By TV Rao
Good lighting has become one of the most important part of present day life. Considerable amount of electric power is used up to meet present day demand in day to day life. Thanks to advances in physics and Electronics and their development resulting in bringing out a new breed of lighting products, what is known as LIGHT EMITTING DIODES.
Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. These LEDs are now moving, from present usage in electronic and electrical devices to homes, street and rural village lighting, bringing dramatic saving of electricity.
The incandescent filament bulb originally invented by Mr EDISON has already given way more efficient florescent and mercury/sodium vapour lamps Although first known report made as early as in 1907 of a light emitting diode by a British scientist HJ Round, first practical visible spectrum LED was developed in 1972 by Nick Holonyak Jr of GE.
How Can a Diode Produce Light?
Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light. Photons are released as a result of moving electrons. In an atom, electrons move in orbitals around the nucleus. Electrons in different orbital have different amounts of energy. Generally speaking, electrons with greater energy move in orbital farther away from the nucleus.
For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy level. Conversely, an electron releases energy when it drops from a higher orbital to a lower one. This energy is released in the form of a photon. A greater energy drop releases a higher-energy photon, which is characterized by a higher frequency. Free electrons moving across a diode can fall into empty holes from the P-type layer. This involves a drop from the conduction band to a lower orbital, so the electrons release energy in the form of photons. This happens in any diode, but you can only see the photons when the diode is composed of certain material. The atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively short distance. As a result, the photon's frequency is so low that it is invisible to the human eye -- it is in the infrared portion of the light spectrum. This isn't necessarily a bad thing, of course: Infrared LEDs are ideal for remote control among other things.
Principle Of Led Technology
Like a normal diode the LED consists of a chip of semi conducting material impregnated, or doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode but not in the reverse direction. Charge-carriers-electrons and cathodes-flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy the form of photon.
The wave length of the light emitted, and therefore its colour, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radioactive transition which produces no optical emission, because these are indirect band gap materials. The materials used have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.
LED development began with infrared and red devices made with gallium arsenide Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colours. LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
The refractive index of most LED semiconductor materials is quite high, so in almost all cases the light from the LED is coupled into a much lower-index medium. The large index difference makes the quite substantial. The reflection is generally reduced by using a dome shaped package with diode in centre. The color of light depends on the material of the semi conductor used. Conventional LEDs re made from variety of inorganic semi conductor materials. Appendix I shows available colors with various materials.
Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light. There are two ways of producing high intensity white-light using LEDs. One is to use individual LEDs that emit three primary colors- red, green, and blue, and then mix all the colors to produce white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.
White light can be produced by mixing differently colored light, the most common method is to use red, green and blue (RGB). Hence the method is called multi-colored white LEDs (sometimes referred to as RGB LEDs). Because its mechanism is involved with sophisticated electro-optical design to control the blending and diffusion of different colors, this approach has rarely been used to mass produce white LEDs in the industry.
Nevertheless this method is particularly interesting to many researchers and scientists because of the flexibility of mixing different colors. In principle, this mechanism also has higher quantum efficiency in producing white light.
There are several types of multicolored white LEDs: di- tri , and tetra chromatic white LEDs. Several key factors that play among these different approaches include color stability, color rendering capability, and luminous efficiency.
What multi-color LEDs offer is not merely another solution of producing white light, but is a whole new technique of producing light of different colors. In principle, all perceivable colors can be produced by mixing different amounts of three primary colors, Phosphor based LEDs consists of coating of one color (mostly blue LED) with phosphor of different colors to get white light. They however have lower efficiency.
However the design of light source and light fixtures are much easier . Hence it is most popular technique for manufacture. A new method used to produce white light without the phosphors has been developed recently.
Operation Of Leds
LEDs by their very nature, require constant current with low voltage, as opposed to the electrical grid which supplies high voltage with an alternating current. Unlike incandescent lamp, which can illuminate regardless of electrical polarity, LED can only light with correct polarity. LED can be operated on alternating current but lights only with positive voltage, causing LED to turn on and off at the frequency of AC supply. Care must be taken the reverse voltage is below the threshold.
A CR dropper followed by full-wave rectification is the usual electrical ballast with series-parallel LED clusters. A single series string minimizes dropper losses while paralleled strings increase reliability. In practice usually three strings or more are used. Lighting LEDs on mains A CR dropper followed by full-wave rectification is the usual electrical ballast with series-parallel LED clusters. A single series string minimises dropper losses, while paralleled strings increase reliability. In practice usually three strings or more are used.
Operation on square wave and modified sine wave (MSW) sources, such as many inverters, causes heavily-increased resistor dissipation in CR droppers, and LED ballasts designed for sine wave use tend to bum on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor and rectifier makes a more suitable ballast for such use, and other options are also possible. Dedicated integrated circuits are available that provide optimal drive for LEDs and maximum overall efficiency.
Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. Parallel operation is also possible but can be more problematic. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel. To increase efficiency the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear continuously lit.
Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages.
Operation on square wave and modified sine wave (MSW) sources, such as many inverter, causes heavily-increased resistor dissipation in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor and rectifier makes a more suitable ballast for such use, and other options are also possible. Dedicated integrated circuits are available that provide optimal drive for LEDs and maximum overall efficiency. The light output, measured in lumens, is about 10 lumens per watt in case of filament lamps and 60-80 lumens per watt in case of flourecent/vapour lamps.
One of the key advantages of LED-based lighting is its high efficiency, as measured by its light output per unit of power input. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002, five-watt LEDs were made with commercially efficiency of 18-22 lumens per watt (Im/W). For comparison, a conventional 60-100 W incandescent light bulb produces around 15 Im/W, and standard fluorescent lights produce up to 100 Im/W. (The luminous efficiency article discusses these comparisons in more detail.)
In 2003, a new type of blue LED was demonstrated by a company to provide 24 mW at 20 mill amperes (mA). This enabled to produce a commercially packaged white light giving 65 Im/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescent. Later researchers made it possible to commercially produce LEDs with efficiency about 1301m/W, approaching an order of magnitude improvement over standard incandescent and better even than standard fluorescents. It should be noted that high-power (= 1 W) LEDs are necessary for practical general lighting applications. efficiency of 115 Im/W (350 mA).
Advantages of using LEDs
LEDs have several advantages over conventional incandescent lamps. For one thing, they don't have a filament that will burn out, so they last much longer. Additionally, their small plastic bulb makes them a lot more durable. They also fit more easily into modern electronic circuits.
But the main advantage is efficiency. In conventional incandescent bulbs the light-production process involves generating a lot of heat (the filament must be warmed). This is completely wasted energy, unless you're using the lamp as a heater, because a huge portion of the available electricity isn't going toward producing visible light. LEDs generate very little heat, relatively speaking. A much higher percentage of the electrical power is going directly to generating light, which cuts down on the electricity demands considerably.
Most significant advantage is use LEDs in rural areas powered with Solar PV units for electrification of villages. Long transmission and distribution lines are not necessary and thus reducing transmission losses.
Main advantages of LEDs over conventional light sources are listed below
Efficiency: LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving device.
• Colour: LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• Size: LEDs can be very small (2 mm2) and are easily populated onto printed circuit boards.
• On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds. LEDs used in communications devices can have even faster response times.
• Cycling: LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
• Dimming: LEDs can very easily be dimmed
• Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
• Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt bum-out of incandescent bulbs.
• Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically
are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent lightbulbs at 1,000-2,000 hours.
Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
• Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
• Toxicity: LEDs do not contain mercury, unlike fluorescent lamps
Disadvantages of using LEDs
Some of the main disadvantages are:
High price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources
Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
Voltage-sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.
Light quality: Most white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly badly by typical phosphor based white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
Area light source: LEDs do not approximate a "point source" of light, but rather a Lambert an distribution. So LEDs is difficult to use in applications needing a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.
Blue pollution: Because white LEDs emit much more blue light than conventional outdoor light sources such as high-pressure sodium lamps, than other light sources, it therefore very important that LEDs are fully shielded when used outdoors. Compared to low-pressure sodium lamps which emit at 589.3 nm, the 460 nm emission spike of white and blue LEDs is scattered about 2.7 times more by the Earth's atmosphere. LEDs should not be used for outdoor lighting near astronomical observatories.
Electro migration caused by highcurrent density can move atoms out of the active regions, leading to emergence of dislocations and pointdefects, acting as nonradiative recombination centers and producing heat instead of light.
Short circuit- Mechanical stresses high currents, and corrosive environment can lead to formation of Whickers, causing short circuits.
Thermal runway Non homogeneities in the substrate causing localize loss of thermal conductivity, can causes damage which causes more heat etc. Most common ones are voids caused by incomplete soldering, or by electro migration effects.
The many application of LEDs are very diverse but fall into three major categories: Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning. Illumination where LED light is reflected from object to give visual response of these objects. Finally LEDs are also used interacting with process that do not involve the human visual system.