925 research outputs found

    Multichannel Online Lifetime Accelerating and Testing System for Light-Emitting Diodes

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    LED寿命长达数万小时,具有高效、节能、环保、高可靠性的优点,是当前国家正在提倡和推广的照明光源。然而,LED长寿命的优点却成为寿命评估的短板,需要耗费大量时间。加速寿命可大大缩短实验时间,同时也是探索LED失效机理,进一步提升LED性能的有效手段。传统的LED加速寿命方法往往采用离线测试方法,将样品连同夹具置于高温箱中,通过高温箱控制样品的整体环境温度,但是在老化过程期间需要中断老化,将样品冷却取出对其各个参数进行测试。为了更加便捷地连续性测试,一些科研机构也提出了在线测试方法,将光电探测器置于高温箱内,进行简单的光电在线测试,但系统易受高温影响。目前,加速寿命测试耗费大量的时间和人力,测量...As the advocated and promoted lighting source in China, LED shows excellence in high efficiency, energy saving, environmental protection, high reliability, and extra-long lifetime which can reach up to hundreds of thousands of hours. However, the advantage of long lifetime is the bottleneck for lifetime evaluation, which would consume a lot of time. LED lifetime acceleration experiments are hopefu...学位:工学硕士院系专业:物理科学与技术学院_工程硕士(电子与通信工程)学号:3332014115283

    Reliabiltiy of devices and technologies for solid-state lighting

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    The role of high-power white LEDs in general lighting applications is becoming, day-by-day, increasingly important. The reliability of these devices, compared to that of conventional light sources, represents one of the keys for their development and their market penetration. It is then of fundamental importance to deeply understand the various degradation mechanisms that affect the operation of LEDs, in terms of lifetime, chromaticity characteristics and efficiency. This thesis reports the results of a research activity focused on several issues related to the reliability of LED-based lighting systems. After an initial overview on the most important theoretical concepts necessary for the understanding of the physical results, three main sections can be identified in this thesis, concerning the presentation of research activity: • The first section reports an extensive study on one of the most critical Electrical Over Stress (EOS) phenomena, called “Hot-plugging”, which occurs when an LED module is directly connected to an energized power supply and can generate current spikes up to several tens of amperes that can potentially destroy or damage the LEDs. The aim of this section is to analyze, for the first time, the nature of the current spikes generated during hot-plugging and to present a simplified model to explain the hot plugging phenomenon. The study is based on transient electrical measurements, carried out on several LED modules (fabricated by different manufacturers), connected to three different power supplies. Results reveal that the amplitude and the time constants of the current spikes are directly determined by the number of LEDs connected in series and by the output capacitance of the current driver. • Afterwards, the second section presents an extensive study on the effects of Electrostatic Discharges (ESD) on state-of-the-art GaN based LEDs, based on optical and electrical measurements carried out during the ESD events. ESD events were simulated through a Transmission Line Pulser (TLP), which generates voltage pulses with a duration of 100ns and increasing amplitude: during each pulse, spatially resolved electroluminescence measurements were carried out through a high speed EMCCD camera. These measurements allowed to identify the chip region where the discharge is localized and the change in the damaged area induced by consecutive ESD events. In addition, the current and voltage waveforms at the LED terminal were monitored during the tests; this analysis provided important information about modifications the impedance of the devices. The analysis was carried out on different types of commercially available low-power GaN-based LEDs with several differences in the manufacturing technology. Thanks to these tests, we have identified two different failure behaviors during a destructive ESD event, clearly related to the different defects in the semiconductor lattice and to structure of the chip. • The last section investigates the thermal stability of remote phosphor plates to be used in solid-state lighting systems, for the conversion of the blue light emitted by GaN-based LEDs into white light. A preliminary thermal characterization revealed that in normal conditions of blue light irradiance the phosphor plates could reach temperature levels higher than 60°C, which can affect both performance and reliability. The results of accelerated thermal stress tests indicate that high temperature levels can trigger a relevant degradation mechanism (estimated activation energy is 1.2 eV), that drastically reduces the phosphor conversion efficiency and modifies the photometric and colorimetric characteristics of the emitted white light

    The development of the MIC detector for space applications

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    Since the development of the original Boksenberg IPCS at UCL several innovations have led to a highly compact, lightweight detector, the MIC-IPCS. A version specially designed for space applications is described here. The design has been based around the special requirements of a space based system. The design and operation of each component of the detector is described. The detector incorporates a microchannel plate intensifier specifically designed for photon counting. The properties of the intensifier that limit the detector performance are considered. The output of the intensifier is coupled to a CCD via a fibre optic taper. The characteristics of fibre coupling are discussed and the effects the fibre taper introduces via pin cushion distortion are considered in detail. A fast scanning CCD is used as a readout device and the operation and design of the camera is described. The CCD data is presented to the image processing electronics that perform various important functions on the data. The design of each component circuit is considered in detail, in particular, a new interpolative centroiding technique has been developed. Tests were carried out to assess the performance of the detector. The results are discussed and the performance characteristics of the detector evaluated. I have been personally responsible for the design and testing of the processing electronics, the analysis of the phosphor decay characteristics of the image intensifier and for analysis of fibre taper distortion and efficiency. Additionally, when construction of the detector was complete, I took part in all the subsequent system tests and data reduction together with the other members of the group

    High-Efficiency Nitride-Based Solid-State Lighting

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    Thermal management and humidity based prognostics of high-power LED packages

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    While Light Emitting Diodes (LEDs) hold much potential as the future of lighting, the high junction temperatures generated during usage result in higher than expected degradation rates and premature failures ahead of the expected lifetime. This problem is especially under-addressed under conditions of high humidity, where there has been limited studies and standards to manage humidity based usage. This research provides an analysis of the factors that contribute to high junction temperatures and suggests prognostic techniques to aid in LED thermal management, specifically under humidity stress. First, this research investigates the effects of current, temperature and humidity on the electrical-optical-thermal (EOT) properties. Temperature rises within an LED because of input stressors which cause heat to build up: the input current, the operating and ambient temperature, and the relative humidity of the environment. Not only is there an accumulation of heat due to these factors that alter the thermal properties, but the electrical and optical characteristics are changed as well. By uncovering specific configurations causing the EOT performance to degrade under stress, better thermal management techniques can be employed. Second, this research proceeds to quantitatively link the EOT performance degradation to the humidity causal factor. The recent proliferation of LED usage in regions with high humidity has not corresponded with sufficient studies and standards governing LED test and usage under the humidity stressor. This has led to indeterminate use and consequentially, a lack of understanding of humidity based failures. A novel humidity based degradation model (HBDM) is successfully developed to gauge the impact of the humidity stressor by means of an index which is shown to be an effective predictor of colour degradation. This prognostication of the colour shift by the HBDM provides both academia and industry not only with an indicator of the physical degradation but also an assessment of the LED yellow-blue colour rendering stability, a critical application criterion. Using the HBDM parameters as indicators of the state of the LED, the degradation study is expanded in the development of a Distance Measure approach to isolate degraded samples exceeding a specified multivariate boundary. The HBDM and Distance Measure approach serve as powerful prognostic techniques in overall LED thermal management

    Studies in picosecond chronoscopy

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