LEDLightEmissionasafunctionofthermalboundaryconditions.docVIP

LED Light Emission As A Function Of Thermal Conditions Cathy Biber, Ph.D. Biber Thermal Design, Ltd. 6826 SW 10th Avenue, Portland, OR 97219 cathy@ Abstract LED diode performance is a function of the device thermal conditions. The forward voltage and light emission of the LED vary with temperature and current. This paper discusses how to choose the desired operating temperature, by examining the effect of varying the thermal boundary conditions on the light emission. The relationships are important to making design decisions about the LED thermal packaging. A generalized calculation process is given for implementation. An example compares different thermal design constraints. Introduction LED diode performance is a strong function of its temperature. In a typical use situation, a forward voltage or current is applied to the diode producing a luminous flux. This light intensity varies with the current. However, the forward voltage-current relationship is a function of the heat sink or board temperature, and the luminous flux also varies with the diode temperature. This data is usually found in the LED vendor datasheet in chart form. By fitting curves to the data in these charts, functional relations can be programmed for calculation and design purposes. An analytical investigation of the design saves many physical design iterations, and gives guidance during the design process. This type of investigation also requires nearly zero investment in the design; it is carried out at a conceptual level, and indeed can and should be used to formulate the features of a feasible product. Typical design tasks include deciding on the overall heat sink outline – the maximum space it may occupy; the fin structure and how it is made; cosmetic or radiant coatings on the heat sink; and the air side conditions. The air side conditions are most often natural convection for illumination applications, both because of reliability and ambient noise concerns. For projection applicati