Study On Measurement Method Of LED Junction Temperature And Thermal Resistance

Light emitting diodes(LED)now being widely used in many applications,including lighting,LCD backlighting,display,UV curing,disinfection and sterilization,because of their environmental protection,long life and high energy conversion efficiency.LEDs have serious thermal problems,especially high-power LEDs.The accumulation of heat will lead to the rise of LED junction temperature,which causes a series of reliability problems such as luminescence color cast,internal quantum efficiency and luminous efficiency decline,and shortened lifetime,etc.The accuracy of LED junction temperature measurement data is critical for the analysis of thermal characteristics of LEDs,such as the calculation of thermal resistance and the evaluation of thermal properties of thermal interface materials.The LED junction temperature is usually measured by the forward voltage method.During the measurement process,the current needs to be switched from a large operating current to a small test current.The current switching results in a measurement delay problem,which reduces the accuracy of LED junction temperature and thermal resistance analysis.In view of the above problems in the conventional LED junction temperature measurement,we have proposed a new LED structure within an integrated sensing unit in the previous related research work.Based on the LED junction temperature measurement within the integrated sensor unit,this paper finds two problems,that abnormal rise of apparent temperature and deviation between continuous repeated measurements,which affect the precision of the data in the LED junction temperature measurement,and innovatively proposed a rapid pull-down bias method.The method can alleviate the influence of the above two problems,and improves the accuracy of the LED junction temperature measurement.In addition,from the perspective of photon carriers and material defect energy levels,theoretical analysis and explanation are carried out for the two problems which lead to inaccurate measurement.The main research contents and conclusions of this paper are as follows:1.There are still two problems in the LED junction temperature measurement experiment based on the internal sensor unit structure:an abnormal rise of apparent temperature during the initial measurement period and a deviation between continuous repeated measurements.2.The rapid pull-down bias method is innovatively proposed and applied to the LED junction temperature measurement with integrated sensor unit structure,which better alleviates the influence of the above two problems on the experimental results.3.The KeySight B2902A digital source meter is controlled by LabVIEW programming.And the rapid pull-down bias method is applied to the conventional LEDs junction temperature measurement to achieve high-precision measurement.4.Theoretical analysis and preliminary calculations are carried out on the physical processes involved in the above two problems leading to inaccurate measurement.In the lighting stage of the LED thermal resistance measurement experiment,a large number of photo-generated carriers are stimulated and accumulated,and the accumulated photo-generated carriers will not disappear instantaneously when the illumination stops.A large part of the photo-generated carriers will quickly drift along the barrier,resulting in an abnormal rise in apparent temperature;some of the photo-generated carriers will be captured by the defect energy levels,resulting in deviations between continuous repeated measurements.The bias is quickly pulled down before each measurement is started,on the one hand,the process of photo-generated carriers drifting along the barrier is accelerated,and on the other hand,the number of electrons occupied by the defect level is reset.