As LEDs became the preferred light source for coloured light, the focus is now on improving the performance of white LEDs, to achieve the required levels for applications such as general lighting. One especially interesting automotive application of white LEDs are daytime running lights, which may become mandatory in the near future in Europe. In this application the specific advantages of LEDs like long lifetime, high efficiency, robustness and freedom for the designers can be fully employed.
The most common way to realize a white LED is the approach of a luminescence conversion LED, where blue light of a GaN Chip is partly transformed into yellow light by a phosphor. The combination of the not converted blue and the yellow light results in white light.
A strong increase of the luminous flux becomes possible by the use of High Power LED Chips, high efficient yellow phosphors and a packaging that offers an excellent thermal management.
First of all, the application of a High Power LED Chip allows an increase of the electrical current up to 1 Ampere. While this increases the light output significantly, the thermal management of the LED package has to be improved in order to remove the heat created by the LED chip.
Apart from chip efficiency and driving current the brightness of a white LED also strongly depends on the efficiency of the used phosphor. Today's typically used phosphors are for example YAG:CE, or with Europium 2+ doped compounds.
The required quantum efficiency should be higher than 90% for good results in the luminous efficiency. Furthermore, for certain applications such as automotive front lighting the converter even has to be stable under temperature conditions up to 120°C, be resistant to humidity and guarantee a long lifetime.
In this contribution we will present a comparison of several different types of phosphors with emphasis on their temperature dependent behaviour.