LED's in automotive lamp applications have been utilized effectively for functions with low light output requirements and for styling purposes. In recent years, high to ultra high power LED's are becoming a light source option for virtually every automotive lighting system. For exterior automotive lighting, these light sources have distinct advantages, but also unique thermal issues compared to conventional incandescent light sources. This study describes two methods of determining the junction temperature of an LED or array of LED's, direct and indirect. Both are based on temperature measurements, but the indirect method also requires a thermal resistance specified by the manufacturer. The direct method utilizes the intrinsic behavior of the junction voltage drop at low current (<0.1mA). A computer model for a typical plate finned heat sink design for a high power LED automotive lamp was experimentally calibrated. Design of experiment analysis was performed using a 3 level 3 input factor full factorial test matrix. The factors were defined as an active heat sink surface area, convection coefficient correlated to an airflow, and environment temperature. A quadratic model is generated from the results of the factors and significant interactions to the response.

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