COB (chip-on-board) LEDs offer a similar beam quality and a high color rendering index. With suitable heat sinks and mechanical holders, they are ideal for many applications.
A pleasant and uniform lighting environment requires the application of light: compact dimensions, high efficiency, sufficient luminous flux, high color rendering index (CRI value), high color homogeneity and color consistency, long life, easy to implement in the application is priced acceptable to the user.
Lamp manufacturers and lamp manufacturers have different types of LEDs available for this purpose. For a long time, discrete standard components such as high-power LEDs or lower-cost LEDs in the PLCC package in the small and medium power range dominated. In the meantime, however, chip-on-board (COB) LEDs have been successful in the market for some time, serving the needs of this application segment in particular and targeting its design and performance issues. Typical applications are mainly light emitters, round ceiling lights, retrofits like MR16, GU10 and PAR lamps as well as decorative lamps like candle lights.
The construction of chip-on-board LEDs and their specifications
Chip-on-board LEDs, so-called COBs, in contrast to conventional SMD LEDs with a single or a maximum of two LED chips from a multi-chip arrangement, in which many individual low-power LED chips in series and connected in parallel are. Also, the PLCC housing usual with SMD LEDs is omitted in particular. This results in a comparatively low thermal resistance and high efficiency.
A decisive advantage of a COB-LED is not only the similar beam quality but also the high color rendering index. The following are typical specifications of COB LEDs in the power range of 4 to 15 watts established high-power LEDs and mid-power LEDs are compared: They are characterized in particular by their significantly higher luminous flux, by correspondingly higher electrical operating parameters and by their larger emission areas (apertures) and dimensions.
Discrete LED components mounted on a common MCPCB (Metal Core PCB) do not produce homogeneous emission but, for example, optical hot spots. This can be improved by the use of diffusers, whereby not insignificant loss of luminous flux must be accepted. In contrast, COB LEDs have a singular emitting surface on the outside and ensure a homogeneous intensity distribution without optical hot spots. This makes it possible to realize a much simpler optical design.
In addition, discrete LEDs mounted on an MCPCB are not suitable for coupling into small apertures due to the larger resulting emission area. Thus, a portion of the radiation is shaded by the aperture and is thus lost for the application. COBs can be optimally combined with secondary optics when the aperture is adjusted.
Standard LEDs in the PLCC package have a comparatively high thermal resistance of 20 to 200 K / W. For high-power LEDs on ceramic substrates, the resistance is still 6 to 12 K / W. For COBs, the thermal resistance is about 2 K / W. Here, the chip ensemble is applied directly to the board without going through a housing. The high thermal resistance caused by the housing, in particular with PLCC LEDs, is eliminated. This considerably reduces the thermal resistance of the entire ensemble. This results in a lower chip temperature T j, which in turn ensures longer life of the COB LED.
COB LEDs are particularly used in applications that require a single LED light source. So the focus is on retrofits, light emitters, ceiling lights and partly on candelabras, which are based on a COB instead of standard LEDs. For omnidirectional applications in the field of lamps (A60 Bulbs), the advantage of beam homogeneity is not relevant in particular because the light diffused by multi-LED arrangements is sufficiently uniformly scattered by the diffusers used. Instead, MCPCBs are dominated either by high-power LEDs or PLCC LEDs, which can be used more cost-effectively to meet lighting requirements.
Targeted application with a COB LED
Finally, the typical structure of a COB-based light emitter is described by way of example for a directed application. The COB is mounted in a holder compatible with Zhaga-Book 3, which ensures the electrical connection in addition to the mechanical recording. The back side of the COB is thermally coupled to a heat sink via a thermally conductive adhesive or via a heat conducting film in order to dissipate the power loss. The luminous flux emitted by the COB is concentrated in the reflector whose aperture is matched to the diameter of the COB.