Research On Thermal Property And Standard Of Thermal Resistance For Light Emitting Diodes

As a kind of novel solid-state lighting source,light emitting diodes(LEDs)have some outstanding advantages such as energy-efficient,environmental-friendly and long-lifetime characteristics.On the way to practical applications of LED lighting,thermal property has always been a research hotspot.As the main parameters of thermal property,the accurate tests of junction temperature and thermal resistance are the basis of evaluation and improvement of device performance.However,there is a lack of thermal resistance test standard aiming at LEDs at home.In this paper,the temperature characteristics of high-voltage and low-voltage LEDs are studied.The electrical test method for measuring thermal resistance is optimized and verified.The draft standard of LED thermal resistance test method is worked out.This paper is supported by the project of 863 Program(No.2015AA033304).The main research contents are as follows:(1)The thermal characteristics for 3V and 50 V white LEDs under different temperatures were studied.The differences between high-voltage and low-voltage LEDs in terms of junction temperature and thermal resistance were compared.The reasons why junction temperature increases with higher platform temperature and why thermal resistance increases slightly were analyzed.(2)Photoelectric characteristics under different temperatures of 6V and 9V highvoltage LEDs with the same package were investigated,and the correction from platform temperature to junction temperature was carried out.Combined with theoretical analysis,the reasons why an increment in junction temperature leads to redshift peak wavelength,broadening full width at half maximum(FWHM),decreasing luminous efficiency,and increasing color rendering index were studied.The comparisons of optical parameters of high-voltage and low-voltage LEDs show that,the increasing junction temperature has less influence on photoelectric characteristics of high-voltage LEDs,because high-voltage LEDs perform better in current spreading and have lower heat production.(3)Three factors which influence the accuracy of thermal resistance test by electrical test method were studied.Firstly,the reasonable range of measurement current in the thermal resistance test was studied.The reasons why temperaturesensitive coefficient changes obviously but the junction temperature and thermal resistance remain roughly stable were analyzed.The reference range of the measurement current was given according to experimental results.Secondly,the influence of calculating optical power and not calculating optical power on the thermal resistance test were compared.The results illustrate that the optical power test should be implemented if the luminous efficiency of LED samples are different.Thirdly,the effect of thermal grease coating on thermal resistance test was investigated.The results show that the samples with thermal grease coating have lower junction temperature and thermal resistance than those without thermal grease coating,which verifies the necessity of thermal grease coating.(4)Thermal resistance test standards of semiconductor devices at home and abroad were investigated,and the method of measuring LED thermal resistance was identified as the electrical test method.Then,the "Measurement Method for Real Thermal Resistance and Impedance of Light Emitting Diodes" standard draft was completed through consulting various thermal resistance test standards and combining with the researches of electrical test method of thermal resistance.Finally,the feasibility of this standard was verified by a great number of LED thermal resistance tests.