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.
Of course, lower power consumption is just one reason for the change. LEDs also last longer and are more rugged than the incandescents that they replace. Plus, despite their higher cost per bulb, LEDs can cut system-level lighting costs over the life of the application by their ability to reduce product maintenance and downtime. (For more on the economics of replacing incandescents with LEDs, see "Cost-Effective LEDs Fit Snugly In Today's Energy Conscious World" by Jordan P. Papanier of LEDTronics Inc., Electronic Design, Jan. 24, 2000, p. 93.)
Evidence of LED benefits abound. In the transportation and automotive realms, LED-ready products are showing up in traffic signals and vehicle brake lights, as dashboard backlights, and as roadside messaging displays. LEDs also are catching on in the aerospace industry, where they illuminate airport runways, provide in-cabin lighting, and identify flight obstructions, such as antenna and water towers. Closer to the ground, they light up exit signs, and they're being considered as both indoor and outdoor decorative lighting for many architectural applications. Those large channel letters that spell out names of department stores and other businesses now signify further uses for LED backlights.
In these wide-ranging applications, LEDs have become practical due to several factors. First, LED semiconductor materials are being honed to produce greater levels of light at the standard levels of drive current.
Meanwhile, vendors are developing high-flux chips that can be driven at much higher current levels than previously possible. Light output is rising significantly. Many of these new applications take place outdoors, where the ambient lighting is often intense sunlight. Daylight visibility of the lamp is a must, which means a very bright LED.
Packaging also accounts for much of the progress. It's possible to build very bright light sources by either of two methods. Individually packaged LED chips—discrete LEDs—may be clustered together in one lamp assembly. Alternatively, multiple LED chips can be combined in a single housing. In both methods, but especially in the multichip approach, the buildup of heat within the semiconductor threatens to cut short the LED's long operating life. Creative packaging designs, however, are applying the necessary heatsinking to prevent heat buildup in these LED arrays.
Aside from brightness, there's the issue of color. The availability of high-intensity blue and white LEDs has helped to fill out the LED color spectrum. White LEDs can generate illumination or backlighting akin to incandescents, while blues can complete the red-green-blue (RGB) trifecta needed in full-color displays. But all of these multicolor pyrotechnics would be of little value in the real world without the steady reductions in device cost fostered by developments in the lab, the fab, and the competition-driven marketplace.
Anatomy Of An LED
The traditional discrete LED has a fairly simple structure. Typically, a semiconductor diode chip is mounted within a reflector cup that sits atop the device's two-wire leadframe. Wire bonds connect the chip with the leadframe, which also carries heat away from the chip. In the standard device, the chip, wirebonds, and a section of the leads are encased in a solid epoxy lens (Fig. 1).
Among discrete through-hole LEDs, the T1-þ and T1 package styles are the most common. The numbers in these designations refer to lamp diameter in eighths of an inch. So, the T1-þ package has a diameter of 0.219 in. or approximately 5 mm. Similarly, the T1 package has about a 3-mm diameter. In addition to these popular types, other through-hole packages exist, as well as a range of standard surface-mount options. SMT LEDs can be found in 0603-, 0805-, 1206-, and 1210-sized chips, plus in SOT-23 packages. Many versions of nondiscrete LEDs, like seven-segment displays are available, too. (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.)
But there are other variations as well. In some cases, chips of different colors are packaged together to create different color effects. For example, the combination of red, green, and blue semiconductor chips creates an LED capable of producing light in 256 colors.
Other packaging variations are employed in higher-flux LEDs, which can incorporate significantly larger dies than the typical 5-mm discretes. Consequently, the parts can operate at higher drive currents. But they additionally need more heatsinking than the leadframe and pc board affords. In some devices, a copper heatsinking slug helps keep the die cool.
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.
Benchmarks And Recent Advances
One of the key manufacturers behind the development of high-brightness LEDs is LumiLeds Lighting, a joint venture between Agilent Technologies and Philips Semiconductor. Their LED technology includes dies with high current capabilities housed in application-specific packaging.
This manufacturer emphasizes materials and packaging technology over product lines because of its applications orientation. "We no longer talk about LEDs as discretes. We're providing light sources," explains Doug Silkwood, LumiLeds' director of marketing.
Along this line of philosophy, the joint venture aligned with other LED/display manufacturers to incorporate their technology into popular lighting and signaling applications. Two include brake lights and traffic signals. In the automotive section, LumiLeds developed high-intensity reds and ambers in the Super Flux LED package. This package is sometimes called the Piranha package in the industry, although that term doesn't show up on data sheets.
The automotive-oriented Super Flux LED relies on a transparent substrate and a reflector on the underside of the die to capture more of the light produced by the AlInGaP die. As a result, these parts produce 2.5 times the light output of conventional 5-mm LEDs. A slight modification of the Super Flux is the SnapLED. For assembly in the application, it replaces the solder joint with a clinch or crimp connection that's more rugged and better suited to the mechanical shocks and vibrations of the automotive environment. An updated version of this product, SNAP-150, doubles the light output to five times that of a 5-mm LED.
The company boosted light output to even higher levels for traffic signal applications. Their custom lighting division developed LEDs with 10 to 15 times the light output of typical 5-mm LEDs. But whereas those 5-mm LEDs run at about 20 to 30 mA, the LumiLeds use a larger die (about 10 times the size of one in the 5-mm package) that allows them to operate at 300 to 400 mA! This dramatically reduces the number of LEDs required in the traffic signal lamp.
While a standard 8- or 12-in. signal head might require between 80 and 300 standard LEDs, the LumiLeds' High-Powered Light Sources—sometimes termed the Barracuda package within the LED industry—require only 18 LEDs. This reduces the number of interconnects in the assembly, thereby enhancing reliability.
Nevertheless, heatsinking presents a challenge with so much power concentrated in a small area. Without sufficient heatsinking, the operating life of the LED would be compromised. Therefore, the die in the High-Powered Light Source is mounted to a copper heatsinking slug (Fig. 1b, again). In a traffic lamp solution developed by Dialight Corp., that slug is then mounted to a metal-core pc board, with further heatsinking in the lamp housing (Fig. 3).
The company's technology serves as a reference for many of the LEDs now in development, including a recent one from LumiLeds. Lumileds' latest packaging innovation enables the company to double the light output and power efficiency of the AlInGaN material that it uses to build high-intensity blue, green, and blue-green LEDs.
Whereas industry-standard 5-mm devices generate about 10 mW in the blue region, the new LumiLeds blue generates 100 mW (5 to 8 lumens). The greens produce 40 mW (10 to 20 lumens), and the blue-greens deliver 70 mW (15 to 25 lumens). If it seems odd that a 40-mW green puts out more lumens than a 100-mW blue, consider that lumens takes into account the eye's sensitivity to different wavelengths. We're generally more sensitive to green light than blue.
Another vendor, Ledtech Electronics Inc., is working on a package similar to LumiLeds' Super Flux package. According to Ledtech, the package will push the current rating on the LED from 80 mA up to 175 mA, which the new part should handle comfortably. The company expects an eight-to-one reduction in the number of Super Flux-style LEDs needed in the application. Ledtech also will make this part available without requirements for strategic customer-vendor agreements. Additionally, it plans to price the part competitively.
Meanwhile, Marktech Optoelectronics is developing a Step 3 series of yellow LEDs for employment in roadway "caution" signs. The new devices will be 30% to 50% brighter than the company's current offerings. Their standard LEDs now deliver 3500 to 4000 mcds at a viewing angle of 17° and a drive current of 20 mA. Under similar operating conditions, the new LEDs will produce 5000 to 6000 mcds.
When applied in solar-powered signs, the newer devices will extend battery life from a maximum of 30 days per charge to 45 days. Rather than running the new devices at 20 mA, it will be feasible to obtain the same brightness on 12 mA. This not only increases run time, it permits downsizing of the power supply as well.
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).
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 2). 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.
Green Light For LEDs
The traditional traffic signal contains three 8- to 12-in. heads with incandescent lamps rated at 67 to 150 W. Power consumption varies with color. Red requires lamps with the greatest number of watts, while yellow and green require less power. In part, this reflects that red filters pass less light than yellow or green. But light sensitivity is involved too. The average traffic signal consumes 990 kWh/year with power taken from the ac power lines.
Incandescents last roughly 8000 hours. This translates to a useful life of one to two years for an incandescent. Physically, in addition to the bulb, the lamp assembly includes a color lens and a reflector. Plus, lamp electronics control and drive the lamp and monitor it as a load to make sure the bulb is operational. Although the cost of the replacement bulb is just $2.50 to $3.00, incandescent-based traffic-signal bulbs might need to be replaced annually.
The LED replacements offer alternative solutions. Some rely on the 5-mm discrete lamps, while others adopt a high-flux approach using LumiLeds-style devices (Fig. 3, again). Furthermore, some solutions require rewiring of the lamp while others offer drop-in replacements. Each has potential advantages.
The drop-in may be an easy installation, an attraction of such products as the Type 3 LED traffic lamp from LEDTronics, scheduled for introduction in the first quarter of next year. The lamp is a direct replacement for 67- to 135-W incandescent traffic lamps. LEDTronics' lamp uses the existing reflector and traffic lens, so there's no rewiring or rebuilding of the traffic heads. This approach gives an extra level of flexibility to the customer, who may replace the LED lamp with an incandescent if necessary.
On the other hand, if rewiring is an option, it's possible to replace the high-voltage ac power with a lower-voltage supply. That reduces the risks of exposure to high voltage, which are present when a signal pole is struck by an automobile. This approach also allows someone other than a licensed electrician to perform servicing.
Traffic-signal incandescents are being replaced over time. Initially, only the red lamps were replaced because affordable high-brightness reds came along first. With a red LED lamp, power consumption drops from the 150 W of an incandescent to only 10 to 15 W. According to Gary Durgin, vice president of marketing at Dialight Corp., up to 18% of the red lamps were retrofitted with LEDs in the last five years. Green LED lamps are taking hold now too, although the switch to LEDs for yellow is less common because this lamp is off most of the time.
Although LED lamp life is described as 100,000 hours, this figure may be lower in the field. Consequently, maintenance must be performed a little more often than every 11 years. According to Vincent Forte, chief engineer at Marktech Optoelectronics, LED lamp makers recommend an average replacement time of five years, which still represents a significant incentive for those specifying traffic signals.
The economic incentives for LED traffic lamps continue to improve. Driven by the advances in high-flux technology, the costs of lamps are dropping. Gary Durgin says that volume pricing for the red LED traffic bulb dropped from $120 a year ago to less than $75 today. Pricing for green went from $230 to $240 per bulb to $175. Furthermore, some utilities in California are picking up the tab for the bulbs by offering rebates equal to the cost of the LED lamp replacement.
Of course, the cost equation must also account for other factors. These include rewiring costs, actual reduction in signal maintenance costs (are crews still being sent out on the same schedule to service other equipment?), regional utility costs, and the availability of lower-power incandescents or other alternative light sources.
Visibility is another issue when replacing a critical incandescent indicator with an LED. Even when filtered, the light output of an incandescent produces a fairly wide spectral output with many wavelengths. But LEDs are monochromatic sources with a narrow spectral output. Individuals with some degree of colorblindness could experience less sensitivity to a given LED lamp versus an incandescent.
If a lamp is harder to see, the viewer's reaction time may be longer. Keep in mind that red-green color deficiency occurs in 8% of men and 0.5% of women. The intensity of an LED lamp might need a boost to account for individuals with reduced sensitivity to color. But that need must be balanced with the need to provide appropriate brightness for those with normal color sensitivity. An alternative to simply increasing light intensity is selecting wavelengths that are more effective for those with color sensitivity deficiencies.
As previously mentioned, the variation in LED light output over temperature affects the design of the drive electronics. Gary Durgin notes that when the traffic signal is housed inside a plastic enclosure, the temperature inside the heat-absorbing enclosure can rise by 50°C. If that temperature rise isn't compensated, the lamps will grow dimmer as they bake in the sun (Fig. 4).
Naturally, these are just some of the concerns that crop up in the design of traffic signals. Various environmental factors play a role in signal usability, and it may be difficult to match all requirements of different municipalities. Unfortunately, there's no common standard for traffic signals across the country.
Nevertheless, successfully building LED-based solutions for traffic signaling will likely help push the development of high-brightness LEDs for many other applications. After all, the growing visibility of LED lamps on the road may alert more designers and potential consumers to their practicality in signaling and illumination. When such a revelation arrives, the magical light that goes on above the designer's head might just contain a cluster of chips.
For more information:
- The Lighting Transformations Web site provides a wealth of lighting-related information for a variety of applications, many of which use LEDs. The site map is located at www.lrc.rpi.edu/Ltgtrans/sitemap.html.
- For specifics about the application of LEDs in traffic signals, read "Optimizing the Design and Use of Light-Emitting Diodes for Visually Critical Applications in Transportation and Architecture: Research Issues and Options," by John Bullough, Kun Michelle Huang, and Kathryn Conway.
- For a list of LED and transportation-related LED product manufacturers, go to www.lrc.rpi.edu/Ltgtrans/led/led-mfr.html.
- Compound Semiconductor Magazine includes extensive coverage of LED technology, including news, trends, a listing of industry events, and links to LED chip makers. This information is available in print or online at www.compoundsemiconductor.net/.
- A wealth of LED-related information may be found at two sites. Don Klipstein's Lighting Info Site! at www.misty.com/~don/light.html includes a section titled "The Brightest and Most Efficient LEDs and Where to Get Them!" Klipstein reports his test results for numerous LED models. The main page, www.misty.com/~don/ledx.html, contains an LED primer, an overview of different LED types, more advanced topics, and links to other LED sites.
- Another Internet haven is Craig Johnson's LED page: www.ledmuseum.home.att.net/. Johnson provides test results on a varied and an extensive list of LEDs, and he takes visitors on a historical tour of LED technology in what's billed as "The World's First Virtual LED Museum."
|Companies That Contributed To This Report|
Agilent Technologies Semiconductor
Contact George Lee
American Opto Plus
Data Display Products
Infineon Technologies Corp.
Contact Ingo Kreuschner
Ledtech Electronics Inc.
Mule Lighting Inc.
Nichia America Corp.
Opto Technology Inc.
(847) 537-4277, ext. 235