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Introduction To LED Lighting
- Jun 06, 2018 -

Lighting principle

LED light source, LED by Ⅲ - Ⅳ compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP phosphorus (GaAs) made of semiconductor, such as its core is the p-n junction. Therefore, it has the i-v characteristic of general p-n junction, that is, forward guide pass, reverse cut off and breakdown characteristic. In addition, under certain conditions, it also has the luminescent property. At positive voltage, electrons are injected into P area from N area, and holes are injected into N area from P area. A portion of a small number of carriers (children) entering the opposite region combines with a large number of carriers (children) to glow.

Assuming that luminescence occurs in the p-region, the injected electron is directly combined with the valence band hole and emits light, or is first captured by the luminescence center and then combined with the cavity. In addition, this kind of light composite some electrons are not luminous center (the center between the conduction band, interface with near middle) capture, then with hole compound, every time the energy released is not big, could not form visible light. The higher the ratio of the luminescent composite to the non-luminescent composite, the higher the quantum efficiency of light. Since the composite is luminescent in a small diffusion region, the light is produced only within a density of 30 degrees Celsius near the PN junction.

The theory and practice show that the peak wavelength of light is related to the gap Eg of semiconductor materials in the luminescence area, i.e., near 1240/Eg (mm).

Where, the unit of Eg is electron volt (eV). If visible light can be produced (wavelength: 380nm uv ~ 780nm red light), the Eg of semiconductor materials should be between 3.26 ~ 1.63eV. Light longer than red is infrared light. There are infrared, red, yellow, green and blue light emitting diodes, but among them, blue light emitting diodes are expensive and not widely used.

Lighting features

The limit parameter

(1) allowable power Pm: the maximum value of the positive dc voltage applied to both ends of the LED and the current flowing through it. Above this value, LED heating, damage.

(2) maximum positive dc current IFm: the maximum positive dc current allowed to be added. Exceeding this value can damage the diode.

(3) maximum reverse voltage VRm: the maximum reverse voltage allowed. Beyond this value, the light-emitting diode may be punctured and damaged.

(4) working environment topm: the range of ambient temperature where led can work normally. Below or above this temperature range, the led will not work normally, greatly reducing the efficiency.

Without changing the material, in the limit range of LED, the way to improve the brightness is to increase the current. With the increase of current, the heat generated by LED will increase dramatically. Those who have used the portable projector with LED light source, or those who have used the micro-projector, must have a deep experience. The projector with LED light source is very hot, and there will be obvious noises generally. These products, small fuselage is one aspect, the key or its own heat is larger.

With the increase of power, the LED heat dissipation problem is increasingly outstanding, a large number of actual application shows that the LED can't increase the input power, the basic reason is because the leds emit a large amount of heat in the process of work, make the pipe junction temperature rising rapidly, thermal resistance. The higher the input power, the greater the heating effect. Performance changes will lead to the rise of temperature and attenuation, the radiative recombination increases, the components of leakage current increases, the semiconductor material defect growth, metal electrode electric migration, encapsulation yellowing with epoxy resin, etc., the serious influence of LED photoelectric parameters. It even invalidates power leds. Therefore, it is increasingly important for LED devices to reduce thermal resistance and junction temperature and study the thermal characteristics of LED.

reliability

LED drive power supply is an important reason of lower full drive power lack of aluminum electrolytic capacitor life spans, needed for the primary reason is to work for a long time internal LED lights when the environment temperature is high, the electrolyte of aluminum electrolytic capacitor was quickly dries, taking greatly shortened, usually only work about 5000 hours. The lifetime of LED light source is 50,000 hours, so the operation life of aluminum electrolytic capacitor becomes the short rib of LED drive power.

Some vendors are now tackling this problem by creating LED power plans without aluminum electrolytic capacitors. But not all LED power suppliers support the idea. Wine palm pointed out: "in the current production of the LED drive power supply is not a selection of electrolytic capacitor has been drive plan, because without it, many pass the test specification, such as the EMI test and flicker-free test."

And the LED drive power plan that USES aluminium electrolytic capacitor is very simple normally above test, PCB copy plate if replace film capacitor and ceramic capacitance or tantalum capacitance, what is the situation? Film capacitor to reach the same capacitance (usually the uf) 100-220, the volume will be very big, and capital is too high, ceramic capacitor capacity is too small, usually as complete such a large capacity with multiple ceramic capacitor, the plate area and capital are too big, tantalum capacitor to have such a large capacity, it is too expensive, but the pressure is too low, can not meet the demand, so for any other variety of capacitance, basically is not too big volume, is too expensive, such as for these defects with smaller capacitor capacity, remove the ripple effect wasn't so good, many export products target will not be able to pass the strict certification test, Therefore, the current high quality LED drive power is still throughout the use of aluminum electrolytic capacitors.

Many suppliers say no electrolytic capacitor LED drive power supply plan, probably just remove the AC input side aluminum electrolytic capacitor, constant current output of the aluminum electrolytic capacitor should be difficult to remove or replace.

A company has launched a triac-dimmed 13WLED bulb with a built-in power source, which USES aluminum electrolytic capacitors as part of the SSL2102's drive power plan. When I asked about the bulbs work days, aluminum electrolytic capacitor good homework problem will certainly affect the whole LED bulbs stature, but to adopt some physical methods to alleviate this problem, such as the aluminum electrolytic capacitor on the PCB location close to the tail lamp, generally speaking, the most close to the LED light source, some of the highest temperature can reach 100-100 ℃, the cooling metal case some second, usually about 100 ℃, tail lamp some minimum, usually about 70 ℃, thus only need to pay attention to the orientation of the aluminum electrolytic capacitor close to the tail lamp, the attenuation of stature would not be too fierce. Our experiment indicated, its stature also can be reached 10000 hours, equivalent to 10 years time, it usually family users, ten years to change a LED bulbs are basically able to endure.

Green environmental protection

Do not contain mercury, lead and other heavy metals pollution to the environment a big, light will not produce ultraviolet ray, so the lamps and lanterns of LED lighting are not as traditional as there are a lot of mosquitoes around near the light source to make the environment more clean and who clean; The innovative design of constant current drive of metal shell makes the conversion of electric energy into light efficiency very high.

Unique advantages

(1) energy saving: LED's spectrum is almost all concentrated in the visible light frequency band, and its luminous efficiency can reach 80-90%. The author will also LED lights with ordinary incandescent lamp, spiral energy-saving lamps T5 trichromatic fluorescent lamps and done some comparison, the result shows: the ordinary incandescent light efficiency of 12 lm/w, life is less than 2000 hours, spiral energy-saving lamp luminous efficacy of 60 lm/w, life is less than 8000 hours, T5 fluorescent lamp is 96 alm/w, life is about 10000 hours, and a diameter of 5 mm white LED to 20-28 lm/w, life can be greater than 100000 hours. Some predict that the future life limit for leds will be infinite.

It is generally believed that the energy-saving of energy-saving lamps 4/5 is a great innovation, but LED is more energy efficient than energy-saving lamps by 1/4, which is a greater reform of solid light sources. In addition, LED has other advantages, high light quality, basically no radiation, is a typical green lighting source; Reliable and durable, extremely low maintenance and so on. Because leds have features that no other solid source can match, they will be the dominant light source in the lighting industry in 10 years.

(2) safety and environmental protection: the working power of LED is low, mostly 1.4-3v; The working current of ordinary LED is only 10mA, and the super bright LED is only 1A. Leds don't add "mercury" in the process of production, also do not need air, do not need glass shell, good impact resistance, good shock resistance, invulnerability to breakage and facilitate transportation, very green, known as "green energy".

(3) long service life: the LED is small in size and light in weight, and the shell is epoxy resin encapsulated, which can not only protect the internal chip, but also have the ability of transmission and concentration of light. LED service life is generally between 50,000 and 100,000 hours, because LED is a semiconductor device, and even frequent switching will not affect the service life. Nowadays household lighting is mainly used for incandescent lamp, fluorescent lamp and energy-saving fluorescent lamp.

(4) fast response: LED the response frequency f tau and minority carrier injection of tau MC about life, such as GaAs material made of LED, the tau MC generally range between 1-10 ns, the frequency response of about 16-160 MHz, such a high response frequency for video signal display 6.5 MHz is enough, this is one of the key factors to realize video LED large screen.

LED response time of the lowest has reached 1 microsecond, generally several milliseconds, about 1/100 of the response time of ordinary light sources. Therefore, it can be used in many high frequency environments, such as automobile brake lamp or status lamp, which can shorten the braking time of the vehicle behind the vehicle and improve safety.

(5) high luminous efficiency: the light efficiency of incandescent lamp and halogen tungsten lamp is 12-24lm/w(luming /w), the light efficiency of fluorescent lamp is 50-70lm /w, the light efficiency of sodium lamp is 90-140lm /w, and most electricity consumption becomes heat loss. LED's light effect can reach 50-200lm /w after modification, and the light has good monochromatism and narrow spectrum, so it can directly emit colored visible light without filtering.

(6) small size of LED components: it is more convenient for layout and design of all kinds of equipment, and it can better realize the effect of "only light can be seen but no light source" in night scene lighting.

(7) high energy concentration of LED light: concentrated in a small wavelength window with high purity.

(8) strong directional LED luminescence: the luminance attenuation is much lower than the traditional light source.

(9)LED low-voltage direct current can be driven: it has the advantages of small load and weak interference, and has low requirements on the operating environment.

(10) the composition of the luminous spectrum can be well controlled: thus, it can be well used for local or key lighting in museums and exhibition halls.

(11) it can control the size of the semiconductor luminescent layer and the semiconductor material with forbidden amplitudes: thus, it can emit various colors of light, and the color is higher.

(12) high color rendering: it will not cause harm to people's eyes.

Lighting specification

As LED market gradually into indoor and outdoor lighting, LED lighting prices also fell sharply, but the quality is uneven also cause problems, according to point out that are set around the LED lighting specification, a number of mandatory lighting standards will since 2012 on the road, will eliminate inferior products is expected to accelerate the vicious competition, leading to the LED lighting market reshuffle effect.

Due to LED lighting applications increasingly widespread and diverse, the ministry of economic affairs bureau following a number of LED lighting standard, the LED street light specification for announced steps ahead of the world, and at the end of 2010 issued three common LED indoor light standards, including LED T8 straight tube lamp, LED project-light lamp and light steel frame light (including flat-panel lamps), etc., the industry pointed out that although the relevant standard specification, but the LED street light after years of promotion, in 2012 is expected to expand the installation, because the government import slowly add budgeting assignment, estimate until in 2013 ~ 2014 is expected to release purchasing the case.

Although China LED industry development in full swing, but due to the regional climate difference is too big, so the industry standards have not yet entered the national standard, dominated by the provinces respectively to promote regional standards of LED lighting, but according to point out that China development and reform commission (NDRC) is through the use of LED lighting technology specification, priorities for indoor LED tube light, expected after finalized in technical specification, the 12th five-year plan on the mainland in 2012, driven by the central subsidies LED indoor lighting fiscal plan unveiled for implementation is expected to expand.

As for the LED lighting products enter the European Union must through the CE certification, and consider the electromagnetic radiation may cause harm to human body, the European Union also announced in 2013, the enforcement body electromagnetic radiation safety standards.

As the U.S. environmental protection agency (epa) released in October 2011, the new solid-state lighting lamps and lanterns and light source product specification, the industry pointed out that the droop of high-profile LM - 80 test, is expected to be since April 2012 enforcement, after the new specification on the road, the future north American market of lamps and lanterns manufacturer will give priority to purchase LED products are verified through the LM - 80, for Taiwan LED encapsulation and grain not verify by factory, into the U.S. market could suffer in the future.

As a result of the LM - 80 droop validation should be after 6000 hours test, takes up to 6 ~ 8 months, part of the LED industry thinks, LED product specification and technology changes with each passing day, after a few months later, long LED lighting validation criteria may be will change, so still have doubt for this certification, but being = said that LM - 80 tickets was seen entering the U.S. market, manufacturers must provide LM - 80 (LED lumen maintenance ratio) test report certificate, to obtain the energy star label, lighting engineering society (IES) of North America has also ordered the LM - 80 light attenuation testing standard, It will not only provide the standards for the measurement of LED applications, but also provide quality assurance for consumers. It will become a common test standard all over the world.

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