Jian Hao

Comparison of online and offline tests in LED accelerated reliability tests under temperature stress.

Accelerated aging tests are the main method used in the evaluation of LED reliability, and can be performed in either online or offline modes. The goal of this study is to provide the difference between the two test modes. In the experiments, the sample is attached to different heat sinks to acquire the optical parameters under different junction temperatures of LEDs. By measuring the junction temperature in the aging process (Tj1), and the junction temperature in the testing process (Tj2), we achieve consistency with an online test of Tj1 and Tj2 and a difference with an offline test of Tj1 and Tj2. Experimental results show that the degradation rate of the luminous flux rises as Tj2 increases, which yields a difference of projected life L(70%) of 8% to 13%. For color shifts over 5000 h of aging, the online test shows a larger variation of the distance from the Planckian locus, about 40% to 50% more than the normal test at an ambient temperature of 25°C.

The Design of Two-Step-Down Aging Test for LED Lamps Under Temperature Stress

In this paper, the step-down aging test for light-emitting diode (LED) lamps is designed according to the stopping rule of the IESTM-28 standard and experimental data. The step-down aging experiment is under the temperature stresses of 90 °C, 80 °C, 70 °C, and 60 °C, from which the temperatures for two-step-down aging test are selected. A Nelson model is used to calculate the equivalent time from one temperature to another, and a two-stage method is adopted to establish the reliability model of LED lamps under accelerated temperature. Then, the lifetime at the ambient temperature of 25 °C is deduced by the use of Arrhenius model. It is shown that the temperatures of 90 °C and 80 °C for two-step-down aging test are the best choice, with standard deviation of reliability and standard deviation of relative lifetime less than 11% and 20%, respectively. The accelerated aging time is <;1200 h, which meets the requirement of quick lifetime prediction of LED lamps.

Analysis of junction temperature and modification of luminous flux degradation for white LEDs in a thermal accelerated reliability test

An accelerated aging test is the main method in evaluation of the reliability of light-emitting diodes (LEDs), and the first goal of this study is to investigate how the junction temperature (Tj) of the LED varies during accelerated aging. The Tj measured by the forward voltage method shows an upward trend over the aging time, which gives a variation about 6°C-8°C after 3,000 h of aging under an ambient temperature of 80°C. The second goal is to investigate how the variation of Tj affects the lifetime estimation. It is verified that at a certain aging stage, as Tj increases, the normalized luminous flux linearly decreases with variation rate of microns (μ) (1/°C). Then, we propose a method to modify the luminous flux degradation with the Tj and μ to meet the requirements of a constant degradation rate in the data fitting. The experimental results show that with the proposed method, the accelerated lifetimes of samples are bigger than that of the current method with increment values from 8.8% to 21.4% in this research.

Disparity between online and offline tests in accelerated aging tests of LED lamps under electric stress

The accelerated aging tests under electric stress for one type of LED lamp are conducted, and the differences between online and offline tests of the degradation of luminous flux are studied in this paper. The transformation of the two test modes is achieved with an adjustable AC voltage stabilized power source. Experimental results show that the exponential fitting of the luminous flux degradation in online tests possesses a higher fitting degree for most lamps, and the degradation rate of the luminous flux by online tests is always lower than that by offline tests. Bayes estimation and Weibull distribution are used to calculate the failure probabilities under the accelerated voltages, and then the reliability of the lamps under rated voltage of 220 V is estimated by use of the inverse power law model. Results show that the relative error of the lifetime estimation by offline tests increases as the failure probability decreases, and it cannot be neglected when the failure probability is less than 1%. The relative errors of lifetime estimation are 7.9%, 5.8%, 4.2%, and 3.5%, at the failure probabilities of 0.1%, 1%, 5%, and 10%, respectively.

Determination of cut-off time of accelerated aging test under temperature stress for LED lamps

To acquire a rational minimum cut-off time and the precision of lifetime prediction with respect to cut-off time for the accelerated aging test of LED lamps, fifth-order moving average error estimation is adopted in this paper. Eighteen LED lamps from the same batch are selected for two accelerated aging tests, with 10 samples at 80 °C and eight samples at 85 °C. First, the accelerated lifetime of each lamp is acquired by exponential fitting of the lumen maintenances of the lamp for a certain cut-off time. With the acquired lifetimes of all lamps, the two-parameter Weibull distribution of the failure probability is obtained, and the medium lifetime is calculated. Then, the precision of the medium lifetime prediction for different cut-off times is obtained by moving average error estimation. It is shown that there exists a minimum cut-off time for the accelerated aging test, which can be determined by the variation of the moving average error versus the cut-off time. When the cut-off time is less than this value, the lifetime estimation is irrational. For a given cut-off time, the precision of lifetime prediction can be computed by average error evaluation, and the error of lifetime estimation decreases gradually as the cut-off time increases. The minimum cut-off time and medium lifetime of LED lamps are both sensitive to thermal stress. The minimum cut-off time is 1104 h with the lifetime estimation error of 1.15% for the test at 80 °C, and 936 h with the lifetime estimation error of 1.24% for the test at 85 °C. With the lifetime estimation error of about 0.46%, the median lifetimes are 7310 h and 4598 h for the tests at 80 °C and 85°C, respectively.

Erratum to: Determination of cut-off time of accelerated aging test under temperature stress for LED lamps

In the original version of this article, the second affiliation should be removed, and the correct affiliations are as given above.In the original version of this article, the ORCID for Jian HAO is incorrect. The correct one should be http://orcid.org/0000-0002-2419-2508.