K R Shailesh

Application of accelerated life testing principles to project long term lumen maintenance of LED luminaires

This work presents our effort to predict the long term reliability of LED arrays using the application of accelerated life testing principles. Assessment of long term reliability and performance of LED arrays is a testing exercise but it is also vital for successful acceptance of Solid State Lighting (SSL) systems. The objective of this work is to analyze IESNA LM-80 test data obtained from the LED manufacturers to study how failure is accelerated by stress and fit an acceleration model to the data. This acceleration model can be used to accurately project the reliability of the LED arrays under normal operating conditions. The methodology was to apply statistical analysis to LM-80 test data and obtain accelerated models for life-stress relationships and life-time distributions. The Arrhenius-Weibull, Generalised Eyring-Weibull and Inverse Power-Weibull models were obtained and were compared for their effectiveness in to predicting the reliability of LED arrays.

Measurement of junction temperature of light-emitting diodes in a luminaire

There are vast numbers of light-emitting diode (LED) luminaires to choose from, but not all LED luminaires perform reliably. More than half of the input electrical power to a LED luminaire is wasted in the form of heat. Heat management is the one of the most critical issues faced by LED luminaire designers. The long term reliability of a LED luminaire is mainly dependent on the junction temperature of the LEDs. Hence, accurate junction temperature information during luminaire operation is critical for monitoring and assessing the health of the luminaire. In practice, it is extremely difficult to measure the junction temperature of LEDs in modern day luminaires. With the optical system and heat sink surrounding the tiny LED, measurement of junction temperature with direct methods like infrared cameras and thermocouples becomes more complex. This paper explores the possibility of monitoring and measuring the junction temperature of LEDs in a luminaire by making use of the strong correlation between the forward voltage drop at the LED junction and the temperature of that junction. Results of thermal investigations of a LED downlight are presented. The results suggest that the inherent forward voltage/ junction temperature dependency of LEDs can be used to measure and monitor LED junction temperature in a luminaire under operational conditions.

Measurement of junction-to-ambient thermal resistance of a LED lighting luminaire

Recent advancement in solid state technology has made LED lighting energy-efficient and today it is one of the rapidly-developing lighting technologies. Design of LED lighting luminaires is still repeatedly governed by misinterpretations and inappropriate use of the related solid state lighting technology. Heat is generated as byproduct of the light generation process at the LED junction; this heat needs to be dissipated in an efficient manner. The correct method of thermal management in the LED luminaire is critical for effective heat transfer from the junction to the ambient. Light output, useful life and long term reliability of an LED luminaire are strongly associated with the temperature of the LED junction. Inadequate heat dissipation influences LED life and consequently the long term reliability of the entire LED luminaire. It is therefore important to have the proper knowledge about the light-output and thermal properties of the LED luminaire. This work proposes a methodology for the measurement of the junction-to-ambient thermal resistance of a market ready LED luminaire using the linear relationship between forward voltage and junction temperature of the LEDs. Measurement of the junction-to-ambient thermal resistance of a LED luminaire is essential to assess the quality of the thermal design of the luminaire. Considering two LED luminaire with similar electrical and photometric characteristics, the LED luminaire with lower junction-to-ambient thermal resistance has better heat management system compared to the one with a higher value of the junction-to-ambient thermal resistance. The methodology used in the study and findings of this study are discussed in this paper. If two similar LED luminaires are tested using the proposed method, their thermal management systems can be assessed based on junction-to-ambient thermal resistance.

Analysis of energy savings from replacing HPSV lighting with LED lighting in road lighting application

This paper analyses a project conducted to study the performance of LED luminaires in a road lighting application. Economic performance of the LED lighting as compared to HPSV lighting was also analyzed. This paper analyses the option of replacing HPSV lighting with LED lighting. Road lighting forms an important part of infrastructure in any modern city. Government agencies therefore must take up this responsibility more seriously because of its planned significance for economic and social stability. Well-designed road lighting can give long term qualitative and quantitative benefits for both Government and public.

A review of existing methods for study and analysis of thermal design of LED lighting products

The lifetime of LED luminaires, made of high power LED arrays, operating under hostile conditions is determined by their thermal design, operating junction temperature and thermal transients managed by the thermal system. Thermal transient characterization is a noninvasive study of the thermal resistance characteristics or thermal design of the LED luminaire. This procedure can identify anomalies in the thermal design or irregularities in the heat conduction path from the LED junction to the ambient. Any increase of thermal resistance along the heat conduction path can cause excessive junction temperature build up and cause early failure or increased degradation of device. The scope of this work is study the existing method for thermal transient characterization and analysis using the evaluation of the structure function of the heat conduction path of LED lighting product. It can be concluded that structure function approach is a very superior procedure for thermal transients characterizing the heat conduction path and estimating junction-to-ambient thermal resistances.

LED lighting reliability from a failure perspective

Incredible long-life makes LED lighting systems a good long-term investment The higher capital cost is justified as there is long-term energy and maintenance cost savings. Operational requirements and materials used in manufacturing a of LEDs make then-failures distinctive compared to other microelectronics devices. Significant effort has gone into understanding of the failure modes and mechanisms and reliability of LED lighting. Although still very incomplete, our knowledge of the reliability issues relevant to LED lighting is increasing. This paper provides an overview of LED lighting failure modes and mechanisms that are commonly encountered. It focuses on the reliability issues of LED lighting.

Review of methods for reliability assessment of LED luminaires using optical and thermal measurements

LED luminaires are extensively used in a variety of applications. The LEDs are driven by from fraction of an ampere to nearly one ampere. LED luminaires are required to operate at high temperatures, depending on the application. High-power LED luminaires generate too much heat. If the heat generated is not dissipated away from the LED device it will affect the useful operating life and color characteristics of the luminaire. This work examines the need and rewards of detailed thermal modeling of LED luminaires. The purpose of this work is to develop a thermal model of LED luminaire such that the model represents the actual physical system. This model will be useful in reducing the experimental analysis. The scope of this work is to develop a thermal model of LED luminaire and validate it theoretically.

Computing static state of linear electrical networks using iteratively weighted least squares algorithm

In this paper a new approach for static state estimation of linear DC circuits using iteratively weighted least squares algorithm is discussed. Three cases of erroneous meter readings while measuring currents and voltage in a simple voltage divider circuit are considered for illustration. Large size of electrical networks limits the number measurements available for state estimation. With limited real time measurements and most of the times with pseudo-measurements usually with large error margins are often used instead. This work highlights the method of processing measurement errors in estimating the static state of a linear DC electrical network. The work also highlights the importance of having accurate physical topology modeling of the network and to have known standard reference to estimate magnitude of errors and identify faulty meters. With increase in non-technical losses in T&D systems there is ever increasing chance of technical losses, state estimation can be a very useful tool in minimizing both technical and non-technical losses in the T&D process.

Review of photometric flicker metrics and measurement methods for LED lighting

Adoption of LED lighting in large scale has opened a debate on how to evaluate performance of commercially available LED lighting products. In this paper a review of existing photometric flicker metrics and measurement methods for LED lighting products is discussed. There is an urgent need to characterize potentially visible temporal light artifacts, or popularly known as flicker from light sources. Conventional light sources like incandescent and discharge lamps modulate light output, but the severity and diversity of light modulation exhibited by LED lighting products has generated curiosity in quantifying and understanding flicker and its impact on human health and productivity. Flicker metrics like flicker index and percentage flicker are not made available in datasheets of LED lighting products. It is critical for lighting designer to obtain this important information from LED luminaire manufacturers or conduct independent testing of LED luminaires to measure flicker metrics. The data reported in this paper show great inconsistency in flicker profile waveforms (flicker profiles) of some typically commercially available LED lighting products. Flicker is hence an important characteristic that the lighting designer needs to consider in assessing LED products. This paper presents a traditional method to develop a minimum suggested performance criteria for flicker measurement and reporting, understanding the performance needs of known lighting applications and including results from significant researchers in the field. It is important to minimize the chance of mismatching LED lighting products is installed in areas where flicker can be very serious health hazard or where it can hamper productivity and human comfort.

Understanding the reliability of LED luminaires

The purpose of this paper is to educate both manufacturers and end users on what constitutes the overall reliability of an LED luminaire and the reliabilities of individual subsystems. Wide ranges of LED luminaires are available in the market to meet the needs of a specific applications, hence a potentially wide range of product performance characteristics exist. The different subsystems in an LED luminaire introduce other potential reliability issues that will be critical in deciding the overall system lifetime. The mutual dependency of subsystems and their interactions make the prospect of characterizing LED luminaire reliability challenging. Best design practices ensure that the overall luminaire reliability will be decided by a small number of subsystems, thus reducing the complexity. In this paper, a general theory of assessing the reliability of the optical, electrical and thermal subsystems of an LED luminaire is discussed. How the remaining useful lives of the subsystems are obtained from their degradation profile and how the remaining useful life of the entire system is derived from them are explained. The theory explained in this paper is useful in designing experiments to express the reliability of an LED luminaire and its subsystems in terms of remaining useful life.