The picture is getting brighter for solid-state LED lighting, with applications spreading worldwide for indoor and outdoor illumination, automotive interiors, backlighting for TVs and monitors, mobile phones and other consumer electronics, and even automotive headlamps.
Global sales of packaged high-brightness LEDs jumped 93% last year to reach $10.8 billion, according to analyst Ella Shum, director of LED Market Research at Strategies Unlimited. Much of this growth occured in backlighting for TVs and monitors as well as in mobile appliances.
Shum notes that despite the current global interest in using solid-state LED lighting for illumination, the market for high-brightness LEDs represents only 8% of last year’s total sales. Still, she expects the high-brightness LED illumination segment to grow about 39% between 2010 and 2015.
ElectroniCast Consultants recently released a market study that claims the consumption of LED lamps in the U.S. last year was worth $485 million. The company expects this figure to reach $1.64 billion in installed lamps by 2015 (Fig. 1).
Pros And Cons
There’s no mystery why LEDs are gaining favor over other light sources. According to the Solid-State Lighting Research and Development project of the U.S. Department of Energy (DoE) multi-year program plan, an LED provides higher efficacy levels and longer lifetimes than high-intensity discharge (HID) lamps, linear and compact fluorescent lamps (CFLs), halogen lamps, and incandescent lamps.
A cool white-light LED offers a maximum efficacy of 132 lumens/W (l/W), compared to a measly 15 l/W for an incandescent lamp, and can last 50,000 hours versus 1000 hours. It also uses 20% of the power an incandescent lamp requires for lighting—a very important fact in an energy-conscious world.
But the drawback is a higher price. A Philips AmbientLED A19 replacement lamp costs about $40 compared to $1 or less for an incandescent bulb. This disparity, expected to drop four-fold by next year, is offset by the much longer lifetimes and tremendous energy savings that LEDs provide.
The DoE estimates that if everyone converted to solid-state lighting, then by 2020, enough electricity would be saved to power 32 million homes. LEDs already save power that’s equivalent to taking 250,000 homes offline.
Also, LED output levels have reached 150 l/W and continue to rise. Several LED manufacturers make high-brightness LEDs that provide dazzling illumination in a wide range of colors. One shining example is Cree’s LR6 downlight lamp. Its patented color-mixing method can create light comparable to that produced by incandescent bulbs but uses 85% less energy and lasts 50 times longer (Fig. 2).
And the efficacy levels of LED lamps keep improving, as do their lower prices as energy-saving alternatives to incandescent lamps. For example, the LEDtronics PAR series of LED bulbs offers 90% energy savings over incandescents (Fig. 3).
Operating from either 120 V ac or dimmable in a wide range of 90 to 290 V ac, these LED lamps come in Edison medium screw bases and GU24 bi-pin base bulbs in various PAR20 (7-W), 30 (12-W), and 38 (15-W) styles. They boast 20% higher efficacy than the previous-generation LEDtronics PAR lamps and directly replace incandescent and halogen lamps that consume up to 100 W.
The purchase cost of an LED lamp is just one factor in a larger issue: the total cost of ownership, which involves not only much lower energy costs, but also maintenance and replacement costs. Researchers at Sandia National Laboratories estimate that today’s LED lighting has twice the lifetime cost of ownership of incandescent bulbs and 10 times the cost of ownership of fluorescent lamps. But the lab also predicts that by 2020, the differentials will shift to one-tenth those of incandescent bulbs and one-half those of fluorescent lamps.
The desire of businesses and organizations to save tens of thousands of dollars annually on energy costs is proving to be a major driving force for retrofitting to LED lighting. It’s happening in many malls, stores, offices, and building interiors and exteriors. It is also occurring in structures like bridges, tunnels, highways, streets, parking lots, arenas, and airports.
Lighting Science Group, which designs, develops, manufactures, and markets high-efficiency LED solutions, reports that it is busy installing many of its high-performance and ultra-efficient lights like the Definity MR16 systems in such settings. They’re being used as retrofits in Simon Property Group mall kiosks, replacing halogen lamps. The company says that its fully dimmable Definity line is 80% more efficient than halogen lamps and provides 50% more light output than many competitive light sources.
One of the challenges facing LED designers and users is that there’s no standardized approach for interfacing light sources with drive circuitry. A European consortium, Zhaga, has initiated an industry-wide effort to develop standard specifications for the interfaces of LED light engines. An LED light engine is an LED module with defined interfaces that don’t depend on the type of LED technology used inside the light engine.
Zhaga will enable interchangeability between products made by diverse manufacturers. Interchangeability is achieved by defining interfaces for a variety of application-specific light engines. Zhaga standards will cover the physical dimensions, as well as the photometric, electrical, and thermal behavior of LED light engines.
Optimizing Driver Designs
LED driving is a complex task. That’s because applications for LED lighting and illumination can vary. Performance parameters like efficiency and color accuracy can be greatly affected by how the LEDs are driven. “The manner of driving an LED can impact the efficacy, color temperature, and color shifting (caused by heating) as well as flicker effects,” explains Peter DiMaso, strategic marketing manager for lighting power products at Texas Instruments (TI).
Patrick Carner, TI’s C2000 microcontroller unit (MCU) marketing manager, agrees. He also points out that TI, for some time, has trumpeted the usefulness of its Piccolo MCUs in LED driver solutions, allowing designers “to dig deeper into the hardware and software used for digital power control.” The company’s C2000 dc-dc LED lighting developer’s kits show the proper power topologies needed for driving single and multiple shared power stages (Fig. 4).
“A major barrier for designers using LEDs is how to handle the software, and the Piccolo control kit simplifies all this. It walks you through the whole design process in a step-by-step manner,” points out Brett Larimore of TI's C2000 systems and application engineering.
Driving LEDs with high currents and low stack voltages while meeting Energy Star and Lighting Facts Label requirements, put forth jointly by the DoE and the Environmental Protection Agency (EPA), as well as European standards is becoming a challenge. One solution involves using fewer LED components and driving them at a higher current level for higher light output levels.
Using fewer LEDs means lower product costs, but it isn’t necessarily beneficial for improving mandated EPA efficiency levels. In fact, efficiency levels tend to drop, a situation noticed in early A19 and E27 lamp retrofit applications. Power efficiency levels are too low.
LED driving gets more complicated with increased output light levels. According to Haitz’s Law, every decade, the cost per lumen for a packaged LED falls by a factor of 10 and the amount of light generated per LED package increases by a factor of 20 for a given wavelength (color) of light. As more light output is achieved, a packaged LED’s usefulness will become more limited for certain applications (for distances of ~1 km for residential lighting and ~10 km for commercial lighting).
While today’s technology can readily address these LEDs, in the years ahead they will be constrained to special applications and will have limited commercial relevance, unless cost-effective technological innovations in LED drive technology and thermal management are greatly improved. To that end, the solid-state lighting industry is making impressive and steady gains, but it remains to be seen if it can keep this trend going in the years ahead.
When driven at higher currents, an LED behaves in a dynamic fashion. Higher currents not only deliver higher output lumens, they also drastically increase temperatures well beyond the above ambient temperature range the LED is operating in. Some estimates say 70% of an LED’s efficiency is given off as heat, which must be removed via conduction or convection. This contrasts sharply with the conventional incandescent tungsten bulb the LED is meant to replace.
According to Jeff Perry, senior development manager at National Semiconductor, a 100-W incandescent bulb may be only 2.2% efficient whereas the best production LEDs on the market are only about 30% efficient. However, the incandescent bulb gives off a significant amount of its heat through infrared radiation. Not so with LEDs, which require heat control using heatsinks and fans. Higher temperatures mean shorter lifetimes and decreased reliability levels (Fig. 5).
A multi-disciplinary approach is required, where both the LED enhancement features and the driver circuit design are considered to alleviate this problem. Power-management IC manufacturers are now developing more efficient LED drive circuitry that targets solid-state lighting designs.
Older LED driver topologies found in earlier solid-state products, which were largely modifications of existing voltage-regulation schemes, are lacking for today’s LEDs. Moreover, LEDs used for solid-state lighting are application-specific. For example, LEDs used in retrofit A19/PAR type lamps are different from those used for street lighting and MR16 applications. Recognizing this, LED manufacturers now offer application-specific LED components.
Newer LED drivers are more sophisticated. They can sense drive current levels and alter them when necessary when too much current is being drawn beyond recommended levels. They also can compensate for changes caused by heat and aging to provide a steady light output. And, their thermal protection circuitry minimizes LED damage due to overheating and subsequent LED failure.
National Semiconductor’s LM3464 and LM3424 drivers incorporate a thermal fold-back scheme that tweaks the current by dimming the output, allowing the LEDs to remain within their maximum operating temperature range. National also offers the LM3445 LED driver to cope with the tough European EN61000-3-2 standard, which sets tight restrictions on lighting devices that consume more than 25 W with regard to total harmonic distortion (Fig. 6). To follow power-factor correction (PFC) requirements, the driver uses PFC to keep input current in phase with the input voltage.
The STMicroelectronics HVLED 805 offline LED driver features primary sensing feedback (Fig. 7). The high-voltage pulse-width modulated (PWM) current-mode controller combines a low-voltage PWM controller with a an 800-V power MOSFET in a tiny SO16N 8.75- by 3.99-mm package. Designed to operate from the ac power line, it provides accuracy within ±5% and quasi-resonant zero-voltage switching. It’s designed for E27 and GU10 retrofit lamps.
The ON Semiconductor CS5171 LED driver IC offers high efficiency and can be powered from a source that’s higher or lower than the LED voltage. Its single-ended primary inductor converter (SEPIC) topology allows it to be used as a dc-dc boost regulator. The chip operates at 260 kHz with about 65% efficiency at a 4.0-V input.
IC drivers are also available for a growing number of backlighting applications for TVs, laptops, mobile phones, and other consumer electronics. Advanced Analogic Technologies offers the AAT14xx family of 31-mA step-up single-channel LED drivers for driving up to 10 LEDs in a single string from a 10-pin, 1.15- by 1.55-mm wafer-level chip-scale package (WLCSP). According to the company, these drivers are 80% smaller than a 1.3- by 2.9-mm SOT23 package and 35% smaller than a 2- by 2-mm quad flat no-lead (QFN) package.
Microsemi Corp. recently announced a reference design that enables lower-cost LCD panels to produce higher-quality images. The design draws on Sony’s CXD473063 timing and lighting controller and Microsemi’s DAZL! LED driver technology for next-generation 3D LCD TVs. Microsemi’s two-chip solution includes the DAZL! 2000 series 32-port LX24232 logic chip and the eight-port LX23108L LED driver power chip.
Driving LED strings configured in series, such as those that are as large as 20 or more LEDs in luminaire fixtures, can be problematic. A single LED failure causes the entire string to go dark. To combat this, the Bourns LSP series of shunts protects individual LEDs in a series string.
Manufacturing, Yields Are Key
Manufacturing LEDs on large wafers and achieving higher yields is the key to reducing their cost as more widely used solid-state lighting elements. Most LEDs today are grown 2-in. and 4-in. GaN-based (gallium nitride) sapphire wafers and suffer from non-uniform yields, causing them to be tested and binned for color brightness, forward voltage, and other variations.
When assembly and packaging steps are added, the LED price goes up even higher. Yields of 30% and lower are common today. In fact, experts don’t expect yields of 40% or more until 2020 (see “Expect GaN To Play A Role In Future LEDs And Power Devices" at www.electronicdesign.com).
LED manufacturers are planning to transition to 6-in. wafers over the next few years, followed by 8-in. wafers, using epitaxial metal-oxide chemical-vapor deposition (MOCVD) processes that provide higher yields. These larger wafers will be possible via tighter control over temperature, flow, and materials composition. LG Electronics started producing LEDs on 6-in. wafers last year. Philips Lumileds and Lextar are also close to doing so. Some companies will even be sampling 8-in. wafers next year.
“If the LED industry is to going to continue to grow, it will be with general lighting,” says Tom Pearsall, secretary general of the European Photonics Industry Consortium (EPIC), a non-profit organization that works to facilitate the development of the LED industry in Europe. “And that depends on continued major progress in everything from basic research and materials to device design and manufacturing efficiency.”