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Title: An investigation into the high brightness light emitting diodes (LEDs) system driven by electronic ballast
Other Titles: Gao liang du fa guang er ji guan xi tong ji qi dian zi qu dong qi de fen xi
Authors: Qin, Yaxiao (秦亞霄)
Department: Department of Electronic Engineering
Degree: Master of Philosophy
Issue Date: 2009
Publisher: City University of Hong Kong
Subjects: Light emitting diodes.
Ballasts (Electricity)
Notes: CityU Call Number: TK7871.89.L53 Q25 2009
106 leaves : ill. 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2009.
Includes bibliographical references.
Type: thesis
Abstract: This thesis presents an investigation into the characteristics of and driving methods for the electronic ballast and light emitting diode (LED) lamp system. In this thesis, a general Photo-Electro-Thermal theory is developed to explain the interaction of light, heat and electrical power in a LED system. Based on this theory, suitable control methods are described and an analysis of the structural designs of LED devices and systems is presented. A LED driver without electrolytic capacitor thus having a long lifetime is proposed. A simple method is proposed to measure the heat dissipation of LEDs and fluorescent lamps in an open system which allows light energy to escape. Based on this method, a comparative study on the thermal and luminous performance of high-brightness LEDs and fluorescent lamps is presented. A general theory links the photometric, electrical and thermal behaviors of a LED system together is derived and presented. The theory shows that the thermal design is an indispensable part of the electrical circuit design and will strongly influence the peak luminous output of LED systems. It can be used to explain why the optimal operating power, at which maximum luminous flux is generated, may not occur at the rated power of the LEDs. This theory can be used to determine the optimal operating point for a LED system so that maximum luminous flux can be achieved for a given thermal design. Although the trend in the industry is towards the development of high-density and high-brightness single chip LED, the LED's junction thermal resistance remains a bottleneck in thermal management and maximization of light output. Based on the general photo-electro-thermal theory, the single- and multi- chip LED device structures, and concentrated and distributed LED systems are analyzed and compared. It is confirmed both theoretically and practically that multi-chip LED devices have lower equivalent thermal resistance and therefore higher luminous efficacy than single-chip LEDs. Parallel use of lower-power LEDs in a system design also provides better luminous performance than using a few of high-power LEDs for the same total system power. Based on the general photo-electro-thermal theory, the optimal power point (OPP) at which the maximum luminous flux occurs may be less than the rated power. Therefore, it is necessary to find this optimal power point before the rating power to save energy. A dynamic thermal model and a control strategy for optimal power point tracking (OPPT) in LED system are presented. The control strategy is derived from the general photo-electro-thermal theory. This method is advantageous for fast optimum point determination and easy implementation. Although LED lamps have longer lifetime than fluorescent lamps, the short lifetime limitation of LED driver imposed by electrolytic capacitor has to be resolved. Therefore, an LED driver without electrolytic capacitor in the whole power conversion process is presented. Instead of using an electrolytic capacitor for the filter, a polyester capacitor of better lifetime expectancy is used. The concept is based on applying the principle of duality to derive a two-stage converter with current source output. An experimental prototype has been built and tested.
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