Problem: How to distribute power and control signal in a large RGB LED application

Solution:

When using a large amount of LED’s in an application, it is not always easy to determine how to power them. Distribution of power is key to performance, reliability, cost, and efficiency......

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Exerpted from http://spectrum.ieee.org/semiconductors/optoelectronics/quantum-dots-enhance-led-lighting/0 BY Neil Savage // December 2009


9 December 2009—The next big thing in solid-state lighting may be exceedingly tiny—the quantum dot. Researchers from around the world gathered at the Materials Research Society fall meeting in Boston last week to discuss the progress they're making in using quantum dots to enhance the color and efficiency of light-emitting diodes (LEDs).

Quantum dots are nanometer-size bits of semiconductor material, such as cadmium selenide, that fluoresce when excited by photons or electrons. By choosing a certain material and a certain size, researchers can precisely tune the wavelength of light emitted. In general, a dot that's 2 nanometers in diameter emits blue light, a 4-nm dot emits green, and a 6-nm dot emits red.

Seth Coe-Sullivan, chief technology officer of QD Vision, a Watertown, Mass., start-up working on quantum dots for use in lighting and displays, says the advantage of quantum dots lies in the ability to pick a desired color without losing efficiency. Today's white-light LEDs consist of a blue-emitting LED coated with a phosphor that is excited by the LED and emits a yellow or orange light. The combination of blue and yellow produces a cold white light lacking in red photons, so human skin, among other things, looks unnatural under it. There are phosphors that can produce color closer to that of an incandescent light, but they come with a 30 percent drop-off in energy efficiency.

By contrast, Coe-Sullivan says, QD Vision has produced an optic coated with a thin film of quantum dots and fits over a blue LED lamp. With the optic, the lamp produces light with a color temperature of 2700 kelvins—about the same as that of an incandescent bulb. It has a color-rendering index (a measure of how "natural" colors appear under it) of 90, compared with 95 for an incandescent bulb and less than 75 for most white LEDs. It also produces 65 lumens per watt, a vast improvement over the 15 lm/W of incandescent bulbs and double the efficiency of compact fluorescent bulbs. QD Vision has started shipping that optic to two lighting manufacturers; the lamps should be on store shelves by January.

"The main benefit of the quantum dot is you're able to get a really efficient lightbulb with a high-quality color rendering index," says Vladimir Bulovic, a professor of electrical engineering and leader of the Organic and Nanostructured Electronics Laboratory at MIT. He says the QD Vision optic represents the first practical optoelectronic device based on this technology. Coe-Sullivan did his Ph.D. work in Bulovic's lab, and Bulovic is a founder of QD Vision.

Bulovic and other researchers are working on creating quantum-dot LEDs that are electrically pumped, thus eliminating the need for a gallium-nitride LED as a photon source. But the electroluminescent LEDs produced so far in laboratories are still in their early stages. To make such devices, researchers closely pack the quantum dots in an organic thin film that acts as a transport layer for electrons. But doing so reduces the luminescent efficiency of the dots from more than 90 percent to about 15 percent.

The first practical use of electrically pumped quantum-dot LEDs will likely be in displays, such as computer monitors, where Bulovic says they'll provide more saturated color than organic LEDs do but still retain that technology's ease of manufacturing and flexibility. Electrically pumped quantum-dot solid-state lighting will take longer, because reaching the brightness required for general illumination requires driving the LEDs at higher currents, which reduces their lifetime. But Bulovic is sure that researchers will eventually reach that goal. One way of extending the lifetime would be to replace some of the organic materials in the LEDs with metal oxides or chalcogenides, which won't degrade with exposure to air and moisture.

View this article in total at IEEE

Exerpts from http://www.ledinside.com/LED_streetlight_20091215
By ivan
Created 12/15/2009 - 18:47

"In recent years, cross-strait development of various types of LED streetlights have been booming, particularly in 2008, when Taiwan and China both initiated several LED streetlight demonstration projects, spurring the growth momentum of LED industry.

From LED lamps with 45Lm/W luminous efficiency in 2008, to the 80Lm/W luminous efficiency announced by some manufacturers by the end of 2009, the progress has been phenomenal. Regardless of whether the quality of the 80lm/W LED will pass the CNS15233 criteria announced at the end of 2008, the expansion of each country’s domestic demand in LED lamps, supported by the government, coupled by substantial investments from different industry players, will effectively increase the R&D resources, thereby improving the luminous efficacy of LEDs. In the near future, with its high luminous efficacy, LED streetlight will completely replace existing high-pressure sodium lamps.

Thermal dissipation of LED streetlights

All existing designs of thermal dissipation can be broadly categorized into 3 types, namely aluminum alloy heat sink, pin fin heat sink, and heat-pipe. With most of the aluminum alloy type, the heat sink is exposed outside; the advantage of this design is its low cost, especially in module development. However, in the R&D of high-wattage products, weight is an important factor, because according to public regulations, there are certain limitations to the light poles regarding the pressure it is capable of withstanding, thus most of the high wattage products adopt either fin-type heat sink or heat-pipes for heat dissipation.

Technical barriers of LED streetlights

In addition to the R&D of heat sink, the other important element in an LED streetlight structure is its power supply. Currently, the most popular LED products are DC-driven, thus in the power supply unit, manufacturers have to customize power supply for each type of LED streetlight, and make adjustments in the circuit to comply with each country’s product safety standards. Based on our experience in the LED streetlight projects, the majority of LED streetlight malfunctions are caused by unstable power supply. Hence, the stability of power supply directly affects the extent of LED streetlight’s luminous decay during the first three years.

Optical design – the Achilles’ heel of LED streetlight

The other technical barrier in the development of LED streetlight is the optical design capability. Good optical property in the lighting fixture allows LED streetlights to be uniformly illuminated on the road surface. Taiwan’s NS15233 criteria specifies that, the glare for streetlights should be <30 at a vertical 80-degree angle, and <10 at a 90-degree angle. In addition, at a 65 ~ 95 degree angle, there are specific brightness specifications. As for all other lighting fixtures, it depends on the optical design capability of individual developers, as some projects specify their own luminous requirements on the road surface. A good optical design can effectively improve the lighting quality of LED streetlights that have lower luminous efficacy .

So, when both quality and reliability of the products are equal, the comparison of LED streetlights depends on its lighting performance. The majority of current LED streetlights adopt OSRAM’s bidirectional optical design, but each manufacturer still endeavors to enhance the luminous efficacy with its own technological know-how.

Establishment of criteria for LED streetlights

Taiwan's national LED streetlight criteria has been officially announced in December 2008, and China's streetlight standards will be announced separately by each province, and product performance tests also vary accordingly. A nation-wide LED streetlight standard will likely be announced in the near term, and the criteria should be in line with both China’s and Taiwan’s CNS standards, to establish a uniform standard for LED streetlight quality across the strait. As for the U.S. and Europe, LED streetlight standards also specify criteria for the lighting angle, quality, and light decay, etc. With the development of LED streetlight technology, we believe better LED streetlights will be introduced, attracting more LED procurement projects from public sectors, thereby improving the overall market visibility of LED streetlights."

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