A light-emitting diode ( LED ) is a semiconductor (diode) that emits a specific color (wavelength) of visible light under the forward bias of a voltage/current source, the brightness of which depends on its own parameters and the parameters of the power supply. The first LED was invented by General Electric researchers around 1962. It is a low-power device that produces low-brightness red light, but at a very high price.
In 1968, the LED price bottleneck was broken, and Monsanto and Hewlett-Packard began mass production of red LEDs using cost-effective gallium arsenide phosphide (GaAsP). This red LED was originally used in large numbers to replace incandescent and neon indicators, such as on/off/standby indicators, and was soon used in digital displays.
With the rapid development of TV-like surprises, today's LEDs offer a wide range of colors, with a single device in multiple colors, brightness and power levels, and are available in a variety of unique package types. There are also countless devices capable of emitting a range of invisible light from infrared (IR) to ultraviolet (UV).
The rapid development of device types often leads to revolutionary applications. Visible LEDs are no longer just used as indicator lights, they are also used to replace incandescent and fluorescent lamps in almost all lighting (utility and decorative) and marking applications because of their low power/low heat characteristics and significantly longer Life and lower costs in long-term and short-term operating conditions.
After years of development, LEDs have also penetrated into some professional non-illuminated designs, such as waveform controllers in audio circuits (Figure 1). Infrared and UV LEDs can also be used in many areas from remote control to medical.
Today's LED products are already very versatile, available in a variety of sizes, colors, shapes and types, as well as a wide range of electrical specifications and parameters, as well as standard and unique packaging, of course, the price is very different. Each LED can be used for one or more applications, such as general lighting, flash in digital cameras, LCD backlighting, and the most basic indicators.
Popular low-cost medium-power LEDs (Figure 2) for basic indication purposes, prototyping, and hobbies are still very rich, and the materials used to make these LEDs are typically aluminum gallium arsenide (AlGaAs), gallium arsenide phosphide (GaAsP), Aluminum gallium indium phosphide (AlGaInP) or gallium phosphide (GaP). Although red is still the most versatile color, it can also be available in green, orange, yellow and blue. These LEDs have a forward voltage drop of between 1V and 2V and a forward current of approximately 20mA.
Figure 2: Common low-cost LEDs typically require 1V forward voltage and 20mA quiescent current to provide a wide range of colors, sizes, and shapes.
In addition to a single device, other general purpose LEDs include digital displays, two- and three-color LEDs, red-green-blue (RGB) components, and flash LEDs. For general designs that do not have much power and size constraints, these LEDs can basically meet budget requirements.
High-power LEDs (HPLEDs) and HPLED modules are used primarily in industrial, commercial, residential or decorative lighting applications and are rapidly replacing traditional incandescent and fluorescent lamps, especially as the price of these products has fallen. . A significant advantage of these LED products is their long life, which can generally reach more than 50,000 hours, more than 10,000 hours longer than fluorescent lamps, and more than 1,000 hours longer than incandescent lamps, even in frequent on/off conditions. HPLED's brightness can reach 105 lumens / watt or more, so the effect is also very good.
The XLamp MPL EasyWhite LED from Cree (Figure 3) is a good example of an alternative to inefficient technology. Compared to conventional light sources, these LEDs offer higher performance, color consistency and lumen density, and are optimized for directional lighting applications, including PAR or BR bulbs. Through careful system design, this LED can provide the same light output as the 3,000K, 75W grade BR-30 bulb, and consumes 78% less power than incandescent technology.
Figure 3: Cree's XLamp MPL EasyWhite LED delivers up to 1,500 lumens of illumination at 250mA.
The MPL EasyWhite LED is available in a 12x13mm package and provides up to 1,500 lumens of illumination at 250mA. In addition, such LEDs can provide 2,700K, 3,000K, 3,500K, and 4,000K color temperatures at the center of the corresponding ANSI C78.377-2008 color category. Of course, other viable options are also available when characterizing LED packages for special lighting applications.
One of the barriers to overall efficiency is that LEDs require DC power, so power converters are needed in many lighting applications. In addition to the increased number of devices, these converters need to be carefully designed to increase the cost of the LED topology.
This problem was partially solved by Seoul Semiconductor's Acriche LED (Figure 4), which works directly from AC power. This 100 lm/W Acriche source is 25% more efficient than existing LED products. According to the company, the carbon emissions of such LEDs are less than one-tenth that of incandescent lamps because they do not require an AC/DC converter.
Figure 4: Seoul Semiconductor's 100 lm/W Acriche LEDs operate from AC power and do not require an AC/DC converter.
LEDs typically emit very little heat, but when many LEDs are brought together for higher-brightness lighting applications, or for high-density boards and particularly compact areas, heat dissipation issues can become prominent. Also in order to share the big cake of LED, the semiconductor company Vishay has launched the VLMW321xx and VLMW322xx surface mount white LED series in the thermally enhanced PLCC-4 package (Figure 5). To achieve greater pin compatibility with similar devices, the VLMW321xx provides three anodes and one cathode, while the VLMW322x LED provides three cathodes and one anode.
Figure 5: Vishay's VLMW321xx and VLMW322xx surface-mount white LEDs are packaged in a thermally enhanced PLCC-4 package.
These devices have thermal resistance as low as 300K/W and consume up to 200mW. Both types of devices are certified to the AEC-Q101 standard for automotive applications. Other features worth sharing include: illuminance from 1,400 to 3,550 mcd, luminous flux from 7,000 to 8,900 mlm, half-brightness angle of 60°, and an illumination ratio of less than 1.6 per package unit.
Infrared and UV LEDs operate at wavelengths greater than 750 nm and less than 400 nm, respectively, and are widely used in optically isolated signal paths in remote control (television, home entertainment centers, etc.), communications, and medical applications. Infrared LEDs are typically fabricated from GaAs or AlGaAs, and the materials of the ultraviolet LEDs may be diamond, boron nitride, aluminum nitride, aluminum gallium nitride (AlGaN), and aluminum gallium indium nitride (AlGaInN). Examples of applications for these LEDs include: Infrared LEDs used in the sensor strips of the Nintendo Wii, as well as UV LEDs used in certain bacteria, adhesive curing and biosynthesis.
Night photography is one of many applications for infrared LEDs. LEDtronics offers infrared LED lamps (Figure 6) with wavelengths from 850nm, 880nm and 940nm. With industry-standard pedestals and multiple illumination angles, image capture is possible in a completely dark environment. This LED lamp is resistant to ambient light and electromagnetic interference (EMI) and can be used with all standard domestic and international voltages.
Figure 6: LEDtronics' infrared LED lamps use wavelengths of 850, 880 and 940 nm for full black photography.
It is worth noting that a luminaire using multiple LEDs provides sufficient light even in the event of failure of one or more LEDs, and has an average life of more than 100,000 hours. In addition to a single device, other general purpose LEDs include digital displays, two- and three-color LEDs, RGB components, and flash LEDs.
At the other end of the spectrum, Lumex offers the QuasarBrite range of UV LEDs (Figure 7). Compared to other similar devices, this LED has a 10-fold longer life (over 50,000 hours) and offers tighter beam angles, higher durability and up to 50% cost savings, Lumex said. These devices are available in a variety of wavelengths including 385 nm, 405 nm, and 415 nm. Applications include surface sterilisation, industrial control related to leak and biohazard detection, drug detection and body fluid flow analysis, and fluorescent inks.
Figure 7: Lumex's QuasarBrite UV LEDs have a lifespan of more than 50,000 hours and can be used in sterilization, industrial control and analytical applications.
Organic LEDs (OLEDs) seem to have good prospects for development. In December 2009, DisplaySearch pointed out in its OLED shipment and forecast quarterly report that global OLED revenue in the third quarter of 2009 broke the last record, reaching $252 million, an increase of 31%.
The OLED uses an organic compound as a light source between the anode and the cathode. Depending on the configuration, light can be emitted from the top or bottom of the device, which is achieved by a transparent electrode.
There are currently three types of OLEDs: transparent (TOLED), stacked (SOLED), and reverse (IOLED). TOLEDs rely on transparent electrodes on both sides of the device so they can emit light from the top or bottom of the device. SOLED stacks red, green and blue together to achieve a full color display. The IOLED uses a bottom cathode connected to a thin film transistor (TFT) backplane that can be used to implement an active matrix OLED (AMOLED) display.
Since OLEDs are superior to conventional LCDs, they are being increasingly used in many display devices. For example, OLEDs do not require a backlight, so the display panel can be lighter and thinner. In addition, the OLED display can completely turn off the pixels to display true dark black. Sony's XEL-1 is said to be the industry's first OLED TV (Figure 8), with a panel thickness of only 3mm and a contrast ratio of 1 million to 1.
The above LEDs are only a few of the many LEDs on the market, and there are many other varieties. Describe the characteristics of the correct LED type is very easy, but you still need some skill when you have to choose the right LED in a given topology.
Some general guidelines apply to most designs around LEDs. Rob Harrison, Engineering Technology Manager for Solid State Lighting Business Unit at OSRAM, is very representative and he believes that the first priority is a complete understanding of the application requirements. Because of the variety of LEDs, designers must determine the range of key parameters including voltage, current, power, heat dissipation, thermal resistance, color, color temperature, color sensitivity, brightness, environmental conditions, package and lifetime. "Be prepared to cut your options from hundreds of LEDs to about 10 or fewer," Harrison pointed out. At the same time, he also believes that thermal resistance is the most critical factor. Obviously, the heat generation not only affects the overall design performance, but also affects the service life of the LED.
In addition to careful system design for heat dissipation, energy efficiency is also a priority. More and more designs need to meet many energy efficiency standards, including Energy Star. Due to the increasing emphasis on environmental issues, product energy efficiency issues will become more prominent in the near future.
A global home appliance manufacturer wants Lumex to help them replace incandescent lamps used to illuminate the interior of an ice-selling machine. This application requires higher light intensity, average light distribution and higher efficiency for better energy savings.
The manufacturer wants to use a high-power 1W LED with a cool white color temperature. The light must illuminate the active paddle, the water dispenser, and the ice machine. It is best to have the same light intensity and color. In addition, the lighting module must be easily replaced in the field.
For the first Harrison view of heat dissipation, the Lumex design team believes that high-power LEDs will create many challenges, especially thermal management, shorter life and uneven light distribution.
As an alternative, Lumex has developed a small cast module integrated with a printed circuit board (PCB) with three low-power 5mm white LEDs mounted on the board (Figure 9) and one at the end of a line assembly. Quick disconnected two-prong connector. The color and brightness of these LEDs are matched, and each LED has the same 2,700K cool color temperature. This solution additionally allows light to reach the exact location of the inner chamber.
According to Lumex, this solution effectively reduces service costs by replacing incandescent lamps with LEDs with a 10-year lifetime. In addition, energy costs are significantly reduced because the power consumption of LED modules used is lower than traditional solutions. This solution is in line with the original concept of using 1W LEDs, driving three LEDs with an operating current of 18mA, which is equivalent to using a 1W device.
No matter what design challenges you encounter, people always have a way to win – or at least somewhere already planned. For example, Bayer MaterialScience recently revealed a unique light scattering technology that hides LED hot spots while emitting more light.
The company's solution is to create a soft LED light effect with minimal reflection, and to emit translucent white light at a level that is unattainable. This technology allows for almost unlimited freedom to implement light diffuser packaging and a wide range of colors for perfect customization of the application.
“This is an exciting time for colorists because we are able to provide product designers and OEMs with design solutions that meet their specific needs,†said Terry Bush, senior chemist at Bayer MaterialScience.
To create a unique diffuser package using this technology, designers can choose Makrolon polycarbonate resin products for their specific application requirements. “The better the base resin, the higher the overall performance of the diffuser package,†said Gerald DiBattista, head of the IT/Electronic Polycarbonate IT market at Bayer MaterialScience.
Available resins include Makrolon LED 2643 for indoor and outdoor applications, which are resistant to UV light, water spray and immersion. Makrolon FR7087 is perhaps the first clear polycarbonate to meet UL 94 5VA rated burn rating for lenses and covers. Makrolon 6717 is a flame retardant grade resin that supports extrusion applications such as light strips and light guides. Makrolon 3103 is a high viscosity, UV resistant polycarbonate suitable for many applications including automotive and consumer electronics.
Based on the fact that the total design solution was introduced, DiBattista reiterated: “No matter what kind of lighting application is ultimately, there is always a suitable solution that can meet the needs of the application.â€
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