Derating as Thermal Protection for LED-Based Lighting Applications

This paper investigates the concept of derating in light-emitting diode (LED)–based automotive lighting systems, emphasizing its role in enhancing LED longevity, performance, and reliability under varying operating conditions. Derating is introduced from a general perspective and is modeled as an approximately linear function of the driving current with respect to the temperature measured by a negative temperature coefficient thermistor (NTC). The NTC serves as a temperature probe in this context. We demonstrate that poorly designed derating strategies can negatively impact luminous flux, lifespan, and overall system reliability. These theoretical insights are applied to a simplified lighting system, which is analyzed using both steady-state and transient computational fluid dynamics (CFD) simulations to illustrate the practical effects. It is shown that the distance between the NTC and the temperature-critical region primarily determines the slope of the derating curve, while having minimal influence on thermal protection on dynamic response. Finally, the precision of NTC-based temperature measurements defined by the thermistor specifications is briefly examined. It has an additional influence on the accuracy if the applied derating.