EDN, May 26, 2011

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Figure 2 Osram's Oslon white diode in a 1W package uses the Brilliant Mix ap- proach, which combines red and white LEDs to create a warmer white than pos- sible with just blue LEDs and a phosphor to create a white light for general lighting. nologies, LED-based solid-state lights have emerged as energy-efficient light- ing that's easily controllable over a local network and lend themselves to intelli- gent-building environments that auto- matically adjust to a building's occu- pancy and use. However, the cheapest white LEDs, which commonly available LED lights currently use, have a blue hue. Designers of solid-state lights and lighting networks are learning about matching the right LED to the right use and can justify using the more expen- sive but also more congenially red-hued warm-white LEDs. LED manufacturers are responding to the lighting requirement for warm lights through a variety of approaches. Cree's TrueWhite modular lighting adds two small red LEDs that kick in depend- ing on the current to the white LEDs. A bulb from system manufacturer Pharox, on the other hand, uses a matrix of dis- crete LEDs that balances continuously on white and red LEDs. LED manufacturer Osram takes another route with its Brilliant Mix approach to creating a white light for general lighting. Brilliant Mix technol- ogy adds red light from a red LED to a greenish-white LED, which comprises a blue LED exciting a green phosphor. Mixing the two yields a white with a color temperature of 2700K with high efficiency and improved CRI. Osram uses this scheme in its Oslon SSL (solid- state-lighting) diodes (Figure 2). Most discrete white LEDs comprise a blue LED covered by a dollop of phos- phor that emits white light when the blue LED's light strikes it. It's difficult 50 EDN | MAY 26, 2011 to decipher the technology that many white LEDs use, and manufacturers are not always forthcoming about what's inside the seemingly discrete LED packages. One way to check out LEDs' warm-light performance is to look for peaks in the light-power-versus-fre- quency charts for a part. Avnet's Kelly points out, for example, that Seoul Semiconductor adds red LEDs to its warm-white Acriche A4 ac LEDs to achieve a high CRI; the spike at 620 nm in the A4's spectrum provides evidence of this approach (Figure 3). Another technological hurdle to replacing the incandescent light is the requirement that replacement lights be compatible with the more than 150 million currently installed TRIAC- based dimming switches. Joel Spira, co- founder and former chairman of Lutron Electronics, invented the solid-state dim- mer switch in 1959 (Reference 8). Now, at least 150 million dimmers are in use worldwide—most likely the reason for the Energy Star requirement that future CFLs (compact fluorescent lights) and LED lights must be compatible with the installed base of dimming switches. This compatibility requirement is difficult to meet because there is no universal speci- fication for the performance character- istics of dimming switches. Therein lies the rub for LED-light designers. TRIAC dimmers are simple in their essence: They stop the ac line from reaching the load during part of the cycle. Less power 1.4 1.2 1 RELATIVE RADIANT POWER (ARBITRARY UNITS) 0.8 0.6 0.4 0.2 0 300 400 500 600 WAVELENGTH (nm) Figure 3 The sharp peak at 620 nm indicates that Seoul Semiconductor uses a red LED in its ac-powered Acriche SAWX4A0X. Both the color spectrum and the radiation pattern are at ambient temperatures of 25°C. 700 800 FOR MORE INFORMATION Avnet Electronics Marketing www.avnet.com Cree www.cree.com Energy Star www.energystar.gov Hasbro www.hasbro.com Lutron Electronics www.lutron.com Osram Opto Semiconductors www.osram-os.com Pharox www.pharox-led.com Seoul Semiconductor www.acriche.com means less light. This approach is fine when you're dealing with a purely resis- tive load, such as an incandescent light, but when you're dealing with an LED that expects constant current, handling the chopped line input from the dimmer can prove challenging. Two characteris- tics are their triggering voltage, or the minimum amount of line voltage it takes to cause the TRIAC to fire, and their holding current, which is the minimum current necessary to make the TRIAC remain on. In addition, dimming switches require a load of 25 to 40W—not a problem for incandescent lights, but CFLs and LEDs typically require 7 to 13W. Thomas Shearer, design and development leader at Lutron, says that a common tactic for CFLs and LED lights is to incorporate active circuits to sense load variations and draw the current necessary to keep the TRIAC happy, even though this current does nothing to drive the light itself and is a source of inefficiency. "Here's where we get into the con- COOL WHITE WARM WHITE PHOTOPIC-SENSITIVITY CURVE

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