Abstract
The usage of Light Emitting Diodes (LEDs) for lighting applications is significantly increasing. White light is the standard for these applications, it is divided in different types of light: cold white, neutral white and warm white depending on the application. Currently, there is no semiconductor material available emitting white light directly. Thus, other coloured LEDs are used and the emitted light of them is mixed to white light. As blue light emitting LEDs have the best efficiency today, they are the most promising base for white light. A part of the blue emitted light is converted into yellow light by a colour conversion layer placed on top of the semiconductor structure. The resulting mixture of both; the blue and the yellow light is recognised as white light by the human eye. By using this technique, a high efficient white LED can be realised. Nevertheless, LEDs still convert about 30 % to 90 % of power into heat. This power loss increases the LED device temperature which often results in a significant decrease of the lifetime of the LED and a colour shift or lumen reduction. Common design criteria of lights integrating LEDs lead to small size and low thermal conductivity, which enhances overheating of lighting systems. Therefore, a thermal management for LEDs is mandatory to ensure long life with constant light quality of the LED, which is the scope of this thesis. Part of this management is the determination of temperatures at different parts of the LED device. The sophisticated, complex and compact LED structures prevent a direct temperature measurement at key spots. A thermalphysical model of the LED is needed to determine temperatures at key points. Multiphysics simulations of typical LED structures are performed to develop a model of the thermal behaviour inside the LED. Results of the simulation are taken as basis for the development of a thermal measurement. The forward voltage of a pn junction changes over temperature and can thus be used as an existing temperature sensor in the LED. Beside the semiconductor temperature, the temperature of the colour conversion layer need to be tracked too. Usually, the layer consist of phosphor embedded into silicone. Overheating changes the emitted colour of the phosphor and lumen loss can increase. For all these reasons a thermal management including temperature determination is developed. It is integrated into the LED driver and tested in a discreet development. Different approaches have been tested, evaluated and optimized using the discrete structure.
The optimized approach was transferred in an ApplicationSpecific Integrated Circuit (ASIC) design combining LED driver, temperature measurement and thermal management in a single chip to achieve a small size, low cost highly integrated solution. The resulting chip controls the current of the LED while determining and controlling the LED temperature even at critical operation regions, which minimises risks of thermal hotspots.