No description available.

No description available.

Nichia Corporation announced blue Light Emitting Diodes (LED) by 1993. They were widely used in markets by 1996 after combining blue LED with yellow phosphors to emit white lights. There¡¦re two keys to utilize LED as replacement light energy; one is to increase the chipset brightness, while another is to use LED arrays instead of single LED. Around 15 to 20% of LED illuminant will be transformed to visible light, while up to 85% of the LED illuminant will be transformed to heat. Therefore, before there¡¦s obvious breakthrough on LED constructions to heat, thermal management of LED is relatively important. The purpose of this research is to do value simulation by slightly change the construction of low power LED and increase its power (150, 350mA), to investigate the differences of high power LED in thermal transmission by single LED and LED arrays under different parameters, and learn if the emitted heat can be tolerated by its key materials. This research can be used as the reference to design LED products for engineers. According to the analysis result, under environment temperature of 25 to 80 Celsius Degree, the temperatures of a 0.5W LED chipset, mounted board and packing materials will increase around 3 to 4 Celsius Degrees when the environment temperature will increase one Celsius Degree. If we increase the LED chipset power to be 1W, the temperature increase for chipset and mounted board is around 3 to 4 Celsius Degrees while the temperature increase for packing materials is 3 to 9 Celsius Degrees. Regarding high power LED arrays, according to the analysis result, when the distance between two LEDs is too small, the temperature will increase dramatically; when the P value (see report content) is over 5mm, per 1mm distance increase, the chipset temperature decrease will become 1 to 2.5 Celsius Degrees from initially 3 to 5 Celsius Degrees. If we further increase the two LEDs distance, there¡¦ll be no significant effect from chipset itself but only the mounted board.

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 162-172).

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Dennis G. Deppe. Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2009. / Committee Chair: Weck, Marcus; Committee Member: Christopher Jones; Committee Member: Jean-Luc Bredas; Committee Member: Joseph Perry; Committee Member: Laren M. Tolbert. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Material properties, coupled with typical device structures of GaN-based light-emitting diode (LED) wafer give rise to Lambertian emission patterns with large beam divergence. However, this pattern may not be useful or beneficial to many applications. In some specific applications, such as spot lighting or light sources for fiber coupling, emission with narrow beam divergence is required, whereas in general lighting such as the street lamps and indoor lighting, a diffused light source rather than a point source is needed. By manipulating the optical emission of LEDs at the chip level, some performance metrics of LEDs can be enhanced and their applications can be extended into new fields rather than merely for lighting. Additionally, the need for external optics can be eliminated, thereby increasing the flexibility of design. In this thesis, five implementations are reported to achieve emission control, namely chip design, optics design, package design and system design, which are ordered according to the LED fabrication process flow. Manipulation of optical emission can be observed by comparing the proposed devices with the conventional devices, or the successful demonstration of a new application. By chip shaping via laser micromachining, a three-dimensional truncated-conic LED (TC-LED) is proposed to cut off efficiently lateral emissions from the LED sidewall, thus enhancing color uniformity from its top quantum-dot coated surface. The optical properties of TC-LED are investigated: the beam divergence is reduced by 32o and the power in the normal direction is enhanced by 21.7%. After applying quantum dots to achieve white-light emission, the top emission color uniformity is improved by 37%. By including optics on the chip level, beam divergence can be narrowed down. The hemispherical lens LED (HL-LED) with directional beam is proposed, achieving a 53.8% enhancement of fiber coupling efficiency. On top of a flip-chip-packaged TC-LED, a hemispherical BK-7 lens is capillary-bonded onto the sapphire surface. Compared with TC-LED, the divergence of HL-LED is significantly reduced by 50o. Vertically-mounted LED (vmLED) is proposed to broaden the emission pattern at the packaging level. By mounting the LED die upright to expose two large illumination surfaces instead of the traditional way of bonding the die flat down, the optical emission pattern is converted from Lambertian to a two-lobed pattern. Both the optical properties and thermal properties are investigated and it is found that there is a trade-off between the heat dissipation and light output. A sapphire-prism-mounted vmLED is further proposed to improve the heat sinking. In the last two chapters, micro-LED arrays with smaller illuminated active regions are introduced and the combination with external optics, including optical fibers and projection lens sets are used to demonstrate novel LED applications. By coupling a bi-linear micro-LED array into a fiber bundle, a portable microdisplay system is demonstrated and this comprehensive system can be used for image projection. Another application involved a linear UV-micro-LED array coupled with a projection lens set; this optical system has been demonstrated as a direct-write lithographic tool for the fabrication of polymer microlens arrays on InGaN LEDs. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

This thesis presents a study on several reliability issues for LED lighting systems. Firstly, a full survey on exsiting LED ballast has been conducted, and critical design challenges are classified on power level basis (low/ middle/ high power). Specifically, reliability issues have been highlighted, and three major factors have been stipulated: issue of electrolytic capacitor; issue of current imbalance in parallel LED strings; issue of LED junction temperature. The information revealed in the whole survey provide important design criteria for existing LED system designs and guidance for further research directions by pointing out the critical design problems. Two possible solutions for Electrolytic-Capacitor-Less LED Ballasts are proposed regarding the first reliability issue. A series of novel passive LED ballasts are proposed. They are found to be suitable for outdoor applications, such as street lighting applications, where the ability to withstand extreme weather conditions are of major concern. When compared with those in switched mode power supplies, these passive ballasts have good power factor performance and comparatively high efficiency. In addition, an active solution has been developed for indoor applications. Its circuit topology is derived from existing differential inverter topologies and inherits same merits such as simple structure, reduced size, and low cost. Self-configurable current-mirror techniques have been derived and developed afterwards to cope with the current imbalance issue for system with parallel LED strings. In contrast with traditional current sharing methods (either linear type or switched type), the proposed techniques offer a simple solution without the need of independent current references, complicated controllers and auxiliary power supplies. These features are favored by outdoor applications and such re-configurable mirror circuits are originally designed for passive LED ballast as post-current regulators. The techniques are further extended with the ability to tolerate possible circuit failure, such as short circuit and open circuit fault. Then, a new non-contact method for the measurement of both junction-to-case thermal resistance and junction temperature in a LED device has been proposed, with respect to the third reliability issue. Traditionally the direct measurement of junction temperature in LED is not easy without the help of sophisticated methods such as laser or expensive equipment like TeraLED Transient Thermal Tester system. In contrast, the proposed method requires only the external LED power, luminous flux, and heatsink temperature information. The method can be further adopted as a simple tool by engineers to check the internal temperature states in a practical LED system for regulation and evaluation purpose. Finally, a thermal design methodology has been developedfor an LED street lamp system powered by a weakly regulated renewable small power grid. It has been successfully incorporated in the proposed passive LED ballast, such that the LED system can provide the least output luminous fluctuation against line voltage variation. It is envisaged that, with the proposed design methodology, such lighting system will not only provide high reliability, with potential lifetime exceeding 10 years, but with a proven feature of reduced light fluctuation, furthermore, it is found that the passive LED system can act as a smart load and has the potential of reducing the energy storage requirement for smart grids. These merits are attractive to public lighting systems powered by future renewable power grids. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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published_or_final_version / abstract / Physics / Master / Master of Philosophy