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36 The PCB Design Magazine • October 2014 Comprehensive LED Characterization The first step is to measure the LEDs that are generally suitable for the lighting application and to evaluate them by thermal and radiomet- ric characterization. The LED must be measured as it transitions from a hot to cold state of opera- tion to be able to thermally characterize it using the electrical test method. The results of such measurements are LED package thermal metrics and descriptive functions that will help design engineers understand the structure. The prop- er thermal design of the cooling solution can be created when the latest JEDEC LED thermal testing standards [1, 2] are used in this approach to identify the real thermal resistance and the real thermal impedance of the package. Also, not only the radiant power is measured and used in the thermal resistance-impedance calculations, but the temperature dependence of other light output properties such as luminous flux or color coordinates can also be measured. This way the best suitable LED can be selected from the vari- ous LED vendors for the design of a particular lighting application. These testing standards were used in the development of LED-specific testing and mea- surement systems to provide comprehensive LED characterization, including thermal tran- sient measurements and measuring almost all light output properties of LEDs. Figure 1 shows an example of an LED junction temperature transient measured on a cold plate—as the JESD 51-51 and JESD 51-52 standards recom- mend. The measured junction temperature tran- sient is turned into thermal impedance if it is divided by the applied heating power. In the case of LEDs, this is the supplied electrical pow- er (forward voltage ´ forward current) less the emitted optical power, also known as total radi- ant flux. The LED under test must be character- ized optically to account for this. If the emitted optical power is not considered, the resulting thermal resistance will be smaller than reality, ThERMAL ChARACTERIzATION OF LEDS continues article Figure 1: Example of measured junction temperature transient of a device.