。而推導的數學模型也會透過模擬軟體與實驗結果進行驗證。本文的研究成果對於設計昇壓型電流模式LED驅動電路相當有幫助。

In recent years, light-emitting diodes (LEDs) have increasingly been used for back-light illumination for high-performance digital television applications. This is usually a wide-screen application in which serially-connected LED strings and boost converter configuration are normally the choices. For such an application, the lighting evenness and local area dimming are two major requirements. Therefore, a constant-frequency current-mode (CM) driver scheme is usually used for this application. Current-mode control scheme allows better performances in light- evenness and local dimming capability for a large-screen television application. To achieve dimming, the industry has in general adopted pulsating scheme, or sometimes called burst-mode control in which the LEDs are powered by a specified current level for certain time and completely shut off for the rest of the time in a low-frequency on-off cycle. Compared to the scheme to reduce LED current level but with continuous operation , this allows better conversion efficiency and better for both the driver circuit and the LEDs. In doing so, however, there are two issues that need to be addressed. Since the LED current, is a stepped waveform under the burst-mode dimming mode, the voltage across the LED strings can have overshoot oscillation which must be restricted to certain level to avoid the shortening of LED life time. The other issue is about the driver circuit feedback stability. The focus of the thesis is to design a proper feedback compensation to meet the goal. In the thesis, a control model is first established by using small-signal averaging method. Then feedback stability issue and the transient voltage overshoot issue were addressed using the current-mode control block diagram and the output impedance characteristics. The analytical results seem complicated , but it was found that the complexity of the analytical results can be simplified into a second-order equation for normal practical situations. This makes the design of the feedback compensation much easier to deal with using the proposed model. The model was verified by simulations and experimental results. The results from the efforts leading to this thesis should be very helpful in designing the feedback circuit for a current-mode boost LED driver circuit.