Chip-on-board LEDs for directional high power applications
In order maintain 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.
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.
Fig. 2: Optical properties
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.
Fig. 3: 10W COB which is comparable to PCB
Thermal properties
Standard LEDs with a PLCC package have a relatively high 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.
Fig. 4: Comparing the thermal properties of standard LEDs (top) and
COB LEDs (bottom)
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.
Fig. 5: Setup of a COB based spotlight
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.
Dr. Christopher Keusch is Senior Technical Sales Manager EMEA at Everlight Electronics – www.everlight.com – Dr. Keusch can be reached at christopher.keusch@everlight-eu.de
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