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38 The PCB Design Magazine • October 2014 sient simulations because the thermal capaci- tance is also included in the model. However, the more detailed RC model requires more data of the LED, which often cannot be found in datasheets. In this case, the CFD tool makes it easier with an interface to the thermal charac- terization system. A file can be exported out of the thermal transient tester's post-processing software that can be read by the CFD simulation tool with all the necessary data for the RC model in form of a Cauer-type ladder model. This file contains not just single thermal resistance and capacitance from junction to bottom (R jb and C jb ) values of the LEDs as a bulk value representing a single thermal time constant for the package, but rep- resents the heat-flow path structure in details appropriate for accurate transient simulation using CFD analysis. For the proper prediction of the LED's hot lumens (luminous flux at op- erating junction temperature), an LED model in CFD also contains simple models for the radi- ant flux and luminous flux for constant drive currents of the LED. These models use the mea- sured junction temperature sensitivity of these light output properties. Including the tempera- ture sensitivity of these parameters is important to account for the complex, multi-domain op- eration of LEDs. Figure 4 shows results from the LED Compact Model in FloEFD provided for a given forward current using thermal data from thermal transient measurements by the T3Ster system. Based on measured results from a validation experiment [4] to test this approach, it was clear that a simple reliance on datasheet values is in- sufficient when trying to determine the thermal performance of an LED system. Other meth- ThERMAL ChARACTERIzATION OF LEDS continues article Figure 3: Example of a chromaticity diagram for an LED generated by the TeraLED measurement system.