Hristo Ganev

Degradation of high-power LEDs — Luminous flux and TM 21 life time

The reliability and the life time prediction are still one of the most important issues when we think about LEDs. The examination of these topics represents one of the goals of a joint project between research institutions and German lighting industry companies. We try to give answers to these questions using the state of the art approaches of accelerated life time tests and try to verify how the life time prediction methodology used at the moment fits with reality.

Analysis of accelerated LED degradation by statistical methods

In the fast developing LED industry, new generations of LED packages are reaching the market in small time spans. In contrast to that some luminaires, especially in street lighting, should be long lasting and uses the same package over many years. That is one reason why standards are developed to be able to provide lifetime values for the customers. The standard method of calculating the lifetime of an LED package uses the mean values of the luminous flux of a certain number of samples. In a time interval (for example 1000 hours) these values are plotted and fitted by a function, in most cases the exponential function of TM-21 [1] is applied. Our results of LED aging show that often there is no such simple function that is able to describe all the degradation curve. Especially at different conditions (temperature, forward current) the course differ and are not always smooth. Another method to investigate the LED aging is based on statistics. This has been successfully applied in the semiconductor technology (e. g. transistors). In that case every LED package itself gives information, because distributions are the main component of the analysis. The Weibull statistic is tested to the data at different conditions. In order to get a failure rate, the value of a certain percentage of the radiant flux has been taken to define this parameter. The acceleration in the aging test is driven by temperature and forward currents. Both parameters have other effects to the different parts of the LED package. Finally, the acceleration is investigated by the Arrhenius method.

LED aging acceleration — An analysis from measuring and aging data of 14,000 hours LED degradation

In the LM 80 testing standard method the lumen maintenance [1] of light sources is measured during their lifetime. As long as the spectral behavior is not changing over time, there is no doubt in that method. Since the integral value of the luminous flux is weighted with the V(λ)-function, degradation in the red and blue parts of the spectrum are represented less than changes in green and yellow spectral ranges. For this purpose spectral bands and their ratio over lifetime are analyzed and compared to each other. Data of 14,000 hours are presented in this study. Aging curves of the luminous flux und optical power are compared and analyzed by the described methods. Extrapolation and accelerated lifetime tests are used to predict spectral changes and their characteristics.