LED亮度恆定之回授控制系統設計

Abstract

在色彩的表現與使用的壽命上，發光二極體(LED)遠勝於冷陰極射線管(CCFL)，因此在液晶顯示器(LCD)的背光應用上，以LED做為背光源已逐漸取代了CCFL燈管，然而因環溫增加使LED溫度增高，或長時間點亮LED，其亮度會有衰減之現象。本研究係為改善亮度不一致之弱點，而設計一個能使LED亮度恆定的回授控制電路，以紅、綠、藍發光二極體(RGB LED)為主，對三色LED之亮度進行監控。 本研究主要之系統架構包含了回授控制電路、驅動電路板、LED燈源與光感測器四個部份。回授控制電路提供LED驅動晶片需要的輸入訊號，以及接收從光感測器回傳的數值，經運算後與預定值進行比對，一旦有偏差，便對亮度灰階值作補償。在運算與補償的部份，提出一遞廻逼近法，以自動回授控制完成亮度校正。驅動電路板採用三個驅動晶片對RGB LED分別作控制，驅動晶片具有內建16位元之PWM功能，在16位元解析之下，亮度可由調整灰階達到目標，RGB LED的色偏值(∆u’v’)可控制在0.003以內(人眼恰可接受的色差範圍是0.005)。由光感測回授控制可使亮度與目標值的誤差能低於1%，以使LED的亮度在不同狀況下都能維持穩定的功能。

LED (Light Emitting Diode) is superior to CCFL (Cold Cathode Fluorescent Lamp) in color performance and lifetime. Therefore in LCD backlight application, LED substitute for the CCFL as the backlight source gradually. However, the brightness of LED gets weak and drops down as the ambient temperature increases or as LED lights up in long time. In this thesis, in order to improve the luminance inconsistency, a feedback control circuit is proposed to stabilize the light output of LEDs. Here, take RGB (red, green, and blue) LEDs as light source. There are four parts in the whole system, including the control processor, LED driver board, LED light source and the light sensor. Control processor provides the input signal to LED driver IC and receives the information from the light sensor feedback. Then it compares and computes with the expected value by proposed recursive approach. It makes the compensation by 16 bits PWM (pulse width modulation) which is build in driver IC as the deviation produces. Feedback control system is proved that color difference (∆u’v’) of RGB LEDs is kept within 0.003, whereas the human’s noticeable color difference is 0.005. Luminance error rate can be kept lower than 1% by light sensor feedback control. It can make the brightness stably under different conditions.