Multichannel Online Lifetime Accelerating And Testing System For Light-Emitting Diodes

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 hopeful to solve the current bottleneck of LED properties,further enhance the LED performance and explore the LED failure mechanisms.Traditional lifetime experiments commonly use off-line tests,which builds LEDs along with fixtures inside a high temperature chamber to control the overall environmental temperature.Because the testing system is independent of the high temperature chamber,the aging process needs to be repeatedly interrupted to test the LEDs optical degradation.For more convenient and continuous testing,several online testing systems have been proposed.The photodetectors are put into the high-temperature chamber to conduct photoelectric measurement.However,the photodetectors are susceptible to high temperature.Considering the high cost of time,manpower and unnecessary measurement errors,the traditional methods restrict the progress of industry and science researches.Aiming to solve the shortcoming of traditional system,a new multichannel online lifetime accelerating and testing system for LEDs is proposed in this dissertation,mainly containing the following aspects:1.Build up hardware platform of the online system.The system can be segmented into heating and sampling module,temperature and current controlling module,and online comprehensive measuring module.The specific works include:Both electrical current and temperature stresses are adopted in this online system,among them LED sink can be directly heated by an integrating heater into the current stressing fixtures.To measure the junction temperature online,a method of junction temperature based on the electrical pulse is combined into this online system;A sampling fixture printed by 3D printer is used to fix the optical fiber and to keep the distance from the detector to the sample at 100 mm;Combining spectrometer,optical multiplexer,current source,and electrical matrix switch,the online system integrates optical,electrical,colorimetric,and thermal properties in LED lifetime tests,to realize multiple LEDs online lifetime test and evaluation conveniently and automatically.For the purpose of ensuring the precision,we use a metrological certificated halogen tungsten spectra-irradiance lamp and a spectrometer(Spectro 320,Instrument systems Inc.)to calibrate the system.2.A supporting software based on Delphi is developed to run the system more intelligently.A USB hub is used to send and receive messages with multiple interfaces including USB,GPIB,and RS-232.The software starts the online test and captures the relevant parameters according to settings automatically.Besides,a new file format of HDF5 is used for big volume and multi-type data processing.3.An accelerated lifetime test for high-power LEDs has been carried out to verify the system.The InGaN blue LEDs coated with YAG:Ce phosphors are selected as samples.After pre-aging and testing junction temperature,these samples are divided into three groups applied by high-current stress(500mA)at three temperature stresses(50 0C,65?,and 80?,respectively).After online aging,the lumen degradation can be divided into two categories:monotonic variation and non-monotonic variation.The main degradation mechanism is due to the high temperature caused yellowing or possible carbonization of silicone lens by phosphors self-heating.It can be seen from this experiment that the on-line test system can give satisfactory results to accelerate the experimental results.Compared with offline test results,the test results from online test are more stable and the data are more abundant.