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Fig. 4: Comparing the thermal properties of standard LEDs (top) and COB LEDs (bottom). thermal resistance in the range of 20 to 200K/W. Even High Power LEDs on ceramic substrate still provide 6-12K/W. The thermal resistance of COBs is less than 2K/W. Here, the multichip ensemble is directly mounted onto the substrate without any interface, eliminating in particular the high total thermal resistance of PLCC packages – see figure 4. Thus, the total thermal resistance of the ensemble is reduced considerably. This results in a lower chip temperature Tj, which ensures a longer product life of a COB LED. COBs are specifically applied in applications which require a singular LED light source. Thus, the COB focus is on directional applications like spotlights, downlights, retrofits and partly on decorative applications. For omnidirectional lamp applications like A60 bulbs in particular, the advantage of beam uniformity is not relevant because here more cost-efficient multi chip ensembles based on mid-power or high-power LEDs are usually applied along with a diffuser which creates a homogeneously diffused light output of the bulb. In a typical setup of a spotlight for directed applications, the COB is installed in a compatible Zhaga Book 3 holder, which ensures not only the mechanical clamp but also the electrical connection. The backside of the COB is thermally coupled to a heat sink heat via a thermally conductive adhesive or foil. The heat sink is responsible for proper heat dissipation. The emitted luminous flux of the COB is focused by a reflector whose aperture is adapted to the diameter of the COB – see figure 5. Fig. 5: Setup of a COB based spotlight. www.electronics-eetimes.com power. precision. excellence. Highly-integrated SCALE-2® ASIC chipset IGBT driver with plug-and-play functionality Advanced Active Clamping (A2C) driven by prime performance www.IGBT-Driver.com


EETE DEC 2013
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