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October 2014 • The PCB Design Magazine 37 ThERMAL ChARACTERIzATION OF LEDS continues misleading the designers of the cooling solu- tion. Measurement of the light output proper- ties in combination with thermal tests as sug- gested, for example, by the JEDEC JESD 51-52 standard provides useful information about their temperature dependence (Figure 2). Figure 3 shows how the chromaticity of the measured LED changes with forward current and temper- ature. The data processing software of the mea- surement system, T3Ster, derives structure functions from the transient measurement, which are then converted into models that are accurate in the case of packages that possess one-dimensional heat-flow path such as power LED packages [3] . Such models can be created as "side products" when the R thJC junction-to- case thermal resistance of the package under test is identified according to the latest JEDEC transient measurement standard (JESD 51-14), based on the so-called transient dual interface method [3] . New Model for Simulating LED Thermal Re- sistance and Capacitance This transient tester data can be used to more accurately simulate LED thermal charac- teristics by 3D CFD software, such as FloEFD, through an LED model, the so-called JEDEC 2R thermal resistor model in an extended format. For the 2R model, the necessary information can be found easily in the datasheets. In case of LEDs, the junction-to-bottom resistor of the 2R model is relevant; it is more or less equal to the R thJC junction-to-case thermal resistance of the package. For the junction-to-top resistance of the 2R model, the junction-to-lens thermal resistance would be needed. This is usually not provided and it hardly can be correctly tested, and usual- ly a sufficiently large value obtained from CFD simulations is provided. The way the standard 2R model is extended in the CFD software is that the junction-to-bottom part of the model is represented an RC model, which allows tran- article Figure 2: Example of luminous flux vs. junction temperature at different current settings.