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COB LEDs for directional, decorative apps

Posted: 26 Dec 2013     Print Version  Bookmark and Share

Keywords:directional  decorative lighting  colour rendering index  LEDs  chip-on-board 

In order to preserve a pleasant and uniform light environment, directional and decorative lighting applications require a light source that meets 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-on-board (COB) 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.

Figure 1: 15W COB including internal electrical configuration.

Figure 2: Optical properties.

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.

Typical specifications
The table 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.

Table: Typical specifications of COBs versus high-power and mid-power LEDs.

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.

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