Thermal Characterization and Lifetime Prediction of LED Boards for SSL Lamp

This work presents a detailed 3-D thermo-mechanical modelling of two LED board technologies to compare their performance. LED board are considered to be used in high power 800 lumen retrofit SSL (Solid State Lighting) lamp. Thermal, mechanical and life time properties are evaluated by numerical modelling. Experimental results measured on fabricated LED board samples are compared to calculated data. Main role of LED board in SSL lamp is to transport heat from LED die to a heat sink and keep the thermal stresses in all layers as low as possible. The work focuses on improving of new LED board thermal management. Moreover, reliability and lifetime of LED board has been inspected by numerical calculation and validated by experiment. Thermally induced stress has been studied for wide temperature range that can affect the LED boards (-40 to +125°C). Numerical modelling of thermal performance, thermal stress distribution and lifetime has been carried out with ANSYS structural analysis where temperature dependent stress-strain material properties have been taken into account. The objective of this study is to improve not only the thermal performance of new LED board, but also identification of potential problems from mechanical fatigue point of view. Accelerated lifetime testing (e.g., mechanical) is carried out in order to study the failure behaviour of current and newly developed LED board

RELIABILITY TESTING & BAYESIAN MODELING OF HIGH POWER LEDS FOR USE IN A MEDICAL DIAGNOSTIC APPLICATION

While use of LEDs in fiber optics and lighting applications is common, their use in medical diagnostic applications is rare. Since the precise value of light intensity is used to interpret patient results, understanding failure modes is very important. The contributions of this thesis is that it represents the first measurements of reliability of AlGaInP LEDs for the medical environment of short pulse bursts and hence the uncovering of unique failure mechanisms. Through accelerated life tests (ALT), the reliability degradation model has been developed and other LED failure modes have been compared through a failure modes and effects criticality analysis (FMECA). Appropriate ALTs and accelerated degradation tests (ADT) were designed and carried out for commercially available AlGaInP LEDs. The bias conditions were current pulse magnitude and duration, current density and temperature. The data was fitted to both an Inverse Power Law model with current density J as the accelerating agent and also to an Arrhenius model with T as the accelerating agent. The optical degradation during ALT/ADT was found to be logarithmic with time at each test temperature. Further, the LED bandgap temporarily shifts towards the longer wavelength at high current and high junction temperature. Empirical coefficients for Varshini's equation were determined, and are now available for future reliability tests of LEDs for medical applications. In order to incorporate prior knowledge, the Bayesian analysis was carried out for LEDs. This consisted of identifying pertinent prior data and combining the experimental ALT results into a Weibull probability model for time to failure determination. The Weibull based Bayesian likelihood function was derived. For the 1st Bayesian updating, a uniform distribution function was used as the Prior for Weibull á-â parameters. Prior published data was used as evidence to get the 1st posterior joint á-â distribution. For the 2nd Bayesian updating, ALT data was used as evidence to obtain the 2nd posterior joint á-â distribution. The predictive posterior failure distribution was estimated by averaging over the range of á-â values. This research provides a unique contribution in reliability degradation model development based on physics of failure by modeling the LED output characterization (logarithmic degradation, TTF â<1), temperature dependence and a degree of Relevance parameter `R' in the Bayesian analysis

Operational Stability and Charge Transport in Fullerene-Based Organic Solar Cells

Organic photovoltaic cells are approaching commercially-viable levels of performance for a variety of applications--particularly those which make use of the unique transparency, flexibility, and ultra-thin form factor that organic solar cells can achieve. With state-of-the-art solar to electric power conversion efficiencies now exceeding 15%, operational stability of organic photovoltaics is perhaps their most significant remaining challenge, as the presence of intrinsic photochemical and morphological degradation modes have thus far limited device lifetimes to several years or months. Thermally evaporable fullerenes (C60 and C70), with their remarkable optical and semiconducting properties, have enabled many of the most efficient and reliable organic photovoltaic cells over the past 15 years and remain central to state-of-the-art devices today. After introducing the fundamentals of organic photovoltaic cell operation and characterization, this thesis focuses on the discovery and exploration of extremely long-range electron diffusion currents in fullerene-based organic heterostructures. It is shown that an energy barrier can be used to confine photogenerated electrons to a thin channel of C60 or C70, where they can persist for several seconds. During this time, the electrons can diffuse laterally over centimeters, which allows for unprecedented study of charge diffusion processes in an organic semiconductor. Organic photovoltaic cells are demonstrated that make use of these channels to achieve high transparency by employing a C60 layer to collect and transport electrons to a sparse metal grid in place of a conventional continuous metal electrode. The remainder of this dissertation explores the reliability of fullerene-based organic solar cells by monitoring their performance during long-term aging, and studying the stability of the individual layers which comprise the cells. The performance of organic solar cells with planar C60 layers degrades rapidly under illumination, which is found to result from photo-oligomerization of adjacent C60 monomers. An analytical model based on reduced exciton diffusion length in the oligomerized C60 layer is developed to describe the device degradation, which fits the observed loss. Blending C60 with a second material and replacing C60 with C70 are both found to effectively stabilize photovoltaic performance. The stability of blended tetraphenyldibenzoperiflanthene (DBP):C70-based organic photovoltaics is found to follow the morphological stability of the device's non-photoactive cathode buffer layer. Stable cathode buffer layers based on 2,2',2''-(1,3,5-benzenetriyl tris-[1-phenyl-1H-benzimidazole] (TPBi):C70 are developed, which produce the most robust organic photovoltaics reported to-date. Even under constant simulated illumination at temperatures up to 130C, no performance degradation is observed over more than 2500 hours. Under exposure to high intensity illumination (up to 37 suns equivalent), the devices degrade slowly, with an extrapolated outdoor lifetime of 54+/-14 years.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147648/1/qcb_1.pd

Some Reliability Aspects of Photovoltaic Modules

Solar cells and photovoltaic modules are energy conversion components that produce electricity when exposed to light. The originality of photovoltaic energy as we understand it here is to directly transform light into electricity. Thin-film silicon in particular is better at low and diffuse illuminations and decreases less than the crystalline when the temperature increases while reducing the amount of material and manufacturing costs. However, the quality of the material and the efficiency of the conversion limit their use on a large scale. If the light absorption of the ultra-thin layers of the active material could be improved, this would lead to low recombination currents, higher open-circuit voltages and higher conversion efficiency. PV systems often communicate with utilities, aggregators and other grid operators over the public Internet, so the power system attack surface has significantly expanded. Solar energy systems are equipped with a range of grid-support functions, which—if controlled or programmed improperly—present a risk of power system disturbances

An Extended Photoelectrothermal Theory for LED Systems: A Tutorial From Device Characteristic to System Design for General Lighting

published_or_final_versio

Methods for electronic components reliability improvement

Katedra mikroelektronik

The IceCube Neutrino Observatory: Instrumentation and Online Systems

The IceCube Neutrino Observatory is a cubic-kilometer-scale high-energy neutrino detector built into the ice at the South Pole. Construction of IceCube, the largest neutrino detector built to date, was completed in 2011 and enabled the discovery of high-energy astrophysical neutrinos. We describe here the design, production, and calibration of the IceCube digital optical module (DOM), the cable systems, computing hardware, and our methodology for drilling and deployment. We also describe the online triggering and data filtering systems that select candidate neutrino and cosmic ray events for analysis. Due to a rigorous pre-deployment protocol, 98.4% of the DOMs in the deep ice are operating and collecting data. IceCube routinely achieves a detector uptime of 99% by emphasizing software stability and monitoring. Detector operations have been stable since construction was completed, and the detector is expected to operate at least until the end of the next decade.Comment: 83 pages, 50 figures; updated with minor changes from journal review and proofin

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

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