- 學位論文
電流模式固定開關截止時間控制LED驅動電路之穩定度研究
Stability Investigations of a Current-mode Constant Off-time LED Driver Circuit
摘要
近五年內，照明應用中最常使用LED。也在這幾年各式各樣的LED驅動電路相繼被研究出來。依照應用需求的不同，LED驅動控制法也隨之改變，而不變的是LED驅動電路的共通特性，「控制LED的亮度即是控制電流的大小」，一般皆採用數位調光或類比調光。 在此論文中，焦點將著重於應用在展示裝置的背光板，LED高精準亮度控制就格外重要。固定開關截止時間控制法，一開始是由業界IC公司所提出的。這控制法不僅提供精準的LED電流控制，至於功率轉換效率也相當的高，並且電路也很簡單。但是應用於實際電路中有迴授不穩定的問題。因此，本論文將討論此電路的穩定度分析。 取樣保持(Sample-and-Hold)元件的不理想特性是造成這不穩定的原因。本論文將提出斜率補償的方式緩和這個問題。最後將由電路模擬以及實作結果作驗證。
並列摘要
Light-emitting-diode (LED) applications for illumination have enjoyed a tremendous growth in the last five years. There are a variety of LED driver circuits reported in recent years. Depending upon the application requirements, the driver circuit scheme may vary, but the general feature of a driver circuits is to control the LED current level for light level or dimming control. In this thesis, the focus is on the LED backlight applications for display units in which an accurate light-level control is important. This is a control scheme first reported in a commercial integrated circuit for LED driver, the constant off-time control scheme. A constant off-time peak-current-mode LED driver circuit provides not only accurate LED current control, but also relatively high power conversion efficiency, and is simple in circuitry. However, there is a feedback instability problem associated with the scheme in practical circuits. A stability analysis of the circuit is the focus of the paper. It’s discovered that the sample-and-hold device, used in the control circuit causes this instability problem. A “slope compensation” scheme is proposed in the thesis to alleviate this problem. Circuit simulations and experimental results show that the proposed scheme works in solving this problem.