Number of applications using Light Emitting Diodes (LEDs) is continuously increasing; LEDs are becoming much more widespread and their relevance in illumination is growing. In terms of efficacy and expected product life time modern high-power LEDs already provide better performance than incandescent and fluorescent lamps and nowadays are competing with gas discharge lamps. Limit to their application in mainly due to the thermal management constraints resulting from high dissipation density. For end users thermal constraints are lumped into a single parameter, the allowed maximal junction temperature. Junction temperature of LEDs affects not only the emitted total luminous flux but has strong influence on reliability which is usually expressed as expected product life time in terms of depreciation of the emitted luminous flux. Therefore LED models capable of depicting device operation in the electrical, thermal and optical domains are important throughout the entire supply-chain of solid-state lighting (SSL) products from LED packages up to complete LED based lighting installations.
This thesis work deals with multi-domain characterisation issues of LEDs with a focus on multi-domain modelling and product aging.
After a brief introduction of modern LEDs it shortly describes thermal and optical testing methods, considering self-heating of the device under test. Long term aging and failure modes are discussed as well as some reliability testing methods.
Combined thermal radiometric and photometric characterisation (fulfilling the JEDEC JESD 51-5x and series of standards), and process of an IES LM-80-08 aging test of Philips Luxeon Z LED samples are described here. Aging and measurement results are discussed on application level. Parameters of a Spice-like model are extracted from the measured data. Potential model parameters that can indicate various aging and failure mechanisms of such light emitting devices are highlighted.