Efficiency and Thermal Management of LED Lighting Systems

Introduction 6 Introduction Light emitting diodes, LEDs, historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs (HB-LEDs) with white light and monochromatic colours has led to a general illumination by means Solid State Lighting (SSL). The luminous efficiency of LEDs is expected to reach over 80 lumens/Watt, about 6 times higher than an incandescent tungsten bulb and thermal management of LEDs is very challenging due to compact size, cost, and long-life. Anyway, SSL is one of the most promising markets in the electronic industry due the possibility for lower power consumption and consequently lower CO 2 emissions. HB-LEDs are opening new market areas where long product life and low power consumption are the key requirements. Typical applications are seen in architectural lighting, street lighting and automotive lighting. Within the SSL design, the lifetime, light-output, efficiency and colour stability are directly linked to the LED-die temperature. LEDs generate a lot of heat; almost 90% of the electrical energy is converted into heat and a rule of thumb in the industry says that every 10°C temperature rise will half the life-time, since a good operation requires at least a life of 50,000 hours it is obvious that this cannot be achieved without taking care of the thermal aspects in lighting design. Purpose of this thesis work is to understand phenomena related to the junction temperature that affect LED lighting systems in terms of efficiency and reliability. In chapter one, how junction temperature affects the LED characteristics will be discussed, focusing on luminous flux (brightness and chromaticity) then a thermal model will be introduced.