DesignNews

Design News, March 2013

Issue link: http://dc.ee.ubm-us.com/i/112771

Contents of this Issue

Navigation

Page 12 of 89

Is it possible for a tiger to change its stripes? Perhaps it can! I'm referring to a cleverly-designed high-brightness LED array just released from Sharp Microelectronics. design a lighting product with a variable CT. Until today. Sharp Microelectronics' new "Tiger Zenigata" LED combines individually-controlled arrays of warm and cool LEDs in the same package. Because the two arrays are intermeshed and each is covered with its corresponding phosphor, the LED appears to have tiger stripes. Internally, the warm white array is comprised of 8 parallel strings of 12 series-connected LEDs (96 total); the cool white array is 6 parallel strings of 12 series-connected LEDs (72 total). Each array's forward voltage is about 37 V. By controlling the current through each array (0 to 700 mA), you can continuously vary the color temperature from a relaxing warm 2700 K to a productive and focus-friendly 5700 K. Light output will range from approximately 1,800 to 2,000 lumens, depending on the CT selected. Figure 1 – Bengal tiger, panthera tigris tigris In many applications, color temperature (CT) is an important criterion when selecting a light source– be it a white LED, incandescent bulb, fluorescent tube, plasma discharge system or other technology. Essentially, CT is a measure of the reddish/bluish hue in white light. At first, the concept seems a bit counterintuitive because a high CT value signifies a "cool" white (bluish), and a low signifies a "warm" white (reddish). We naturally would expect hot objects to have higher temperature values than cooler ones. CT is actually analogous to the temperature of an idealized body if it radiated light of that hue. Think of a bar of iron held over a flame– at low temperatures, perhaps about 1500 Kelvin, it would have a reddish glow. As the temperature of the iron bar increased to 3000 K, 6000 K and beyond, it would appear "yellow hot", "white hot" and ultimately "blue hot". Thus, light from a white LED at a CT of 3500 K corresponds to the slightly-reddish hue of a bar of metal heated to 3500 K. (Iron isn't an idealized black body radiator, but you get the idea.) In some environments, warm white lighting is preferred. It enhances the appearance of food in restaurants, evokes imagery of sunsets and fireplaces, and physiologically sets us at ease. Cool white has its place as well; it improves our alertness, accentuates visual contrast, and is sometimes preferred in professional work spaces. Our homes are dynamic spaces; at times we might prefer cool white when preparing a meal and warm white when enjoying it. Can individual LEDs change their CT as needed? No. This is due to the fact that color temperature is primarily determined by the composition of the phosphor material placed on the LED die during manufacturing– thus it is fixed. Unless you arrange warm white LEDs with cool white LEDs and control each set individually, it is not easy to Figure 2 – The Sharp "Tiger" High-Power LED array from Avnet and Silica. A single package with individually-controllable warm and cool LED arrays. Because the 168 LED tiny die beneath the orange/ yellow phosphor are placed together in close proximity, in operation the overall appearance is a smooth and well-mixed light emanating from a dime-sized area. The white substrate under the LEDs is alumina ceramic; it's both thermally conductive and electrically insulated. A matching Tiger "holder" (retainer/connector) is available as well. It secures the Tiger against a secondary heat sink surface in the end-product and provides four springloaded, wire-retaining "poke in" solderless contacts. Once mounted with two screws, the holder accommodates optional metalized snap-in reflectors. Very convenient for rapid assembly. For more information on the Sharp Tiger Zenigata variable color temperature LED array, contact Silica LightSpeed in Europe (http://www.silicalighting.eu) or Avnet in the Americas (www.em.avnet.com). Cary Eskow is Global Director of the Solid State Lighting and Advanced LED business unit of Avnet Electronics Marketing. An ardent advocate of energy efficient LEDbased illumination, he has worked closely with LED manufacturers, advanced analog IC and secondary optics vendors since his first patent using LEDs was issued two decades ago. Avnet works with customers through their national team of illuminationfocused sales engineers who are experienced in thermal, drive stage and optics design. Prior to his LED lighting focus, Cary was Avnet's technical director and managed Avnet's North American FAE team. To submit questions or ideas, e-mail Cary at LightSpeed@Avnet.com Your questions and comments are always welcome. This and other Light Matters articles are downloadable at www. em.avnet.com/LightSpeed. To learn more about designing an LED-based illumination system, go to: www.em.avnet.com/LightSpeed

Articles in this issue

Links on this page

Archives of this issue

view archives of DesignNews - Design News, March 2013