Brighter, Simpler, Lower-priced Lighting – The CoB LED Revolution
With efficient, homogenous light output from a spot source, CoB technology has made LED lighting a candidate for many new applications, but perhaps its biggest impact on the industry has been its ease of use, simplifying LED design and reducing time to market for modern LED lighting solutions.
History and Development
To understand the impact of CoB technology, it is necessary to understand what came before it. LEDs were invented in the ‘60s and started out in a dual in-line package
(DIP) format. These translucent epoxy enclosed devices are still used today, but mainly for signage purposes. The relatively bulky packaging of DIPs and their low lumen output make them unsuitable for the small form factors of most modern electronics.
Surface mount technology changed the face of LEDs as we know them, shrinking the dimensions of individual chips down to the order of millimetres. Designed for mass production by the same equipment used to create printed circuit boards, surface mounted device (SMD) LEDs come in a vast variety of types, lumen outputs, and colours, but generally just a few standard sizes ranging from 1.1 mm to 5.7 mm square are the most popular.
CoB LEDs
Seeing an opportunity in the deficiencies of SMD technology, LED manufacturers in recent years have come out with chip-on-board (CoB) LED technology. CoB LEDs forego the standardized packaging of SMD LEDs in pursuit of superior thermal efficiency and lumen density. Just as SMD technology shrunk LEDs, CoB LEDs shrink them even further to where they’re often less than a millimetre thick and just barely visible to the naked eye. Anywhere from just a dozen, to dozens, or even hundreds of these tiny chips will popular a single array. These LED dies are directly attached to the PCB substrate and wire bonded. Instead of individual phosphor coatings for each diode, a single large phosphor coating covers the entire array for creating a uniform spread of light.
CoB LEDs are generally larger than SMDs and range from a few millimetres to a few centimetres square. Instead of being soldered onto a PCB like SMD modules, they are attached directly to a heatsink, creating better thermal performance. Wiring is very simple with just two contacts on each CoB array.
Brighter, simpler, cheaper
CoB LEDs may seem like an evolution of SMD technology but in fact their impact has been revolutionary. By densely packing LED chips into a single array, CoB LEDs achieve not only better lumen density, but better quality light, reduced costs, and greatly simplified lighting design.
Better quality, spot lighting
Through better lighting characteristics, CoB technology have enabled LEDs to effectively replace traditional spot light sources that have proven troublesome for SMD packaging. Individual SMD LEDs generally don’t create enough light to power a luminaire, so several LEDs have to be combined in an array, with optics to diffuse the light. In the end this light is still not a great spot source. Illumination is not uniform, with point sources often visible, creating glare and multiple shadow effects.
In contrast, CoB LEDs have a uniform light emitting surface (LES) that make them a great replacement for traditional spot sources such as downlights, bay lights and floodlights. As a single, pre-binned unit, CoB LEDs also have a more consistent colour temperature than luminaires made with multiple discrete LEDs. Finally, with more luminous output from a smaller area than a group of SMD LEDs, CoB lights are not only more space efficient but can create a tighter beam angle.
Traditional LED luminaire design with SMD chips involves figuring out how many LEDs are required to create the target lighting output, designing a custom circuit board for the luminaire which takes into account space, thermal and power requirements, and hoping all your LEDs are the right colour. However, with CoB LEDs one generally uses a single module, so no custom boards are needed, greatly simplifying the design. The larger size and simplified wiring of CoB packages makes them easy to assemble without complex and expensive solder reflow equipment and a fully functioning LED luminaire can be created with little more than an appropriate heatsink, LED driver, CoB LED module, and optics. CoB LED luminaires will generally achieve better lumen per euro figures than SMD-based designs. Besides reducing tooling costs through their ease of assembly, CoB LEDs benefit from reduced packaging costs and costs associated with designing and building the circuit board needed to mount, drive and cool a group of SMD LEDs.
Considerations for CoB luminaire designs
The discrete light points of SMD based lighting groups create hot spots that for most lighting applications have to be corrected through diffusion, but this can reduce lighting efficiency considerably. CoB lights produce homogenous light from a single source, allowing for simpler optical design for better efficiency and beam control. Whereas SMD based luminaires will usually custom optics, popular CoBs, such as the Cree XLamp® CXA or Philips Luxeon LED arrays, have off-the-shelf optics readily available. These often even have connectors or holders available to simplify assembly to the CoB.
CoB High Uniformity & Efficiency LED Arrays
Optics
There are two basic types of optics – those that create a wider beam than the source, known as diverging optics, and those that create a narrower beam than the source, known as collimating optics. As a flat light source with a wide Lambertian light distribution, CoB lights need collimating optics for most applications. The optics not only narrow the beam angle but also control light distribution to better suit the application. This can include creating more diffuse lighting with uniform illumination, more focused beams for increased throw and intensity, or beams with sharp cut-offs.
Collimating optics include both lenses and reflectors. Lenses are generally more efficient than reflectors and provide better beam control. For collimating light, Total Internal Reflection (TIR) lenses are generally used. TIR lenses cover the entire LES to gather all light from the LED and use both refractive and reflective principles to achieve high efficiency. While well suited for LEDs with a smaller LES, with larger CoBs, TIR lenses can be impractically big or uneconomical.
As a result, larger CoBs, tend to use, reflectors to provide the right balance of performance and economics. While cost effective and lightweight, reflectors are only able to affect light emitted laterally from the LED which touches the reflective surface. Light is emitted directly from the LED without touching the reflector isn’t affected. While that forward facing light is transmitted very efficiently, there is no control over its light distribution. Some applications can tolerate this, but for applications requiring more complete beam control, reflectors may not be sufficient.
Cree XLamp CXA2011 & MT-G
EasyWhite™ LEDs
Hybrid optics have recently come on the market that combine a reflector design with a TIR lens to direct the light that would normally be transmitted without passing through any optics in a pure reflector. These designs, like the Saga from LEDIL, can achieve total beam control without excess bulk and weight.
Thermal Considerations
In a CoB LED, the die is attached directly to the substrate, which is mounted directly to the heatsink together with a thermal interface material. This design improves thermal performance compared to SMD LEDs which are separated from the heatsink by their packaging and a comparatively thicker PCB layer. However, the low thermal resistance of CoB LEDs does not mean cooling is less important, as the high-density nature of the arrays generates a lot of heat over a small area.
As with all LEDs, the junction temperature of the CoB LED must be kept below operating limits specified in the data sheets at all times through the use of a heatsink with the proper thermal resistance. The substrate material of the CoB LED will also affect its thermal resistance. Aluminum substrate CoBs are cheaper but CoBs with a ceramic substrate like the Cree CXA family offer better thermal performance.
Electrical Considerations
Rather than using custom circuitry, CoB LEDs are usually powered with an external LED driver that provides a constant current power supply. Choosing the right driver for CoB designs is generally very straightforward as there’s usually only one LED to drive, and for popular CoB models, driver compatibility tables are readily available.
The CoB advantage
It’s hard to overstate the impact that chip-on-board LED technology has had on the lighting industry. By foregoing the standardized packaging constraints of SMD based LEDs, CoB technology has managed to achieve higher lumen density while reducing material costs. With a single, larger LES, CoB LEDs also provide a homogenous light source well suited to downlights, spot lights and floodlights.
The benefits of CoB technology go beyond affordable, high quality illumination. By packing high lumen counts into a single package, many standard lighting applications can be powered by a single CoB array, greatly simplifying luminaire design and assembly. For many of the popular CoB LEDs, entire ecosystems of compatible optics, drivers and heatsinks have also risen, making it extremely easy to create a complete CoB based luminaire. CoB LEDs still can’t do it all – lighting strips, bulbs, tube lights and architectural lighting are better served by traditional SMD based LED solutions, but for many common lighting applications they provide a strong value proposition for both end users and lighting designers.
About the author:
Mark Patrick is Supplier Marketing Manager, EMEA at Mouser Electronics – www.mouser.com
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