Study On LabVIE-Based Measurement Of Junction Temperature Of LED And Its Photoelectric Properties

With the development of semiconductor technology, LED (Light Emitting Diode) has become a hot research issue because of its energy saving, high efficiency, long life span and high reliability, and it has been widely applied in the field of illumination. However, junction temperature has a great influence on the performance of LED, and directly affects its illumination efficiency, color development index, color temperature, main wavelength, color coordinates and service life. The junction temperature of LED has become a critical hindrance and a prominent problem in the development of high power LED,. Therefore, it is a significant task in engineering application to measure the junction temperature of LED rapidly and accurately and maintain it in an allowable range so as to obtain a steady light output and prolong the life-span of LED.Among various methods of measuring the junction temperature, the direct voltage method, based on the temperature effect in the electric transportation of LED as well as the measurement the positive voltage, is considered the most accurate measurement method. In this thesis, on the basis of the positive voltage method, a virtual instrument of LabVIEW software was compiled and a measurement platform was established with the essential components of Data Acquisition System (DAQ) and thermostatic box. With the system, the voltage temperature coefficient (K) and the junction temperature of LED of different power and type were measured respectively under different current level. As a result, it was concluded that the voltage temperature coefficients (K) of different types of LED were different, so was those of the same type of LED under different current level, though differed insignificantly. Under constant current level, the junction temperature calculated with the correlation K coefficients calculated by linear equation obtained with the least squares fitting of K coefficients was consistent with the result deduced from the theoretical formula with initial measurement parameters as well as the result obtained with the widely-applied small current fast switching method, and the error was relatively insignificant. Therefore, in practical application, the junction temperature of LED can be inferred from the junction temperature's linear equation deduced from the working current values obtained by measuring the positive voltage of LED.In the thesis, the influences of junction temperature of LED on its photoelectric characteristics were also studied and discovered that when the effective current value remained constant, the junction temperature of LED would differ under different voltage variation. The temperature increased sharply when the variation range of voltage was wide, and correspondingly, the light of LED would attenuate more rapidly and its life span would be shortened. Variation in the junction temperature of LED could lead to variations in its luminosity, chromaticity, life span and other parameters.Measurement results also revealed that some photoelectric parameters tended to vary with the junction temperature of LED in a linear way, and hence such effects could be utilized to explore other methods for the measurement of junction temperature.