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EETE DEC 2013

LED LIGHTING Chip-on-board LEDs for directional high power applications By Dr. Christopher Keusch In order maintai n a pleasant and uniform light environment, directional and decorative lighting applications need a light source which has to meet several key criteria. It should be compact, have a high efficiency with sufficient light output, a high colour rendering index (CRI), a high colour uniformity and colour consistency and a long service life. The light source should also be easy to implement into the application at an acceptable price for the user or the consumer. A lot of different types of LEDs are available for lamp and lighting manufacturers. For a long time, standard discrete components such as high power LEDs or, alternatively, more cost effective PLCC package LEDs in the small and medium power range played the dominant role. Meanwhile, however, chip-onboard LEDs successfully serve the requirements of this application segment and specifically address design and performance issues. Typical applications mainly include downlights, spotlights and retrofits such as MR16, GU10 and PAR lamps as well as decorative lamps such as candle lights. Setup of Chip-on-Board LEDs Conventional SMD LEDs mostly consist of a single or maximum two LED chips. Chip-on-board LEDs, however, are based on a multi-chip assembly with many individual low power LED chips connected in series and in parallel. Mechanically, the COB can be mounted directly on the lamp heat sink, eliminating the SMT processes required by traditional discrete components on an MCPCB. This provides more direct thermal dissipation, higher efficacy, and ease of assembly. Fig. 2: Optical properties. Typical specifications Table 1 shows typical specifications of COBs within the power range from 4W to 15W vs. established High Power LEDs at 1W and Mid Power LEDs at 0.5W. COBs in particular are characterized by much higher luminous fluxes, higher electrical parameters as well as by their larger emission surfaces (apertures) and dimensions. Discrete components mounted onto a common MCPCB cannot provide a homogeneous emission, but show optical hot spots such as the ring pattern shown in figure 2. Performance can be improved by use of diffusers but considerable light power losses have to be taken into account. COBs in contrast provide a singular light spot ensuring homogeneous intensity distribution without any optical hotspots. Hence they allow for a much simpler optical design. Furthermore, discrete LEDs mounted onto an MCPCB (LED ensembles) cannot be coupled into small optics apertures due to the larger resulting emission surface. Thus, a part of the radiation is shaded by the aperture and is lost for the application – see figure 3. COBs, however, are ideal for combination with secondary optics providing matched optics aperture. Thermal properties Standard LEDs with a PLCC package have a relatively high Table 1: Typical specifications of COBs versus high-power and mid-power LEDs. Fig. 1: 15W COB including internal electrical configuration. Dr. Christopher Keusch is Senior Technical Sales Manager EMEA at Everlight Electronics - www.everlight.com - He can be reached at christopher.keusch@everlight-eu.de Fig. 3: 10W COB which is comparable to PCB 20 Electronic Engineering Times Europe December 2013 www.electronics-eetimes.com


EETE DEC 2013
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