New Applications and New Standards for High-Brightness LEDs

New Applications and New Standards for High-Brightness LEDs

Key point: The overall lighting system efficiency includes the efficiency of power conversion, current regulation and the HBLED (high-brightness light-emitting diode) device itself, not just the efficiency of the HBLED die.

HBLED lighting systems are less efficient now than the most efficient fluorescent lamps, but HBLEDs will catch up within three years.

Other advantages of HBLEDs, such as their clean light source, ruggedness, and dimming control capabilities, will help them find their way into new medical and lighting applications.

The power of HBLEDs (high brightness light emitting diodes) is increasing: for example, Cree now offers 88lm/W devices and plans to offer 100lm/W devices by the end of this year, and 150lm/W devices within five years. As independent components, HBLEDs are more efficient than incandescent lamps and even fluorescent lamps. However, at the system level, their advantages are weakened because the power losses they bring (including AC/DC and DC/DC conversion and current regulation) must be considered. In addition, LED lighting devices (or light sources) will cause losses, and the LED components themselves will also have heat losses.

As high-brightness LEDs increase in power, they are enabling many new applications, from architectural lighting to medical products. Energy Star lighting standards are increasingly focusing on overall system efficiency.

The US DOE (Department of Energy) recently completed a new Energy Star specification for SSL (solid-state lighting) light sources, so that system designers can use the same comparative figures for light sources and devices. The new specification does not focus on the luminous efficiency of HBLEDs at the component level, but on the overall light source efficiency.

The light intensity of a current-driven LED is directly proportional to the forward current. These devices have the same steep voltage/current curve as diodes, and a small change in voltage will result in a relatively large change in current and brightness, so controlling current is more important than controlling voltage. Many IC manufacturers (usually engaged in the power controller business) have entered the LED driver current regulation market. These suppliers include: Texas Instruments, National Semiconductor, Intersil, Cypress, Maxim, and Linear Technology.

In addition to the current regulator, the lighting system may also have to include an AC/DC converter, and battery-powered systems may also require a DC/DC boost converter. In summary, 10% to 15% of the system power may be lost in conversion alone. In addition, due to reflections and lens losses, the brightness of the SSL in the device may be lost by up to half.

The Energy Star SSL specification will take effect on September 30, 2008. The specification has two parts. Category A covers devices that exist today. It states that a recessed light fixture (or "downlight") that meets Category A must be 35 lm/W. Category B will cover high-efficiency SSL devices that appear in three years. At that time, SSL will be able to compete with today's most efficient lighting systems using traditional light sources. For example, the most common high-performance T8 fluorescent lamps and electronic ballast systems today can produce about 100 lm/W. High-quality products of these fluorescent ballast systems can achieve an efficiency of about 70%, resulting in a light source efficiency of 70 lm/W. HBLED light sources cannot achieve the minimum luminous efficacy of Category B when using today's commercial SSL technology. However, LED technology is developing rapidly and will meet the requirements of Energy Star Category B in the future. But LEDs have other advantages besides efficiency and long life, and it is worth continuing to track their development before higher efficiencies are achieved. For example, in fluorescent lighting, dimming is very difficult, while for LEDs it is just a matter of directly reducing the current. In addition, you can use multiple cool white LEDs and warm white LED arrays to dynamically change the color of a room.SSL is expected to become an important technology for home and industrial lighting within the next five years.

Most HBLEDs available today require DC voltage and current, so most SSL systems contain conversion circuits to convert AC power to regulated DC power. However, Seoul Semiconductor recently introduced the Acriche HBLED, which can work directly from AC power (Figure 1), with a surface mount resistor setting the input voltage, ranging from 100V to 110VAC and 220V to 230VAC. In the die, the LED contains multiple layers of LED semiconductor junctions. The diode junction is established when the total forward voltage is close to the AC voltage of 110V or 220V. These devices have two strings of LEDs back to back. The first string is turned on and conducts during the positive half-voltage cycle, while the second is turned on during the other string cycle, so the LED can emit light during the entire AC voltage cycle. Direct AC operation simplifies the power conversion circuit, increases system reliability, and reduces design time. Acriche HBLED has two models: AW3200 for 100V/110V and AW3220 for 220V/230V. Both models have an efficiency of 59lm/W, which is lower than the luminescence of DC-powered HBLEDs, but not much different.

As energy costs continue to rise, lighting efficiency becomes increasingly important. The U.S. DOE estimates that lighting consumes 20% of a building's electricity. However, in developing countries, where reliable power grids are often lacking, people can use solar SSL to power batteries for reliable nighttime lighting. The only other option is kerosene lamps, which are both dangerous and expensive.

HBLEDs have other advantages beyond their obvious efficiency and lifetime benefits that make them attractive for non-traditional lighting applications. For example, they have a narrow spectrum, making them ideal for use as bilirubin lamps. Bilirubin is a red-yellow organic compound that is derived from hemoglobin produced when red blood cells break down. Excessive bilirubin causes hyperbilirubinemia, a condition characterized by jaundice, a yellow tint to tissues such as the sclera (i.e., the “whites” of the eyes) and bodily fluids. While low levels of bilirubin are generally not a concern, large amounts can circulate to brain tissue and may cause neonatal seizures and brain damage. Fortunately, light therapy is usually effective for this condition because bilirubin absorbs blue light, breaking it down into a water-soluble form that is excreted from the body (Figure 2). Narrow-band blue light in the 458nm to 462nm region is most effective. In the past, bilirubin treatment light has been provided using custom blue fluorescent tubes and colored filters. However, fluorescent lamps have a relatively broad spectrum. Although the center of the distribution is at the desired wavelength, the spectrum drops off at both ends, meaning that less light is available to destroy bilirubin, so treatment time will be extended. In contrast, blue LED bilirubin light sources can have the correct frequency and waste almost no light energy. In addition, LED bilirubin light sources have mechanical stability, longer life, and are cheaper than fluorescent devices. In the future, light therapy will be able to provide light through "light suits", providing more effective treatment methods than light boxes.

One significant difference between HBLEDs and traditional incandescent or fluorescent lighting is that HBLEDs have more package variations. Therefore, it might be expected that some standards on packaging would emerge, but this will not happen for at least five years. Mark McClear, director of business development at Cree, predicted: "Every time we make HBLEDs brighter, more portable and more efficient, it has led to more applications. We are on a steep curve, trying to accomplish three goals at once, and an important part is packaging. We have one type of package; our competitors may have another... Not to confuse customers, but because (one type of package) has more light output, and light is the most important."

Cary Eskow, director of Avnet's LightSpeed ​​SSL and LED business unit, said that the rapid development of HBLEDs has brought about issues related to eye safety, and designers of these devices generally have to solve this problem. He said: "The rapid development of HBLEDs may have surpassed the design of safety." The most direct threat to the eyes comes from the intensity of light: some HBLEDs can produce up to 150lm of light from a small chip core. In some cases, this light intensity can damage the eyes through photothermal and photochemical processes. Our body's blinking response does not have much protection against these types of damage because this response does not occur at the end of the visual spectrum where damage occurs.

Photothermal damage occurs when the temperature of the retina increases by approximately 10°C. This damage is directly related to the beam spot size due to the way heat is transferred across the retina; it increases with focal size, just as a needle pressed against the skin causes much more pain than a finger with the same pressure. This type of damage becomes increasingly dominant as the wavelength extends from approximately 550nm (green) through yellow, orange, and red to IR (infrared).

On the other hand, strong UV and blue light can cause photochemical damage to the eyes. Blue light and short wavelengths are 1,000 times more dangerous than IR radiation. Again, the blink response is useless in this case. Eskow strongly recommends that designers make HBLED safety a major design consideration, as these devices are finding their way into more and more applications, from lighting to medical devices and even toys.