Brighter LEDs Signal Longer Life And Lower Power For Lighting Applications
Improvements in chip material and packaging enable LED arrays to supplant many incandescent lamps.
by David Morrison
Power, Packaging & Components Editor
Electron Design
elecdesign.com
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What CMOS circuits did for the world of power-hungry electronics, LEDs may be doing for the world of lighting. Just as CMOS-based ICs reduced the energy necessary for a given electronic function, LEDs have slashed power requirements in many lamp applications. Thanks to advances in chip material and package design, these solid-state lamps, long viewed solely as status indicators, have shined their way into a host of applications that traditionally relied on incandescent lamps and other light sources.....
....Aside from these packaging effects, the package plays a large role in determining the brightness, or luminous intensity, and viewing angle of the LED. Furthermore, these two parameters are intrinsically related. Brightness in a visible product also is a function of the sensitivity of the human eye to the different wavelengths that make up the light spectrum.
It isn't surprising, therefore, that with LEDs, specifications for luminous intensity often appear in different units that reflect differences in measurement approaches. Before an LED chip is packaged, its light output might be rated in terms of radiant power (mW) or radiant flux. That value reflects the total light output. But once packaged, LEDs are typically rated in lumens (lm) or millicandellas (mcds).
Lumens is a rating of light power integrated over the spectral response of the eye, so it reflects perceived light to some extent. Millicandellas take perceived brightness a step further by taking into account the viewing angle. The general guideline is that an LED which casts its light over a narrower viewing angle is brighter than one with the same number of lumens but a wider viewing angle. The term viewing angle refers to the angle over which the pattern of radiated light is at least half of its peak value.
The viewing angle is largely determined by packaging-related factors. The shape of the reflector cup, the lens, and the distance between lens and chip all affect the viewing angle. Some other factors include the size of the chip and the clarity of the lens. A clear lens—one that lacks tinting or diffusion—produces a narrower viewing angle as well as a brighter beam.
Although packaging affects both the viewing angle and brightness, in most cases, the chip alone determines color. When a visible-color LED is forward-biased and driven by a sufficient level of current, it produces monochromatic light somewhere in the range of 400 to 700 nm. White LEDs are an exception because white light has components across the visible spectrum.
It's possible to create white light by combining the light from red, green, and blue chips. This method, though, is unpopular because the semiconductor materials used to make the three chips differ, as do their responses to temperature variations. This situation requires that the red, green, and blue chips be individually compensated for temperature to maintain the same hue of white. In the standard method, white LEDs are fabricated by applying a white (or yellowish) phosphor coating over a blue LED chip.
The chips themselves can be fabricated from one of several semiconductor compounds. Over the years, vendors have developed semiconductor compounds with higher levels of luminous efficiency. As a result, LED performance, measured in lumens/watt, has risen steadily in time (Fig. 2). So for the same level of drive current, the newer compounds are much brighter than the old ones. Typical voltage drops vary from about 1.5 to 4 V, depending on the chip material. A 20-mA drive current is fairly typical. Therefore, power dissipation for the usual discrete LED is normally a value in the tens of milliwatts.
One of the turning points for LEDs resulted from work on aluminum gallium arsenide (AlGaAs). This compound was used to build the first daylight-visible red LEDs, which were then put to work in the first LED-based vehicle brake lights, traffic signals, and exit signs. Later, aluminum indium gallium phosphide (AlInGaP, often pronounced as "allen gap") chips produced even brighter reds as well as bright oranges, yellows, and lighter shades of green. Plus, the development of gallium nitride (GaN) led to similarly bright blues, bluish-greens, and whites. The current crop of high-luminous-intensity LEDs employ AlInGaP and GaN/InGaN. .......
....Further news of ultrabright devices comes from Cree Inc which recently introduced the Ultra Bright series of blue and green LED chips. Typically, the 470-nm blue delivers 5 mW or 440 millilumens, while the 525-nm green puts out 3 mW or 1400 millilumens. Fabricated from InGaN on a SiC substrate, these devices are said to narrow the performance gap that still exists between SiC and the brighter sapphire-based devices. In addition to these new components, other LED developments have expanded the range of high-intensity LED options (see Table 1; PDF reader required).
Lamp replacement modules represent a category of LED packaging in which discrete LEDs or chips are clustered to generate light output comparable with the bulbs they're meant to supplant. Usually, those bulbs are incandescents in industry-standard bases (see Table 1; PDF reader required). LED vendors are developing lamp replacement modules as both standard and custom products. Although they cost more than incandescents, LED lamp replacements offer compelling reasons to switch. LEDs consume 80% to 90% less power and last much longer than incandescents. LED vendors cite a life expectancy of 100,000 hours or roughly 11 years for many of their products.
Lower failure rates as well as greater durability also set LEDs apart. When it comes to withstanding shock, vibration, and temperature extremes, LEDs have the advantage over incandescents. But temperature is an issue with LEDs. As semiconductor devices, they can be susceptible to thermal runaway. This concern can be addressed in the drive electronics and in the chip material. Another concern is the decrease in light output that occurs at higher ambient temperatures. (More on this later.)
When the light source is battery powered, there's an added benefit of more usable light at the end of battery life. Northe Osbrink, technical editor of the Semiconductor Products Group at Agilent Technologies, points out that when the cells in an incandescent flashlight run down, the weak light emitted by the bulb turns a shade of orange. Unfortunately, you can't use this light because it's in a region where our light sensitivity is weak. Contrast this with a white LED flashlight. The color of the white LEDs remains practically constant as the light becomes dimmer with decreasing drive current.
Yet cost is often the central factor in determining whether an LED solution can replace an incandescent. Two issues come into play. If the cost of the lamp over the life of the application is lower with LEDs (taking maintenance and downtime into account), then the higher initial costs of the product are justified. In some cases, initial costs are further mitigated by utility-sponsored rebates.
But in the long run, better and cheaper LED technology will make LEDs more feasible. As Jim Sloan, president of SloanLED, points out, price erosion of high-intensity LEDs is making them more attractive as replacements for incandescents. According to Sloan, ultrabright reds are running at about 10 to 20 cents a piece in volume, while white and blue LEDs cost about 65 to 85 cents for comparable brightness.
New approaches to packaging, such as one developed by Ledtech, may increase the popularity of LED lamp replacements. In most lamp replacement modules, the light source clearly is an LED array. Ledtech has developed a device with an epoxy fill that diffuses the light, creating an incandescent-like effect. Its 360° viewing angle makes this lamp more noticeable as an indicator.
For now, LED lamp replacements are being adopted in those applications where replacing an incandescent bulb is either difficult or costly. But increasingly brighter LEDs bring hope that "one of these days, LEDs will replace lamps in the home," says Silkwood of LumiLeds. Immediate applications might include lighting for exterior landscapes, accent or shelf lighting, and under-cabinet lighting. Silkwood speculates, "The first LED-based products for these applications are one to two years away."
But before LEDs can be applied in these home lighting applications, some technical challenges must be addressed. White LED lamps have to be developed that are bright enough and sufficiently low in power consumption to justify their use. These LEDs must also have a hue fairly close to that of halogen because those are the lamps that might warrant replacement.
While standard lamp replacement modules provide some off-the-shelf options, much of the progress occurring in the area of lamp replacement involves customization. This allows lamp designers to fully take advantage of the differences between LEDs and incandescents. Traffic signals illustrate this point as well as some challenges of transitioning from light-emitting wires to light-emitting diodes. <snip>
(A more comprehensive list of LED-related terms and their meanings, along with a primer on device operation, appears in "Light-Emitting Diodes 101" on the MCD Electronics Web site: www.mcdelectronics.com/led101.html).
Jim |
