On driving techniques for high-brightness light-emitting diodes

Pao Yue-kong Library Electronic Theses Database

On driving techniques for high-brightness light-emitting diodes

 

Author: Lun, Wai-keung
Title: On driving techniques for high-brightness light-emitting diodes
Degree: M.Phil.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Light emitting diodes.
Department: Dept. of Electronic and Information Engineering
Pages: xv, 98 p. : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2425042
URI: http://theses.lib.polyu.edu.hk/handle/200/6080
Abstract: In recent years, with the enormous improvements achieved in light-emitting diode (LED) technology in terms of lifetime, luminous efficacy, power rating, and color property, LEDs have become one of the most promising candidates to replace conventional light sources in various illumination applications. The advances in the LEDs have stimulated great interests in developing efficient LED drivers with optimized control circuitries. There are two driving techniques currently employed in most LED drivers; namely, the amplitude-mode (AM) and pulse-width modulation (PWM) driving techniques. The AM driving technique uses DC current to drive the LEDs and change its current amplitude to control its luminous intensity. It is found that the use of AM driving technique can provide a higher luminous efficacy over that of the PWM driving technique. For a given luminous intensity, less electrical power is required for LEDs. However, the AM driving technique suffers from the problems of controlling the luminous intensity and color instability due to the dominant wavelength of LEDs shift with forward current. The PWM driving technique is therefore preferred for illumination control as the aforementioned problems can be alleviated. Moreover, the PWM driving technique can provide more flexible dimming control and stable chromaticity on LEDs. Based on the PWM driving technique, we proposed a bi-level driving technique to improve the efficacy of LEDs. The thesis begins with the investigation on luminous efficacy of LEDs under different operating conditions. The differences in luminous intensity and efficacy by the AM and PWM driving techniques for LEDs are analysed. The advantages and disadvantages of these two driving techniques are studied in detail. A bi-level current driving technique is proposed in order to attain the advantages of the two conventional driving techniques. Based on the LED's intrinsic current-to-luminous natures, the luminous efficacy of LEDs is improved by introducing a dc-offset current in PWM-mode driving current. The proposed driving technique is implemented and verified through experiments. A thorough investigation into the problem of color variations on LEDs due to forward current and junction temperature has also been conducted. By using the current-to-chromaticity and electrical-thermal characteristics of LEDs, the color of LED generated by different driving currents can be predicted through simple calculations. The color differences of LED with three driving techniques; namely AM, PWM and the proposed bi-level current driving, are studied. An adaptive control approach of the newly developed bi-level current driving technique is proposed to alleviate the color variations on LEDs that induced from dimming. By changing the current levels of bi-level current, the color stability is improved. The proposed adaptive bi-level current driving technique is implemented and successfully demonstrated its effectiveness on LEDs through experiments.

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