Research On Online Measurement Of Junction Temperature And Soft Measurement Of Thermal Resistance For High Power LEDs

High power LED rapidly rises in the field of lighting, it becomes the fourth generation of green light source with the advantages of low power consumption, high efficiency and long life. In recent years, though the LED luminous efficiency improves continually, there are still about 70% of the electrical energy is translated into heat energy, junction temperature has become a bottleneck for restricting the development of the LED, so it need to improve the cooling ability of the LED device. Thermal resistance has great important influence on the cooling ability. Generally, the technology of LED thermal resistance can measure the overall thermal resistance justly, and it cannot make evaluation for the LED encapsulation structure. In this paper, we study the online measuring of high power LED junction temperature and the soft measurement of thermal resistance, by the means of calculation on the transient cooling temperature data of the LED device, so we can detect thermal resistance of the internal structure of each layer of the LED devices nondestructively. The main work and conclusions are as follows:(1) The measurement theory of LED junction temperature and thermal resistance is studied, three basic modes of heat transfer in LED are analyzed: heat conduction, heat radiation and convection. With the analysis of the resistance-capacitance network with the thermal resistance-thermal capacitance network analogy, put forward the use of electrical RC network model equivalent analyzed the LED thermal resistance of thermal capacity.(2) This paper put forward the small current K coefficient method to measure the LED junction temperature, and introduced using the method of transient thermal response measure LED thermal resistance of each layer structure measurement process, the mathematical model of thermal resistance soft measurement algorithm and two key calculation of algorithm that includes the deconvolution and model transformation is deduced, the author put forward using Fourier deconvolution and Foster network model transformation to Cauer model method.(3) The soft measurement software of measure LED thermal resistance is developed, Import the data of instantaneous cooling temperature to calculate the LED devices thermal resistance of each layer structure is achieved. To prove the correctness of the thermal resistance soft measurement algorithm and the reliability of the measurement software, through compares the result with the thermal resistance of structure function curve which the international standard thermal test instrument T3 Ster measurement.(4) Three kinds of LED sample device which commonly used on the market is selected, using high power LED junction temperature online measuring and thermal resistance measurement technology to study the thermal resistance measurement problem, obtained the thermal resistance structure function curve of the sample. The thermal resistance of the LED chip, solder layer, heat conduction layer and the substrate layer can be read from the curve. Each layer of heat conduction ability is decided by size of the thermal resistance, points out which layer should be optimized in design of this kind of LED lamp bead encapsulation structure.