As the name implies, a light-emitting diode (LED) is a semiconductor (diode) that, when forward biased via a voltage/current source, radiates visible light of a particular color (wavelength) and at a brightness level determined by its parameters and by the parameters of its power source. The first LED, attributed to General Electric researcher Nicholas Holonyak circa 1962, was a fairly low-power device capable of producing low-intensity red light, but with a hefty price tag.
Breaking price barriers by 1968, the Monsanto Company and Hewlett Packard began mass production of red LEDs in 1968 using cost-effective gallium arsenide phosphide (GaAsP). Initially, red LEDs found fruitful employment as replacements for incandescent and neon function indicators such as on/off/standby lights and shortly after as segments in alphanumeric displays.
Evolving on an upward flight path not unexpectedly similar to television, LEDs are now available in a wide range of colors as well as single units capable of producing multiple colors, brightness, and power levels and in various unique package types. Myriad devices also can deliver non-visible light from the infrared (IR) and ultraviolet (UV) ends of the spectrum.
Naturally, the rapid evolution of device types often leads to revolutionary applications. No longer just performing as indicators, visible-spectrum LEDs are supplanting incandescent and fluorescent components in almost all lighting (practical and decorative) and signage applications because of their low-power/low-heat characteristics, significantly longer lifespan, and lower cost in both long and short runs.
Over the years, LEDs also have wandered into esoteric, non-lighting designs such as wave shapers in audio circuits (Fig. 1). IR and UV LEDs are proving to be viable in numerous applications ranging from remote control to medical as well.
VISIBLE LIGHT LEDs
Today, LEDs are available in a wide variety of sizes, colors, shapes, and types with diverse electrical specs and parameters, in standard and unique packages, and all with varying price points. Each addresses one or more applications such as general lighting, flash functions in digital cameras, LCD backlighting, and, getting back to basics, indicators, plus many more.
Usually made from aluminium gallium arsenide (AlGaAs), gallium arsenide phosphide (GaAsP), aluminium gallium indium phosphide (AlGaInP), or gallium phosphide (GaP), generic, low-cost moderate-power LEDs (Fig. 2) for basic indication chores, prototyping, and hobbyist interests are still around and plentiful. With red still being the most common color, these devices are also available in green, orange, yellow, and blue and operate with a forward voltage drop in the realm of 1 to 2 V with a forward current around 20 mA.
In addition to the standalone device, other general-purpose LEDs include alphanumeric displays, bi-color and tri-color LEDs, and red-green-blue (RGB) components and flashing LEDs. For no-frills designs with few power and size restrictions, one of these should fit the bill and budget.
Primarily targeting lighting applications, be they industrial, commercial, residential, or decorative, high-power LEDs (HPLEDs) and HPLED modules are rapidly replacing traditional incandescent and fluorescent fixtures, especially as their cost recedes. These LED alternatives are notable for their long life of more than 50,000 hours, exceeding the 10,000 hours or more for fluorescents and 1000 hours or more for incandescent bulbs even under inordinate on/off cycling. Power efficiency is an equally desirable benefit with the HPLEDs delivering brightness levels beyond 105 lumens/W.
One example of the LED supplanting inefficient technologies is the XLamp MPL EasyWhite LED from Cree (Fig. 3). Promising better performance, color consistency, and lumen density than conventional light sources, it’s optimized for directional lighting applications, including PAR-style or BR-style light bulbs. With attention to system design, it can deliver the same light output as a 3000-K, 75-W equivalent BR-30 light bulb while consuming 78% less energy than incandescent technology.
In a package measuring 12 by 13 mm, the MPL EasyWhite delivers up to 1500 lumens at 250 mA. Additionally, it’s available in 2700-K, 3000-K, 3500-K, and 4000-K color temperatures that are in the center of the respective ANSI C78.377-2008 color bins. Naturally, when characterizing LED packages for a particular lighting task, other viable options are out there.
One of the roadblocks to overall efficiency is that LEDs require a dc power source, which entails the use of power converters for many lighting applications. In addition to more parts, these converters need to be well designed, upping the cost of the LED topology.
Seoul Semiconductor may have this solved in part with its Acriche LED bulb (Fig. 4), which operates directly from an ac power source. The 100-lumen/W Acriche light source specifies 25% greater efficiency than existing LED products. Requiring no ac-dc converter, it generates less than one-tenth the carbon emissions of an incandescent bulb, the company says.
LEDs in general run pretty cool. But when they’re grouped en masse for brighter lighting apps or restricted to heavily populated boards or extremely tight quarters, heat does become a concern. Also putting its fingers in the LED pie, semiconductor company Vishay offers the VLMW321xx and VLMW322xx surface-mount, white LED families (Fig. 5) in thermally enhanced PLCC-4 packages. For wider pin compatibility with similar devices, the VLMW321xx has three anodes and one cathode while the VLMW322xx LEDs offer three cathodes and one anode.
The devices exhibit a thermal resistance down to 300 K/W and a power dissipation up to 200 mW. Groomed expressly for automotive applications, both families are AEC-Q101 qualified. Other shared features include a luminous intensity from 1400 to 3550 mcd, luminous flux from 7000 to 8900 mlm, a 60° angle of half-intensity, and a luminous intensity ratio per packing unit of less than 1.6.
You can’t see it, but that doesn’t mean you can’t use it. IR and UV LEDs operate at wavelengths above 750 nm and below 400 nm, respectively. These devices find gainful employment in remote control (TVs, home entertainment centers, etc.), communication, and optically isolated signal routing in medical applications.
IR devices are generally made of GaAs or AlGaAs, while UV parts come in diamond, boron-nitride, aluminium-nitride, aluminium-gallium-nitride (AlGaN), and aluminium-gallium-indium nitride (AlGaInN) flavors. Some examples include the use of IR LEDs in the sensor bar of Nintendo’s Wii game system and UV LEDs for sterilization of certain bacteria, curing of adhesives, and plant synthesis.
Night photography is one of the many applications for IR LEDs. Enabling image capture in total darkness, LEDtronics offers IR LED lamps (Fig. 6) in 850-, 880-, and 940-nm wavelengths with industry-standard bases and several angles of emission. The lamps resist ambient-light and electromagnetic interference (EMI) and are available in all standard domestic and international voltages.
Notably, the use of multiple LEDs allows the lamp to provide adequate light even if one or more emitters fail. Other advantages include an average life span beyond 100,000 hours. In addition to the standalone device, other general-purpose LEDs include alphanumeric displays, bi- and tri-color LEDs, and RGB components and flashing LEDs.
Jumping to the other end of the spectrum, the QuasarBrite family of UV LEDs (Fig. 7) from Lumex lasts 10 times longer (more than 50,000 hours) and provides tighter beam angles, greater durability, and up to 50% cost savings over comparable devices, the company says. Available in 385-, 405-, and 415-nm wavelengths, applications include bacterial and superficial sterilization, industrial control related to leak and biohazard detection, forensics such as counterfeit detection and analysis of bodily fluids, and ink fluorescing.
A category unto themselves, organic LEDs (OLEDs) appear to be the wave of the future. In December 2009, DisplaySearch indicated in its Quarterly OLED Shipment and Forecast Report that worldwide OLED revenues broke the last record, reaching $252 million in revenue for the third quarter of 2009, up 31%. Notably, OLED shipments totaled 21.7 million in the same quarter, showing a 19% increase over the prior year.
OLEDs use a layer of organic compound as a light source between their anode and cathode. Depending on the configuration, light can be emitted either from the top or the bottom of the device, enabled by a transparent electrode.
There are three types of OLEDs: transparent (TOLED), stacked (SOLED), and inverted (IOLED). TOLEDs rely on transparent electrodes on both sides of the device, so they can emit light from the top or bottom, while SOLEDs stack red, green, and blue to achieve full-color displays. The IOLED exploits a bottom cathode that interfaces with a thin-film transistor (TFT) backplane to create an active-matrix OLED (AMOLED) display.
Increasingly, OLEDs are finding their way into many display applications due to their advantages over traditional LCDs. For example, they don’t require a backlight, resulting in lighter and thinner panels. Also, OLED displays can turn pixels completely off to display true, deep black. Sony’s XEL-1, which the company calls the industry’s first OLED television (Fig. 8), features a 3-mm thick panel and a contrast ratio of 1,000,000:1.
These are just a few of the types of LEDs available, not to mention their numerous variations. Characterizing the right type of LED is quite easy, but it gets a bit tricky when you have to choose the right one within the given typology.
Some general guidelines apply to most of the design gantlets surrounding LEDs. One would be hard pressed to disagree with Rob Harrison, engineering manager of OSRAM’s Solid-State Lighting Business, when he points out that the first thing to grasp is a complete understanding of the application requirements.
Since variables abound, designers must establish the boundaries of critical parameters: voltage, current, power consumption, heat dissipation, thermal resistance, color, color temperature, color sensitivity, brightness, ambient conditions, packaging, and lifespan. “Be prepared to whittle down your choices from hundreds of LEDs to about 10 or less,” Harrison says. He also points to thermal resistance as the most critical factor. Obviously, heat will affect not only overall design performance but also the lifespan of the LEDs.
In addition to good system design for heat dissipation, energy efficiency is a priority. More and more designs need to meet a number of efficiency standards such as Energy Star. With the increasing focus on environmental concerns, this will become even more critical in the very near future.
A CASE STUDY
A global appliance manufacturer approached Lumex looking to transition away from incandescent bulbs to illuminate the cavity for its ice and water dispenser. The application required a higher light intensity, even light distribution, and high efficiency for energy savings.
The manufacturer’s vision entailed using a high-power, 1-W LED with a cool-white color temperature. Light had to hit the activation paddles, water dispenser, and ice dispenser, preferably with the same light intensity and color. Also, the illumination module had to be easily field replaceable.
Echoing Harrison’s pinpointing of thermal factors as a primary concern, the Lumex design team concluded that the high-power LED would create numerous challenges, particularly heat management, shorter lifespan, and uneven light distribution.
As an alternative, Lumex developed a small molded module integrating a printed-circuit board (PCB) supporting three low-power, 5-mm white LEDs (Fig. 9) with a quick-disconnect two-pin connector at the end of a wire assembly. The LEDs were color and intensity matched so each had the same 2700-K cold color temperature. This approach additionally allowed light to be aimed at exact locations in the cavity.
According to Lumex, the solution reduced service costs by replacing the incandescent bulb with an LED with a 10-year service life. Also, its energy costs were lower since the module consumed less power than a traditional solution. It bested the initial concept of using a 1-W LED, driving three LEDs at 18 mA versus using a 1-W device as well.
EMERGING LED TECHNOLOGY
Whatever the design challenge may be, something is usually coming out to address it—or, at the very least, it’s on the drawing board somewhere. For example, Bayer MaterialScience recently unveiled a unique form of light-diffusion technology that hides LED hotspots while transmitting higher light levels.
The company’s approach creates the effect of softened LED light with minimal reflection, allowing the diffusion of translucent white colors at normally unattainable light-transmission levels. This technology promises nearly limitless freedom for light diffuser packages and a broad palette of colors to customize the application.
“This is an exciting time to be a colorist because we are able to offer product designers and OEMs a design solution specific to their needs,” says Terry Bush, senior chemist at Bayer MaterialScience.
To create a unique diffuser package using the technology, designers select a Makrolon polycarbonate resin grade that suits their particular application. “The better the base resin, the better the overall performance of the diffuser package,” says Gerald DiBattista, market segment leader, IT, Electrical/Electronics Polycarbonates, Bayer MaterialScience.
Available resins include Makrolon LED2643 for indoor and outdoor applications. The formulation resists UV light, water exposure, and immersion. Perhaps the first clear polycarbonate to pass UL 94 5VA flame-rating requirements at 3 mm, Makrolon FR7087 suits lenses and covers. Makrolon 6717, a flame-retardant grade resin, supports extruded applications such as light bars and light guides. Makrolon 3103, a high-viscosity, UV-stabilized polycarbonate, handles a number of applications including automotive and consumer.
Attesting to the fact that there will always be a design solution on the horizon, DiBattista reiterates, “No matter what the final lighting application, there will likely be a solution that meets the application’s needs.”