Design driven LED degradation model for opto-isolators

Results from a matrix of temperature and current stress testing of opto-isolator LEDs (light emitting diodes) are presented. Extensive statistical analysis of this large database is shown along with the method used to define the shape of the LED degradation curves. A basic equation was developed based on the Arrhenius model for temperature-dependent effects and on the author's experience with the physics of LED degradation. Also shown are the results of multiple regression analysis of the plotted points and how they were used to resolve the constants associated with this equation. In addition, explanations are presented of unusual findings and their causes. This equation can be used by circuit designers to predict LED degradation for any time, operating current, and ambient temperature. A graph of percent degradation versus time is shown which was derived by plugging into the equation typical use currents and temperatures. A further refinement is presented that describes degradation in terms of a six-sigma distribution, giving the ability to encompass variations encountered during production.<
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