LEDsDigest_Nov2013_SSLFixtures

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2 Moldable optical silicones enable LEDs

12 Aluminum extrusions match SSL thermal needs

20 Challenges of matching SSL and control technology

32 COB LEDs simplify SSL manufacturing

editorial digest

Components combine with manufacturing technique for quality ssl fixturesDeveloping and manufacturing LED-

based solid-state lighting (SSL) systems is

a multidisciplinary art where electronics,

packaged LEDs, optics, and thermal elements

are sculpted on advanced manufacturing lines

to yield great lighting products. This digest will

span the gamut of the technologies that go into

top-performing products.

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* This article was published in the October 2012 issue of LEDs Magazine.

LEDs Magazine :: EDITORIAL DIGEST

Moldable optical silicones enable LED lamps and luminaire designs

Silicone materials can withstand high heat and help deliver higher lumen density in SSL product designs while simplifying the manufacturing process and enabling more complex architectures.

Moldable siliCones are emerging as a viable option in LED-

based product designs for use as secondary optics, light pipes, light

guides, and other optical components. Indeed new formulations

designed specifically for solid-state lighting offer the ability to

withstand high temperatures associated with the LED semiconductor junction

with no optical degradation.

The material can also be

molded into complex shapes

offering great flexibility to the

product developer.

The global lighting

market is on the verge

of a transformation as

LEDs increasingly replace

conventional light sources.

According to analysts at

research firm McKinsey &

Company, the market for

LED lighting will explode at

30% per annum to exceed

Fig. 1. Moldable silicones enabled a design by LEDiL, a leading optics supplier, to incorporate secondary optics that integrate dual functions – the optics shape the light and seal the electronics against water ingress.


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$81 billion by 2020, when it will

represent close to 60% of the overall

lighting market.

Such projections are supported

by the accelerating adoption of

LED technology in more and more

general lighting applications,

ranging from low-power, low-lumen

fixtures like downlight replacements

– where LEDs are increasingly

replacing low power compact

fluorescent lamps and halogens – to

more challenging applications like

street lights, industrial lighting,

office lighting, high power halogens

or illumination of sport venues. As

LED-based light sources further

penetrate applications that demand

higher lumen density and power,

physics will demand they operate

at higher temperatures – even as

lamp and luminaire designers seek

to reduce the number of LEDs, or pack the LEDs closer in order to develop sources

that are comparable or smaller in size than previous generation devices.

At the same time, LED designers are innovating modules, lamps and luminaires

that integrate multiple functions into fewer parts (Fig. 1), or that incorporate

smaller or more complicated features. LED manufacturers are seeking new

materials that accelerate productivity, deliver higher yields – particularly for

larger parts – or reduce waste.

new materials

All of these challenges effectively represent growing pains for an emerging

lighting segment that is quickly evolving past its early-generation designs and

materials. In response, the industry is exploring new materials, such as silicones

Fig. 2. Moldable silicones performed well in tests that compared their thermal and optical stability against conventional materials, such as PC, acrylic and epoxy.

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that, while less familiar in LED applications, bring a rich history of proven

performance in dozens of other industries, including advanced electronics,

automotive and communications. Silicones address several of the challenges

posed by next-generation LED designs, including the ability to withstand higher

temperatures, support higher lumen density, enhance manufacturability and

enable more complex designs. Like LEDs, silicones are evolving too. Recently, several

leading optical and LED manufacturers have been putting a new class of optical-

quality moldable silicones to the test in their designs, and seeing positive results.

While some grades of silicone are transparent, moldable silicones such as

those recently introduced by Dow Corning represent a more advanced material

engineered expressly for LED applications, which is to say they compare

well in performance against today’s best-in-class optical materials. Plus, like

conventional silicone materials, moldable silicones exhibit low viscosity before

cure, enabling them to be molded more easily into complex shapes than either

organic polymers or glass, offering new design options for secondary lenses,

light pipes, light guides and other optical components. This quality can also

help reduce manufacturing costs and cycle times in injection molding and other

processes, and potentially reduce system costs for LED-illuminated lamps and

luminaires. Lastly, compared to many organic materials, the chemical backbone

of silicones makes them particularly well suited to manage the increasingly high

temperatures of today’s and tomorrow’s LED lighting systems.

Hotter led designs

High-heat applications are where moldable silicones shine. As a class of high-

performance materials, they easily withstand temperatures of 150°C and higher

without significant loss of optical or mechanical performance. These qualities

will become more attractive as LED sources increasingly deliver more intense

white light from comparatively smaller package sizes, and as customers seek

smaller lamps and luminaires with higher luminous flux, which will also drive up

temperatures at the device level.

As lumen densities increase, the package temperatures within today’s high-

brightness LEDs are already reaching as high as 150°C. This not only poses

challenges for epoxy encapsulants conventionally used to seal LED packages,

it is also raising heat exposure for traditional secondary optics materials such


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as polycarbonate (PC) and acrylic. In general, the optical quality of these plastic

materials declines over time at temperatures above 125°C and 95°C, respectively.

The same applies to epoxies at temperatures above 150°C.

Such high temperatures cause traditional optical materials used in LED lighting

systems to yellow with age, which diminishes the total system light output. This

can have a profound impact on lumen maintenance and efficiency – dropping the

expected 80% lumen output below acceptable levels earlier than the expected

50,000 hours of an LED light source’s useful lifetime. Further, yellowing can

adversely change an LED’s color temperature over time. Such shifts in a light

source’s color are unacceptable to lighting designers and end-users alike.

In comparison to conventional materials, moldable silicones retain their excellent

optical stability and transparency even after prolonged exposure to temperatures

upwards of 150°C, exhibiting comparatively little or no yellowing and greater

reliability across the visible spectrum. Indeed, this emerging class of silicones

enables LED optical components to maintain their lumen output and efficiency

better over the course of an LED’s useful lifetime.

thermal testing

Thermal aging tests performed by Dow Corning in an air-circulating oven

at 150°C for up to 10,000 hours demonstrated that silicone’s high optical

transmission remained steady – ranging from 90-95% in the visible spectrum –

under such conditions. Moldable silicones also retained their high performance

for other optical qualities during aging under high heat, including reflection, low

haze and stable refractive index.

Moldable silicones also stood up well in related tests that compared their thermal

and optical stability against conventional materials, such as PC, acrylic and

epoxy. Thermally aged for 200 hours at 150°C, moldable silicones retained their

high optical quality (Fig. 2). In contrast, incumbent organic materials exposed

to identical conditions began to show significant yellowing as temperatures

exceeded 125°C.

The outstanding thermal and optical stability of moldable silicones may be

enough to inspire new LED designs. These materials can help resolve issues such


7


as glare control while maintaining efficiency, color temperature stability and

performance over time. But in addition to this, silicone’s low viscosity before

cure further enables designers to consider LED components with more complex

shapes, thinner wall configurations, dual functions or very fine features.

silicone structures

Structures like undercuts, for instance, are difficult to easily achieve with plastics

because they cannot be easily released from the mold. Fabricating parts that

adjoin thin- and thick-wall sections is also more challenging with plastic, which

is more brittle and therefore more prone to cracking or breaking. Lastly, designs

that use plastic materials typically keep them away from the heat of the LED light

source, which precludes configurations that shift plastic optics closer to or even

touching the LED.

Like conventional silicones, optical-grade silicones are well-suited to precision

molding applications. Before cure, the viscosity of silicones decreases as heat

increases. This allows silicone resins to be injected into a mold at lower pressures

than what is typical for other materials, while still achieving good flow and

reproduction. For example, their low viscosity enables replication of micrometer-

sized features on a lens surface that, in turn, offers advantages in enhancing,

focusing or directing light output.

In short, the physical properties of moldable silicones enable new designs that

would be otherwise very challenging to achieve with incumbent materials,

allowing for the exploration of new shapes, styles and sizes of LED lighting, as

well as new applications.

Fabrication techniques

Silicone-based components can be fabricated using a variety of techniques,

including injection molding, casting/cavity molding and others. While naturally

very flexible, their hardness may be tuned to either absorb vibration or deliver

excellent impact resistance. With their low moisture uptake and ability to

withstand harsh environmental effects, conventional silicones are already

frequently used by the electronics industry to protect fragile components against

damage. Moldable optical silicones deliver many of the same advantages.


8


Shrinkage is another familiar challenge for plastics that is not as much of a

problem for moldable silicones, which need not be cooled in the mold as long as

plastic in order to prevent warping. This helps reduce cycle time – particularly for

large parts – which is important since the length of time that a part must remain

in the machine can represent an important portion of its total cost, depending on

the mold, optical part design and process factors mentioned above. In addition,

the comparatively low shrinkage of moldable silicones helps minimize or prevent

warping in components that integrate straight sections, such as the back of

semispherical optics.

The design and manufacturing advantages of moldable silicones cannot be

overstated because they allow issues to be solved early in the LED design chain. A

finished LED lamp might incorporate over ten different silicone-based components,

including adhesives, pottants, secondary optics and encapsulants (Fig. 3). Silicones

are well known for addressing challenges at the package level, and as LEDs

penetrate into general lighting those challenges will become more common.

optics example

Referring back to Fig. 1, LEDiL, a leading optics supplier to the world’s lighting

manufacturers, recently demonstrated this with the development of its

innovative Strada-FT-TPHS lens module. The product of a collaboration between

LEDiL, Dow Corning and other suppliers, the module features secondary optics

comprising an asymmetric lens fashioned from Dow Corning’s moldable silicones.

Notably, LEDiL’s application of moldable silicone technology enabled the

secondary optics to perform dual functions. In addition to creating an

asymmetric forward-throw light distribution pattern, the secondary optics also

provide ingress protection (IP) for the LED package. By using the lens material as

a seal against outside dust and moisture, LEDiL was able to reduce the overall

number of parts and address a challenge that would normally have fallen to

its luminaire customers further down the design chain. Namely, its customers

would have had to seal the entire luminaire.

Moldable silicones enabled further design features for the module, such as an

undercut, and thermal and optical stability from -45o C to 150oC to prevent

yellowing over the course of the module’s lifetime. Neither these features nor

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the secondary optic’s dual

functionality would have been

easy or even possible using

plastics, underscoring the

versatile design possibilities

that moldable silicones offer.

The performance demands

may vary wildly between

one LED design and another

in today’s evolving lighting

industry. But moldable

silicones are finding

application at every level of

the LED design chain at other

top optical suppliers and

lighting designers. Further,

Dow Corning is continuing

to seek new industry

partnerships to develop

innovative new applications

for moldable silicones.

In terms of performance,

moldable optical silicones

combine and often exceed the

best qualities of both organic

polymers and glass. As demand

for LED lighting accelerates over the next decade, moldable optical silicone materials

will play a major role in the development of new high-performing LED light sources,

and help expand design and processing options for LED lamps and luminaires.

Their good thermal stability, moldability and mechanical properties offer benefits

at virtually every stage of the LED value chain – solving challenges to sealing,

protecting, adhering, and shaping light. With the addition of their attractive optical

qualities, moldable silicones can address design issues such as diffusion and glare

control, color temperature variation and performance over time. Moldable silicones

Fig. 3. A finished LED lamp might incorporate over ten different silicone-based components, including adhesives, pottants, secondary optics and encapsulants.


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offer the potential to advance the adoption of LED lighting, drive down cost and

help expand the technology into new markets, such as general and accent lighting

for home, office and retail spaces, traffic lights and other outdoor lighting, mobile

devices and automotive interior lighting. Silicone-based LED lighting could especially

benefit applications that require a cool touch and environmental toughness.

HuGO DA SILvA is Global Market Manager for Lighting at Dow Corning, Lighting

Solutions. Based in Belgium, da Silva leads the optical, thermal and protection

business for solid state lighting devices such as LED, OLED and innovations

related to lamps, modules and luminaires applications.


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* This article was published in the April 2013 issue of LEDs Magazine.

Aluminum extrusions match SSL thermal management needs in many applications

Design teams working on LED-based lighting products must consider aesthetics, cost, and quality when considering thermal elements of a design, but thermal performance is paramount.

WitH led-based solid-state lighting (SSL) technology

revolutionizing the lighting industry, new opportunities for

lighting products are appearing everywhere from residential

to commercial to street lights. However, the dirty little secret

of LED technology is that it presents a thermal management challenge that is

significantly different, and hotter, than any challenge ever presented by legacy

light bulbs. Conducting heat away

from the LED junctions is a requisite

for long product life and consistent

lumen and color maintenance.

There are many material and

manufacturing choices for thermal

management, and aluminum

extrusions can serve in a broad set of

applications.

Architects, lighting designers, and

other specifiers are demanding

fixtures and enclosures that offer great

looks, options for both finish and color,

as well as structural integrity. Design

Fig. 1. The CFD analysis of a heat sink shows the heat source in the center of the heat sink (yellow). The heat dissipates away from the source (green to blue to purple).

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Aluminum extrusions match SSL thermal management needs

engineers are attempting to provide the above, while dealing with the thermal

management issue. Lighting manufacturers want to deliver all of the above, but

in cost-effective products.

The benefits of LEDs are well accepted at this point: higher-quality light, greater

energy efficiency, and

lower maintenance

costs, thanks to the

long life span of

the bulbs. However,

thermal management

continues to be a

challenge. The heat

generated by LEDs is

detrimental not only

to the life of the bulb,

but also to the quality

of the light. Engineers are now challenged with developing products that not only

look great, but also solve the thermal management problem.

the thermal challenge

The objective of thermal engineers is to remove the heat from the source and

dissipate it into the surrounding atmosphere, as far from the electronics and as

fast as possible. The lighting industry prefers to use passive thermal-management

products such as heat sinks as opposed to active thermal-management

techniques. Typically, active thermal management equates to the addition of

a device to assist in moving air over the heat sink, often a fan. Moving air can

increase the effectiveness of a heat sink or even enable the use of a smaller heat

sink in some applications. However, active elements can increase cost, add noise,

and/or decrease system reliability.

Engineers typically use the light-fixture or enclosure materials to assist in

transferring the heat. Most materials have the ability to conduct heat, some

better than others. This ability is also referred to as thermal conductivity and is

measured in watts per meter kelvin (W/mk).

Fig. 2. The thermal conductivity of aluminum varies with the heat sink manufacturing process.

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Different materials offer a broad range of thermal conductivity. For example,

diamonds have a very high level (typically 2200 W/mK), but are obviously too

expensive for use in lighting applications. Copper has a decent level of thermal

conductivity (typically 390 W/mK), but has two significant drawbacks compared

to aluminum — copper weighs approximately three times more than aluminum

and typically costs up to five times as much. Aluminum doesn’t conduct heat

quite as well (237 W/mk maximum), but offers the weight and cost advantages

that are important in many SSL applications.

Computational fluid dynamics

To assist in determining the proper thermal management solution, engineers

typically work with specialized software that models the products and their

thermal characteristics. Computational fluid dynamics (CFD) is used to simulate

the thermal conductivity of the product and finite element analysis (FEA)

examines the structural integrity of the component. An example of CFD is shown

in Fig. 1. There are numerous variables in each product that are dependent on

product size, shape, and application (indoor versus outdoor, for example). By

combining FEA and CFD along with the variables, it is possible to design the

most cost-effective product that meets the needs of both the design engineer and

thermal engineer.

Historically, aluminum has been the material of choice for thermal management

applications in the lighting industry. The variable has been whether the

Fig. 3. A CFD analysis shows a comparison of temperature between similar die-cast (left) and aluminum-extrusion (right) heat sinks. The extruded product realizes a 23% reduction in maximum temperature.

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aluminum is cast or extruded. LEDs, however, have resulted in new thermal

challenges and also diversity in SSL product form factors that can require new

thermal approaches. In response, the lighting industry has started to use thermo-

plastics and graphite in certain thermal management applications.

Through the remainder of this article, we will review the four materials and

manufacturing process combinations that are most often utilized to address the

thermal management challenge in LED fixtures. The candidates are:

• Aluminum extrusions,

• Aluminum castings,

• Injection-molded thermo-plastics, and

• Molded graphite.

There are certain designs, applications, or conditions where each one of these

materials/processes makes sense.

extrusions vs. castings

Aluminum extrusions and aluminum castings are most often used in LED

thermal management. Sapa alone provided millions of pounds of extrusions to

the lighting industry in 2012. That growth is being driven by improving thermal

efficiency, design flexibility, and the cost advantages of aluminum extrusions

versus castings, proof that many designers are discovering that aluminum

castings are not the solution for the majority of the applications.

There are several types of castings used in the lighting industry. For lower volume

applications, such as specialized street lights, sand castings are the product of

choice. Other types of castings include permanent-mold castings, which are used

for mid-volume applications, such as standard industrial or warehouse lighting

applications. Finally, die castings are used for high-volume applications, such as

lights sold through retailers.

When considering the associated costs, sand castings have the lowest tooling

costs, which typically fall within the $5,000 to $10,000 range and offer the highest

piece price. Pricing for permanent mold castings typically ranges from $15,000 to

$30,000. Piece part pricing for both sand and permanent mold products are highly

dependent on the amount of secondary machining required.

17



Die castings have lower

piece prices compared

to both sand and

permanent-mold castings

and usually require

the least amount of

secondary operations,

however tooling can

range from $50,000

to $100,000. Injection-

molded tooling costs are

similar to those of die-

casting tooling.

All forms of aluminum cast tooling have a specified life expectancy after

which tooling needs to be replaced. By comparison, tooling for a large extrusion

generally falls within the $5,000 to $7,500 range, and aluminum extruders

typically cover all replacement tooling costs, giving extrusions a cost advantage

over castings.

thermal conductivity

In terms of thermal conductivity, there is a clear advantage to using extrusions

over castings. Aluminum extrusions can be 53% more efficient than castings

because they contain a higher level of thermal conductivity. The collective

conductivity of the types of castings referenced above is typically within the

120–140-W/mK range, while the conductivity of aluminum extrusions is typically

within a much higher 200–215-W/mK range. Fig. 2 shows the comparative

thermal conductivities of pure aluminum, extruded alloys, and cast alloys.

Numerous lighting companies have found out the hard way that sand, mold, and

die castings can be less efficient than aluminum extrusions. The nature of the

casting process creates problems with gas porosity. If the porosity is near the

area generating the heat, the porosity acts as an oven, holding the heat in that

area, which will then reduce the life of the LED. This is especially a problem with

foreign casters who may have lower quality procedures and standards. Porosity is

not an issue with the aluminum extrusion process.

Fig. 4. Samples of extruded-aluminum heat sinks show the flexibility of the manufacturing process.

18



You can easily use CFD analysis to compare similar aluminum die castings and

aluminum extrusions. Fig. 3 shows such a review, illustrating that the extrusion

process allows the fins of the extrusion to be designed without draft, which is

required for die castings. This fact allows for the longer fins in the extrusion

that provide additional surface area. In general, the greater the surface area,

the greater the natural convection of the heat into the surrounding atmosphere.

The combination of the extrusion’s increased surface area and higher thermal

conductivity over die casting results in a 23% reduction in the maximum

temperature.

The increase in thermal conductivity of extrusions versus castings allows

the lighting manufacturer to use less material to obtain the same thermal

efficiency. Less material plus a smaller footprint usually equates to lower total

costs. Additionally, high-volume CNC machining allows extruders to machine in

features in a cost-effective manner. Another design advantage to extrusions is

that the process allows for a superior surface finish, which can be anodized in

numerous colors, bright-dipped, or painted any color (Fig. 4).

With extrusions, there is also more flexibility in terms of size. Extruders can

create products upwards of 21-inches wide and offer fin ratios of 19:1. Two

methods of providing wider products include a snap-fit design, which is often

used for enclosures or boxes, and a technology called friction stir welding, which

allows extruders to join two or more pieces of aluminum together with no filler

material. Sapa has used this technology to hermetically seal an extrusion by

welding a cover on the top, which can be particularly useful in industrial lighting

applications where the fixtures need to be explosion resistant.

injection-molded thermo-plastics and graphite

There are some situations in which aluminum castings, and injection-molded

thermo-plastics or graphite do have advantages over extrusions. Typically, they

are small applications where the heat sinks need to be attached to the bulb, often

in the case of retrofitting legacy products. Although an extrusion could provide

a better thermal management solution, the machining to create the contour is

slightly more expensive than the as-cast/molded product.

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Injection-molded thermo-plastic products and graphite are legitimate options if

weight is a factor and limited structural integrity is acceptable. Overall, these

options provide limited mechanical properties and have size limitations. Retrofit

lamps are an example of a product that successfully uses aluminum die castings,

injection-molded thermo-plastic products, and molded graphite as the heat sink.

An optimum LED enclosure design should be developed with the help of

engineers that specialize in thermal and quality disciplines. Extrusion engineers

can provide in-depth thermal analysis and also design an extrusion to

simplify the manufacturing process. This allows for working directly with the

manufacturer, from design to finishing, for a customized approach to thermal

management that exactly matches application requirements.

STEvE JACkSOn is the Business Development Manager of Thermal Management

at Sapa Extrusions North America (www.sapagroup.com/na).


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* This article was published in the July/August 2013 issue of LEDs Magazine.

Matching SSL and control technology remains a challenge

Lighting specifiers should consider the state of the industry for LED dimming and control in order to find the paths for success.

tHe rapid adoption of LED light sources is rooted in energy savings,

long life, and new fixture options that enable them to be used in almost

any application. They are highly efficient; deliver a useful lifetime

averaging 50,000 hours; and offer very good color rendering. LED lamps

also emit very little infrared (IR) radiation and contain no mercury. Despite these

clear advantages, issues of compatibility between LED lamps, drivers, and controls

continue to cause confusion for

specifiers and their customers.

If they are paired improperly,

performance will suffer.

The best strategy for selecting an

LED product is a holistic approach

that takes into consideration

a variety of factors including

the application type, required

dimming performance, and

control requirements. Mock-

up installations and expensive,

time-consuming testing may

be necessary before customers

are confident that the proposed lighting system is the best choice. Many LED

component and control manufacturers are investing more time and effort into

FIG. 1. There is a linear relationship for dimming level and consumed power with LED lighting.


21


dimming testing and research, and can provide compatibility information to

ensure successful LED lamp, driver, and control installation.

looking beyond baseline efficiency

LEDs are energy efficient by design. Simply using LED lamps or fixtures can help

a facility meet updated building and energy codes while reducing electricity

consumption and cost. So why worry about dimming your LEDs? For the same

reason you control any light source: to maximize energy savings, extend system

life, enhance flexibility, increase productivity, and provide a safe, comfortable

environment for building occupants. Additionally, many energy-efficiency

standards are being updated to mandate more sophisticated lighting control

strategies, effectively mandating dimming in many applications.

A wide range of controls are available — from a single switch or dimmer to a

centralized lighting control system — to provide maximum flexibility, as well

as measurement and reporting tools to help you effectively analyze the energy

savings being achieved with the lighting and control installation. Sophisticated,

configurable systems can allow tuning of spaces based on actual occupant usage

post-installation, which over time can allow lower energy densities that surpass

design goals. Furthermore, easy-to-install wireless controls facilitate simple

retrofits, reducing installation and programming costs, and improving return on

investment (ROI).

Regardless of the control system you choose, it is critical to work with a

manufacturer who can guarantee compatibility and performance with the

desired LED loads, eliminating many of the common concerns and issues that are

seen with LED installations. Let’s discuss why and how to implement dimming in

more detail.

Maximize savings and system life

Dimming LEDs saves energy at a roughly 1:1 ratio, which matches or even

exceeds the energy reduction of dimming fluorescents. This means that if you

dim LEDs down to 50% of their light output you save nearly 50% of the associated

energy use (Fig. 1). While it is true that LEDs are already very efficient compared

to almost any other light source, you save even more energy by dimming them.

L T F

POWERED BYPOWERED BY

L.T.F., L.L.C. PHONE: (847) 498 5832 FAX: (773) 337 5628 EMAIL: [email protected] Company’s Address: 2905 MacArthur Blvd Northbrook, IL 60062 U.S.A. www.ltftechnology.com

LTF serves OEM customers in the Architectural, Commercial, & Residential lighting industries with innovative designs and technology. We have the most complete selections of low voltage LED drivers including the smallest form factors in the industry.

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We don’t think you should design fixtures to fit power supplies. That’s why we design power supplies to fit your fixtures. We offer a large selection of power supplies in a variety of sizes but we don’t limit your imagination. We encourage OEM customers

to challenge us with innovative fixture designs and we will design and build complete solutions to fit your needs giving you a unique advantage.

Our current selection includes:• 10-300W AC/DC UL rated dimmable

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23


Dimming LEDs also makes

them run cooler, extending

the life of the electronic

components in the driver,

as well as the phosphor

in the LEDs. This will

potentially double or triple

the useful life of the LED

lamp or module. Research is

ongoing to better quantify

the relationship of dimming

LEDs and lifetime extension.

Still, you must choose the

fixture, driver, and control

combination to meet project

needs. LEDs are making great

strides, and LED products

now exist for replacing

virtually any fixture type

including general-purpose

lighting, downlights, cove

lights, and outdoor lighting.

The type of control you

choose will depend on the

results you want to achieve. For example, in a lobby or atrium, a 20% minimum

dimmed light level is typically acceptable. But in a conference room or restaurant,

very low levels of light — dimmed down to 1% — are often desirable.

It is all too easy to neglect the importance of LED dimming range. While 10%

dimming may sound appropriate for most applications, our eyes are sometimes

too smart for their own good. Due to the dilation of the pupil in the human eye,

lower light levels are perceived as brighter than expected. For example, a 10%

light level appears to be about 33%. Even 1% dimming is perceived as a 10% light

level (Fig. 2).

FIG. 2. The human eye perceives much higher light levels than the actual output from dimmed sources.


24


The expected dimming range should always be part of the product specification,

but it is not always provided by the manufacturer. Furthermore, even for a given

LED load, the dimming range may vary depending on the control used. Designers

and specifiers need to be aware of the low-end light level that can be achieved

with the proposed LED load and control combination.

If all the parts and pieces are not carefully evaluated, the result can be dimmable

products that do not work as claimed. For example, you can end up with lights

that never turn off

completely, or that flicker,

pop on, or drop out, leaving

the end user with the

perception that dimming

LEDs does not always work.

Challenges of dimming

It is generally recognized

that LEDs are inherently

dimmable and controllable,

so why are there so

many challenges with

dimming them? It is the result of the physical differences between LEDs and their

predecessors including incandescent and halogen lamps.

Incandescent lamps create light by heating a tungsten wire to high temperatures

in a low-pressure glass envelope, causing it to glow white hot. Electrically

speaking, these are very simple devices — the more voltage delivered to the

source, the hotter it got, and the more light it produced. With legacy sources the

shape of the voltage waveform really didn’t matter. AC, DC, phase cut, or nearly

any other form would provide the same amount of light for the same RMS voltage.

LEDs behave very differently. Light is produced by subatomic processes in

specially designed semiconductor materials. For a given LED device, the amount

of light generated is proportional to the amount of current (not voltage) passed

through the device. Furthermore, the current can flow in only one direction

through an LED, meaning they can only tolerate DC current. (Note that so-called

FIG. 3. LED lamps have integral drivers that limit control options.


25


AC LEDs rely on tricks such as two diodes that are wired such that current flows

through some diodes in one direction, and other diodes in the other direction.)

Finally, LEDs are inherently low-voltage devices, typically requiring a large

reduction in voltage from the mains wiring. These functions — reducing the

voltage, regulating it to DC, and controlling the current — are all handled by a

device called an LED driver.

LED drivers come in a variety of designs, constructions, and feature sets.

One thing they have in common is that they do not have the same electrical

properties as an incandescent load, and this difference is essentially the root

cause of compatibility challenges between controls and LEDs.

Different manufacturers’ drivers prioritize different requirements. Some may

optimize for cost, some for size, some for lifetime, and so on. Part of the design

of the driver determines how well, and how low, it will dim, and using what

controls. This fact leads to two important conclusions:

The design of the driver determines the best possible dimming performance that

can be achieved.

The compatibility of the driver with the control determines how well the driver

will achieve this performance.

In essence, even the best control cannot make an LED lamp dim beyond its design

parameters. Both poor driver design and improper pairing with a control can lead

FIG. 4. Luminaires often allow the specifier to choose a driver with the desired adaptive controls.


26


to undesired aesthetic performance, including flicker, drop-out, dead travel, or

acoustic noise (buzzing).

Additionally, poor driver design and control pairing can lead to reduced lifetime

of the control or load. A good driver will guarantee smooth, continuous dimming

to very low light levels on a wide variety of controls with no negative impact

on lifetime, matching the dimming performance that people expect from

incandescent lamps. Few driver manufacturers in the industry today can reliably

make this claim.

ensuring expected performance

In order to properly align customer expectations with LED system performance,

several factors need to be considered during the course of designing an LED

project:

the load type (retrofit lamp or fixture)

the required control type

the number of loads allowed on a control

the dimming performance of the load and control combination

For assistance, look for trusted manufacturer resources that can assist you with

the selection process, such as the LED Control Center of Excellence offered by

Lutron Electronics. Let’s discuss the factors in more detail.

LED light sources come in two basic types: retrofit lamps (sometimes called

screw-in lamps or LED lamps), and LED fixtures (Figs. 3 and 4). Different

applications call for different solutions, but from the perspective of dimming

compatibility, there is one major distinction. LED retrofit lamps have a fixed

driver built into the lamp that can only be controlled via phase-cut dimming,

while LED fixtures can offer a selection of drivers, available at specification time,

which may provide a range of dimming performance and compatible controls.

If retrofit lamps are being used, even a proven lamp and control combination will

max out performance at the lamp’s published capability. Improved performance

may require selection of a different lamp, perhaps from a different manufacturer.

With fixtures, most manufacturers offer a range of drivers, which allows


27


selection of the dimming performance in advance, without affecting the aesthetic

or optic performance of the fixture.

For general-purpose illumination, there are four predominant control methods

available:

forward-phase control

reverse-phase control

0–10V analog control

digital control (DALI or EcoSystem)

Each of these methods has strengths and weaknesses, making some more

suitable for certain

applications than others.

However, lighting designers

and specifiers often select

a fixture or control system

without a full understanding

of the ramifications of

that selection when it

comes to LED and control

compatibility. Note that while

other control schemes exist,

including DMX or direct

wireless control to lamps or

fixtures, these methods are

not widely used in today’s

general-purpose lighting applications.

Forward phase

By far the most commonly used dimmer today, forward-phase controls are

dominant around the world and 150 million are estimated to exist in North

America alone. Thus, many LED loads claim compatibility with forward-phase

dimmers. Forward-phase dimmers cut, or turn off, the front part of the 60

Hz, 120V (in North America) mains voltage sine wave (Fig. 5). The cut supply

effectively reduces the RMS voltage delivered to the fixture. The more of the

FIG. 5. Forward phase-cut dimmers eliminate the voltage during the initial part of each half cycle of the AC line.


28


sine wave that is cut off, the

lower the voltage, and the less

energy the load uses. This

method is extremely common

because it is easy, generally

inexpensive, and uses the

xisting wiring to the fixture.

The recently released NEMA

SSL7-A standard helps

define the proper behavior

of forward-phase dimmers

and LED loads to ensure

reliable operation; it also

provides a basic expectation

of performance. LED loads and controls marked as SSL7 compliant should remove

much of the guesswork associated with compatibility for this control type.

reverse phase

Reverse-phase dimmers operate much the same way as forward-phase dimmers

do. They reduce the RMS voltage going to the load by cutting off part of the sine

wave. However, unlike forward-phase dimmers that remove the front part of the

sine wave, reverse-phase dimmers remove the back portion of the sine wave (Fig.

6). Like forward-phase dimmers, this reduces the RMS voltage to the fixture and

uses the existing wiring.

This method was originally developed for control of electronic low voltage (ELV)

transformers with low-voltage halogen lamps. These loads make up a much

smaller portion of the market than their line-voltage counterparts. Therefore,

reverse-phase dimmers are much less common than forward-phase dimmers.

Only a small percentage of installed dimmers are compatible with ELV loads.

However, due to the electrical similarities between ELV transformers and LED

drivers, several driver manufacturers design their drivers to work exclusively

with reverse-phase dimmers.

FIG. 6. Reverse phase-cut dimmers cut the voltage at the end of each half cycle.


29


analog 0–10V control

In contrast to the previous methods that rely strictly on existing wiring, the

analog 0–10V scheme requires an additional pair of low-voltage wires to be run

from the control to each fixture. This low-voltage pair provides the signal to the

driver, which determines the target light level. A voltage of 1V tells the driver to

go to the low end of its dimming range, while a voltage of 10V tells the driver to

go to the high end. Generally, a line voltage switch is also included in the system

to cut mains power to the driver when the lights should be off. This behavior is

specified in an IEC standard, 60929, which covers only very basic functionality.

For example, there is no assurance provided by the standard that smooth,

continuous dimming to low light levels will occur. Mixing 0–10V fixtures from

different manufacturers or using long wire runs for the 0–10V signal can cause

noticeable differences in light levels across multiple fixtures.

While 0–10V allows the control wires to be run separately from the power wires,

it has an inherent disadvantage when multiple control strategies are desired. By

definition, all fixtures tied to the same pair of 0–10V wires are controlled together

and will dim together. This fact means that for spaces with multiple control types,

where fixtures must dim to different levels due to differing control inputs (such as

daylight sensors, personal zone controls, and occupancy sensors), the room must be

broken into multiple areas of control, each with their own 0–10V wires. Control of

0–10V loads can become very complex to design and install for all but the simplest

applications. One other disadvantage of 0–10V is that any change of functionality or

fixture zoning requires rewiring of the 0–10V control links.

digital control

Like 0–10V, digital control methods, such as Lutron’s EcoSystem and the Digital

Addressable Lighting Interface (DALI), require an additional pair of low-voltage

wires to be run to each fixture. Unlike 0–10V, however, the pair of wires sends

bi-directional communication signals to each fixture, allowing individual

addressability and control. DALI, a commonly used digital protocol, is also

defined by the IEC, but differing interpretations of the standard can lead to

incompatibilities between devices from different manufacturers even if they

all claim DALI compliance. EcoSystem was developed by Lutron based on DALI

and compatible fixtures are offered by Lutron and other lighting companies


30


such as Cree and GE Lighting. Compatible fixtures provide guaranteed control

compatibility, no matter which manufacturer is selected.

Digital control methods allow easy layering of control schemes. Each fixture can

behave independently from all others if that is desired to achieve lighting goals.

Digital technologies enable advanced functionality and zoning of fixtures through

a simple software interface, with no modification of the wiring required. Digital

controls allow flexibility in space configuration and reuse. Digital fixture control

most effectively unlocks the capability of smart lighting promised by LEDs.

Control ratings

As was described previously, LED loads do not electrically behave like

incandescent loads, leading to differing performance on different controls.

However, beyond aesthetic performance, the electrical characteristics of many

LED loads cause additional stresses on controls, beyond what their wattage alone

may indicate. This complicates determining how many loads — for instance, how

many retrofit lamps or downlights — can be reliably connected to a control.

The high inrush currents, repetitive peak currents, and RMS currents that can

occur when phase-cut dimmers are used with LEDs means that most controls

rated for incandescent loads cannot handle nearly the same wattage of LED loads.

Generally, only detailed electrical testing can determine the proper minimum

and maximum number of loads that can be connected to a control.

What many users do not understand is the effect of the lamp selection on the

regulatory rating of the control. Controls are tested and listed with a specific load

type in mind. For example, UL may rate a control as appropriate for incandescent or

magnetic low voltage (MLV) loads. Using them on other load types, such as ELV loads

even at the same voltage level, means they are being used beyond their original

design and testing, which can lead to unexpected behavior or decreased reliability.

Most existing incandescent dimmers have not been designed, rated, or tested

by UL or another nationally recognized testing laboratory (NRTL) with LED

loads. Fortunately, many control manufacturers have recently released dimmers

explicitly rated for controlling LEDs, such as the C•L dimmer family from Lutron

Electronics. These types of controls have LED-specific ratings, allowing the actual


31


wattage of the LED to be used to determine the maximum loading. In summary,

the manufacturer of the control should always be consulted to determine

whether or not the control in question has been tested on LEDs, and that the

control is not overloaded.

A holistic approach to LED control can help meet and exceed customer

expectations. Technologies are improving; control options, available literature,

and general knowledge are expanding; and LEDs can now be effectively used in

virtually any type of commercial application. By choosing the right manufacturer,

control, and driver, and considering key issues, it will be easier than ever to

provide customers with LED lighting and a control system that meets energy-

saving, performance, and aesthetic expectations.

ETHAn BIERy is the LED engineering leader at Lutron Electronics Co., Inc.


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COB LEDs simplify SSL manufacturing, drive broader deployment

Dense LED arrays can reduce the design complexity and manufacturing cost of solid-state lighting products and ultimately help solve the global energy crisis.

tHe ligHting Market

is evolving rapidly from

traditional lighting sources

to solid-state lighting (SSL)

technology. The main drivers for this

change are the need for greater energy

efficiency due to rising energy costs

and new legislation, both of which have

global implications. With the market

drive to mass adoption of SSL technology,

lighting manufacturers must adapt to

faster product development cycles that

more closely match developments in

LED technology. Developing fixtures

that incorporate chip-on-board (COB)

LED arrays is one avenue toward faster development, although

that choice also comes with challenges in how to implement

the electrical, thermal, and optical interfaces with the LED. This

article provides background on COB technologies, summarizes the

issues and challenges with COBs, and suggests a unique luminaire assembly and

interconnect approach that simplifies luminaire design.

FIG. 1. A chip-on-board (COB) LED packs a dense array of LEDs.


33


The world is facing a wide range of

energy-related issues ranging from oft-

debated global warming to diminishing

fossil fuel supplies. Power generation

limitations due to increasing demand

or resulting from natural disasters

in various regions result in brown-

outs or even black-outs that directly

affect quality of life for residents.

These disruptions can extend to

national productivity due to decreased

manufacturing output at companies located in those regions.

Political, economic, geographic, or logistical issues often limit

adding power generation capacity to address the increased

demand. Given this environment, countries around the world

are fully engaged in trying to minimize their carbon footprints

and decrease energy consumption, thereby lessening the strain on their energy

infrastructure.

One major global effort is to decrease the energy load posed by the inefficient

lighting systems in use today. Increasing global population and the resulting

rise in demand for lighting can no longer be served by incandescent sources that

accounted for 79% of light source sales volume in 2006. Collectively today, lighting

energy consumption accounts for around 18% of the total global generated

energy and cannot be allowed to continue. Simply switching to readily available,

more energy-efficient light sources such as CFLs or LEDs can result in a 40%

energy savings that would eliminate 630 million tonnes of CO2 and 1800 million

barrels of oil. It would further cut down on the power-generating footprint,

eliminating the need for almost six hundred 2-TW/yr power plants. Although it is

neither feasible nor realistic to expect this change to happen overnight, phased

regulations are in place around the world to ban the incandescent bulb. These are

progressing and driving the changeover to incandescent alternatives — primarily

turning to the LED moving forward.

Hb led evolution

Let’s move on to LEDs, the evolution of the technology, and the challenges of SSL

FIG. 2. Companies initially developed custom sockets for each LED from major manufacturers.

13MCTR012 AdSeries-LED_8x10-5.indd 1 3/28/2013 3:18:52 PM

http://www.metalcoaters.com/led


35


development. LEDs have

gone through a significant

transition since their

inception that mimics

the evolution of the single

semiconductor transistor

device morphing into the

integrated circuit we know

today. Early LEDs were

low-power devices used for

indicating purposes and

are still in wide use today

across many applications.

In a manner similar to

transistors, an indicator LED that cost $300 in the 1960s

can now be purchased for less than $0.05 in a number of

different package styles. As with transistors, LED technology

transitioned from its original format as a low-power device to

a high-power device in the late 1990s/early 2000s. While these

earlier devices were considered high power, they were single-

die packaged emitters with very limited light output and

limited practical use.

In an effort to increase light output, companies began placing multiple die in

a lead-frame style package to increase light output and individually placed

phosphor dots on each die to tune color. Around 2005, a new LED package was

developed that eliminated the secondary LED package and placed the die directly

on a metal-clad PCB substrate — collectively called chip-on-board or COB (Fig. 1).

Rather than individual phosphor dots on each die, these COBs are characterized

by a yellow-orange phosphor pool or slurry that covers all die and is typically

centered on a white-colored substrate, giving these LEDs the nickname “fried-egg

LEDs” due to their appearance.

These COB LEDs have undergone a proliferation over the past couple of years as

all major LED manufacturers began offering COB products to the market. Unlike

the smaller packaged LEDs that are considered point source emitters, COBs by

FIG. 3. Hand soldering is one option for making the electrical connection to COBs but can be unreliable and time consuming.


36


their nature are wide area

emitters with up to 204

0.25-mm2 die packed into

a 12-mm-diameter area as

seen in products offered by

Nichia Corporation. In some

instances, larger die are used

in large arrays as seen in the

Bridgelux products that place

64 1-mm2 die in a 35-mm-

diameter area.

With light source areas this large, COBs are clearly positioned as broad angle

emitters — their large source size becomes very difficult to effectively focus into

a small beam angle. As a result, COBs are now found in applications requiring

large amounts of light spread out across a large area as seen in high bay and

street lighting. Still, optics such as reflectors can also enable the use of COB LEDs

in applications including downlights and even reflector-based retrofit lamps.

Connecting packaged leds

Over the years, multiple connector companies developed a number of sockets

for the high-power LEDs that came to market. In fact, the idea has spread that

an LED is just another device to be connected to. Sockets such as the TE Type LS

and Type NL2 (Fig. 2) devices were dedicated to a particular manufacturer’s LED

package and therefore had limited applicability beyond the specific targeted LED.

The emergence of COB LEDs posed different challenges to fixture manufacturers:

how to provide power to these devices and then affix them to heat sinks in the

lighting fixtures. The traditional method was to hand-solder the wires to the pads

on the substrate (Fig. 3) and then secure these assemblies with screws to the heat

sink — a process that was time consuming and subject to variability. As with the

earlier discrete devices, this presented an ideal opportunity for socket solutions.

TE and other connector companies addressed this emergence of initial COB

devices the same way earlier discrete LED packages were addressed: custom

sockets. As other COBs started to enter the market, connector manufacturers

FIG. 4. A pair of holders that reference the corners of the LED can work with many different COB LEDs.


37


realized that the status quo method of developing a socket for each would be

prohibitively expensive and time consuming, resulting in never-ending efforts

forever chasing the next new COB to enter the market.

With well over 50 different

COB products commercially

available around the

world from multiple

manufacturers, finding a

single solution to address

each would be a challenge

to say the least. Aside

from the fact that all were

rectangular, all had two

electrical contact pads,

and most had circular light

emitting surfaces, there

were few other dimensional

similarities. The challenge

for connector companies was how to address these varied but similar COB LEDs

with a minimum of engineering and tooling expenditure while providing a

future-proof and flexible platform based product.

scalable interconnects

One solution to the interconnect challenge is a flexible, scalable, platform-based

socket. An analysis of available COBs yielded a crack in the shell of these “fried

egg” LEDs. While not exactly identical, there are similarities between contact

pad locations that, when combined with the rectangular nature of the devices

and diagonal contact pad placement, form the basis for the platform solution.

As a result, a holder product, referenced off the corners of the COB, could yield a

virtually limitless array of use scenarios with the varied COBs available on the

market as well as those to come in the future.

The substrate differences among COB LEDs from difference manufacturers,

however, added yet another issue that needs to be tackled by socket/holder

suppliers. While most of the early COBs incorporated aluminum substrates,

FIG. 5. The COB holders can directly place force on thicker aluminum substrates.


38


further investigation of

the commercially available

COBs indicated a number

of COBs that are based

on ceramic substrates

(typically aluminum

nitride). While not a

significant differentiator,

these two substrate

variants have physical

features that differ enough

to require special handling

due to their different

material properties.

The aluminum substrate COBs are rather robust and in some instances are

mounted using machine screws. On the other hand, ceramic-based COBs create

some frustration for fixture manufacturers since unlike aluminum-based

substrates where they could simply secure the COBs with a screw to a heat

sink, these ceramic substrates are far too brittle to secure with screws and

thereby mandate some sort of secondary attachment to ensure suitable thermal

performance. This attachment needs to be accomplished using thermal adhesives

or by a mechanical holder that provides normal force to the heat sink or thermal

pad, mechanical attachment, and electrical interconnection.

Holder devices that accommodate the COB nuances mentioned in the previous

paragraphs are appearing on the market. An example of one such platform

solution is the TE Scalable LED Socket connector that utilizes the corners of the

COB as a reference. In this manner, the two datums formed by the sides of the

COB can be used to positively locate the electrical contact on each of the COB’s

electrical pads.

Moreover, the diagonally opposed contact pads on most COBs can allow the

luminaire industry to leverage the symmetry. The symmetry can enable use of

the same socket assembly on both corners of the COB (Fig. 4). By doing this, SKU

(stock keeping units) or model numbers are drastically reduced by eliminating the

FIG. 6. The holders must leave a gap on the thinner and brittle ceramic substrates.


39


need for right and left versions of the socket, providing an advantage for socket

makers, distributors, and SSL manufacturers.

Managing substrate height

The variability of COB thickness poses yet another design challenge for a socket

product. COB substrates are available in a variety of materials and thicknesses

that need to be accommodated. The thinner and more brittle ceramic packages

can easily crack under pressure when used with flexible thermal interface pads

and therefore consequentially use greases as thermal interface materials (TIMs) to

optimize the thermal path from the LED substrate to the heat sink. On the other

hand, the more durable aluminum substrates can use grease or flexible TIMs.

Any holder being

considered by an SSL

manufacturer needs to

be evaluated relative to

how it accommodates

these varying stack

heights posed by COB

applications. One method

is commercially available

that utilizes a combination

of optional thermal springs

and housing ledges to

accommodate both types

of COBs. An example using

an aluminum substrate COB is shown in Fig. 5. The red circle indicates how the

housing is used to secure the COB against the heat sink.

When the same system is used with the thinner ceramic substrates as shown in

Fig. 6, clearance needs to be provided to avoid exerting pressure on the ceramic

substrate. The housing design naturally clears the thinner ceramic COB as

indicated by the red circle. Since the electrical contact spring is inadequate to

provide an appreciable normal force for thermal contact, a secondary thermal

spring is incorporated and is seen in Fig. 7. A pair of these springs can exert just

FIG. 7. Springs can secure ceramic substrates against the TIM and heat sink.


40


the right amount of force on a ceramic device to ensure optimal heat transfer

when used with most commercially available thermal greases.

simplifying manufacturing

While a two-piece socket design affords a significant level of scalability to

accommodate a wide range of COBs, some fixture manufacturers prefer to only

handle a single part. Additional levels of scalability can be incorporated into a

two-piece design by the relatively simple addition of two arms oriented at 90° to

each other. These arms essentially create a scalable, factory-assembled one-piece

housing. A customized holder is then available to fit a specific manufacturer’s

COB LED (Fig. 8).

A scalable approach to the

interconnect enables a more

generic two-piece design or a

customized one-piece design, and

either can be used by the fixture

manufacturer with very little

cost difference between the two.

Consider the option a stepping-

stone approach. A lighting

manufacturer can minimize the

interconnect investment during

prototyping by launching the

fixture using a readily available two-piece holder solution. Once production

ramp starts, the manufacturers can switch to a more manufacturing-friendly

one-piece design.

Ideally, having a single holder to accommodate all COBs would be the perfect

solution. Given the slight differences in pad locations between commercially

available COB products and the different plating styles commonly used, that

perfect solution is still elusive. Nonetheless, with a scalable holder solution,

solderless interconnections to a broad range of COBs are possible with a

very small number of socket SKUs that can accommodate a wide range of

commercially available products.

FIG. 8. By adding arms to the corner holders, companies can supply a one-piece COB interconnect to SSL manufacturers.


41


COB corner holders are also quite adaptable. An additional benefit to the holders

referencing off the corners of a COB is that the applicability of these holders can

extend beyond COBs. By using the same corner holders used to provide power into a

COB, large printed circuit boards containing massive arrays of LEDs can be powered

in a similar fashion through pads located on the corners of the circuit board.

In conclusion, COB holders, in particular those that reference off the device

corners, offer lighting fixture manufacturers a unique and flexible termination

solution for their COB LED attachment and interconnect needs. By utilizing a

holder designed from the start as a scalable platform, fixture manufacturers gain

the flexibility to utilize a broad range of light source options. From a COB LED

manufacturer’s point of view, the availability of a common holder design provides

a stable reference for manufacturers designing a new COB package since the use

of pre-defined contact pad locations ensures the availability of an off-the-shelf

holder solution when a new COB is released to the market.

The global march toward energy efficiency continues. LED lighting will play a major

role and, as with all new technologies, efficiencies are increasing while costs are

dropping. The need for cost-effective LED lighting is putting a renewed emphasis

on cost and manufacturability, which is where both COB LEDs and the COB holders

offer customers an ideal cost-effective solution that will accelerate LED adoption.

ROn WEBER is the industry market manager for lighting and security at TE

Connectivity’s Industrial Business Unit.

LEDsDigest_Nov2013_SSLFixtures

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