了解LED內部熱阻

　　盡管與傳統照明相比，其卓越的功效，LED仍然得到熱。由于過多的熱量是不希望的影響，如色度移位或更糟，災難性故障的原因，熱管理是固態照明（SSL）設計的一個重要方面。

　　對于一組給定的操作條件下，LED的內部熱阻主要決定了該設備將如何熱，以及如何將很快達到峰值溫度。更大的內部熱阻導致更快的溫度上升和更高的峰值溫度。

　　本文著眼于LED的內部熱阻的定義及其對商業產品的結溫的影響。

　　The importance of thermal management

　　The primary source of heat in an LED comes from the junction between the p-type and n-type semiconductor material making up the device. This heat is a by-product of the recombination of electrons and holes at or near the junction. Ideally， recombinations result in photons that exit the LED and contribute to the overall illumination， but often the photon is reabsorbed in the die， generating heat. Tiny vibrations of the LED’s crystal lattice that occur while the device is operating also raise the temperature. Despite the high efficacy of LEDs compared to conventional light sources， some 70 to 80 percent of the electrical energy applied to the device is still converted to heat rather than light.

　　Because the LED junction is small， the energy density is high and the temperature rises rapidly. It is not uncommon for the junction temperature （TJ） of modern chips to rise to 100oC and above， and although contemporary LEDs are far more robust than older devices， prolonged operation at high temperatures is undesirable. Limiting junction temperature reduces chromaticity drift and prolongs life. （See the TechZone articles “Thermal Effects on White LED Chromaticity” and “Understanding the Cause of Fading in High-Brightness LEDs.”）