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In the last half decade LED technology has undoubtedly made its most rapid progresses leading to an enormous growth of illumination applications. Though, this advancement is based on a series of quantum steps in improvement of efficiency, removal of the remaining power loss is a major concern in further developments. In our paper we compare LED demonstrator assemblies with three different types of high-power LEDs mounted on different types of printed circuit boards. The thermal performance of the respective combination is investigated. We found that in a power loss range of a few Watts a satisfying solution can frequently be attained by application of conventional glass fiber epoxy based PCB technology with advanced thermal design (improved heat-spreading capability by thermal pads and thermal vias). For dissipating higher power loss, however, the insulated metal substrates turned out to be superior with respect to their polymer-based counterparts, particularly if the PCB itself has to fulfil cooler or heat spreader function. Moreover, some aspects of the dynamic thermal behavior of different demonstrator boards assembled with the same LED type were studied by recording temperature changes on the LED surface during the heating-up phase.