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LED(Light Emitting Diode) is a solid-state semiconductor device, which can directly convert electric energy into light energy. The core of LED enterprise legend photoelectric LED is semiconductor chip. One end of the chip is attached to the bracket, which is the negative electrode, and the other end is connected to the positive electrode of the power supply. The whole chip is encapsulated by epoxy resin. The semiconductor wafer consists of two parts, one is a P-type semiconductor, in which holes are dominant, and the other is an N-type semiconductor, mainly electrons. But when these two semiconductors are connected together, a "pn junction" is formed between them. When the current acts on the chip through the wire, the electrons will be pushed to the P region, where they will recombine with holes and then release energy in the form of photons, which is the principle of LED light emission. The wavelength (color) of light is determined by the material forming the pn junction.

LED is a semiconductor that can convert electric energy into light energy. It changes the principle of incandescent tungsten filament luminescence and energy-saving lamp tricolor powder luminescence, and adopts electric field luminescence. According to analysis, the characteristics of LED are very obvious.

LED bulbs and lamps have long display life (19), high light efficiency, no radiation and low power consumption. The spectrum of white LED is almost completely concentrated in the visible light band, and the luminous efficiency can exceed 150lm/W(20 10 years). Comparing LED with ordinary incandescent lamp, spiral energy-saving lamp and T5 tricolor fluorescent lamp, the results show that the light efficiency of ordinary incandescent lamp is 1.2 lm/w, and its service life is less than 2000 hours. The power of spiral energy-saving lamp is 60lm/W, and its service life is less than 8000 hours. The power of T5 fluorescent lamp is 96lm/W, its life is about 1000 hours, and its diameter is 5 mm. It is also predicted that the upper limit of LED life will be infinite in the future. However, the working principle of LED lamps makes the heat dissipation problem very prominent in the high-power LED lighting industry. Many LED lighting schemes do not pay enough attention to heat dissipation, or their technical level is limited. Therefore, the actual service life of mass-produced high-power LED lamps is far less than the theoretical value, and the cost performance is lower than that of traditional lamps. In order to prolong the service life of LED lamps and truly make them suitable for commercial mass production, the LED lighting industry is stepping up its efforts to develop new heat-conducting materials such as heat-conducting plastics independently or in cooperation with professional heat-conducting material suppliers.

High-power LED generally refers to more than 0.65W, and different companies will have different standards, because there is no recognized industry standard in the field of high-power LED. The ratio of light intensity to lumen is small, but the heat dissipation is also large. At present, high power is mostly used in a single application, and the effective heat dissipation area is large, so the integrated LED lamp matrix appears, but the heat dissipation effect is not very good. The low power is generally around 0.06W W W. At present, LED flashlights are generally used at low power, and the light does not diverge, which is divided into large angle and small angle depending on the light emitting angle of LED. Small angles don't spread, big angles only spread. Flashlights on the market are generally made of straw hats. The effect is very good. Now I'm worried that some manufacturers don't pay attention to quality and use inferior LEDs as flashlights. It won't be long before there are dead lights.

The brightness of LED must be related to the light emitting angle of LED. The smaller the LED angle, the higher the brightness. Nothing is super bright, it's a lie to children. If it is a good quality LED, no matter which LED manufacturer produces it, everyone's brightness is similar, but the production process is different and the service life is slightly different. Because everyone uses those foreign LED chips. If it is a 5MM LED with an angle of 180 degrees, the brightness of white light is only a few hundred MCD. If it is an angle of 15 degrees, it will reach the brightness of 1 10,000 to 20,000 MCD, and the brightness difference is dozens of times. If it is used for lighting, it is best to use high-power LED outdoors, with higher brightness and single power of 65438+.

Color temperature has nothing to do with brightness, but brightness is related to lumen value.

Let's look at some related concepts:

luminous flux

(lm)

Because human eyes have different sensitivities to electromagnetic waves with different wavelengths, we can't directly measure light energy by the radiation power or radiation flux of light source, but must use the unit based on human eyes' perception of light-luminous flux. The luminous flux is indicated by the symbol φ, and the unit is lumen (lm).

luminescent intensity

(CD)

Luminous flux refers to the total light energy emitted by a light source to the surrounding space. The spatial distribution of luminous flux emitted by different light sources is different. The unit of luminous intensity is Candeira, and the symbol is cd, which indicates the luminous flux emitted by the light source within the unit spherical solid angle (the angle formed by the surface of the object and the point light source). 1CD =1lm/1Sr (Sr: spherical unit of solid angle).

clever

(cd/m2)

Brightness refers to the intensity of light emitted by an object seen by the eyes from a certain direction. The unit is Candeira /m2 [cd/m2], and the symbol is L, which indicates the luminous flux of the luminous body in the unit area of the solid angle in a specific direction, which is equal to the luminous intensity emitted by 1 Candeira from the surface of 1 m2.

colour temperature

(color temperature)

When the light emitted by a light source is the same color as the light radiated by a blackbody at a certain temperature, the temperature of the blackbody is called the color temperature of the light source, which is expressed by the absolute temperature k (Kelvin = 273. 15).

color rendering

In principle, artificial light should be the same as natural light, so that people can correctly distinguish the color of things with the naked eye. Of course, it depends on the location and purpose of lighting.

The degree to which a light source presents the color of an object is called color rendering. Commonly referred to as "color rendering index" (Ra). Color rendering refers to the relationship between the true color of things (their own color) and the color displayed under a standard light source. The determination of Ra value is to compare eight test colors defined in DIN6 169 standard with the measured light source. The smaller the color difference, the better the color rendering of the test light source color.

A light source with Ra value of 100 indicates that the color displayed by things under its light is the same as that under the standard light source.

Basic information

LED(Light Emitting Diode) is a solid-state semiconductor device, which can directly convert electric energy into light energy. The core of LED is semiconductor chip. One end of the chip is attached to a bracket, which is the negative electrode, and the other end is connected to the positive electrode of the power supply. The whole chip is encapsulated by epoxy resin. The semiconductor wafer consists of two parts, one is a P-type semiconductor, in which holes are dominant, and the other is an N-type semiconductor, mainly electrons. But when these two semiconductors are connected together, a "pn junction" is formed between them. When the current acts on the chip through the wire, the electrons will be pushed to the P region, where they will recombine with holes and then release energy in the form of photons, which is the principle of LED light emission. The wavelength of light determines the color of light, and the color of light is determined by the material that constitutes the pn junction. smd led

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LED application

In view of its own advantages, LED is mainly used in the following aspects:

(1) display screen and traffic signal display light source LED lamps have the characteristics of shock resistance, fast light response, power saving and long service life, and are widely used in various indoor and outdoor display screens, which are divided into full color, double color and monochrome display. More than 100 sets are under development and production in China. Traffic lights mainly use ultra-high brightness red, green and yellow LEDs. Because of the energy-saving and high reliability of LED lights, traffic lights are being upgraded step by step all over the country, which is a good market opportunity with fast promotion speed and great market demand.

(2) Application in automobile industry Automobile lamps include dashboard, audio indicator, switch backlight, reading lamp, external brake lamp, taillight, side lamp, headlight, etc. Incandescent lamps for automobiles are not resistant to impact and impact, easy to be damaged, short in life and need to be replaced frequently. 1987, China began to install high-position brake lights on cars. Due to the fast response speed of LED, drivers can be reminded to brake as soon as possible to reduce rear-end collision accidents. In developed countries, the central rear high-position brake light made of LED has become the standard of automobiles, and the LED automobile taillight module introduced by Hewlett-Packard Company of the United States in 1996 can be combined into various automobile taillights at will. In addition, the light source in lighting components such as automobile dashboard can be used as ultra-high brightness light, so LED display screen is gradually adopted. China automobile industry is in a period of great development, which is an excellent opportunity to promote ultra-high brightness LED. In recent years, the annual output value 1 100 million yuan will be 3 billion yuan after five years.

(3) The LED backlight with high efficiency side emission is the most eye-catching. As the backlight of LCD, LED has the characteristics of long life, high luminous efficiency, no interference and high cost performance, and has been widely used in electronic watches, mobile phones, BP machines, electronic calculators, credit card machines and so on. With the miniaturization of portable electronic products, LED backlight has more advantages, so the backlight manufacturing technology will develop towards thinner, lower power consumption and uniformity. LED is the key device of mobile phone. An ordinary mobile phone or PHS needs about 10 LED devices, while a color screen and a mobile phone with camera function need about 20 LED devices. At present, the backlight of mobile phones is very large, and 3.5 billion LED chips are used every year. At present, the output of mobile phones in China is very large, and most of the LED backlights are imported, which is an excellent market opportunity for domestic LED products.

(4) (4) The early products of LED lighting source have low luminous efficiency, and the light intensity can only reach several tens of MCD, which is suitable for indoor occasions, household appliances, instruments and meters, communication equipment, microcomputers, toys, etc. At present, the direct goal is to replace incandescent lamps and fluorescent lamps with LED light sources, and this substitution trend has begun to develop from local application fields. In order to save energy, Japan is planning a light-emitting diode project (called "Lighting Japan") to replace incandescent lamps. The budget for the first five years is 5 billion yen. If LED is used to replace half of incandescent lamps and fluorescent lamps, the energy equivalent to 6 billion liters of crude oil can be saved every year, which is equivalent to the power generation of five 1.35 × 106kW nuclear power plants, and the generation of greenhouse gases such as carbon dioxide can be reduced, thus improving people's living environment. China also invested 5 billion yuan in 2004 to develop energy-saving and environmental-friendly semiconductor lighting plan.

(5) For other applications, such as a kind of flashing shoes popular with children, the built-in LED will flash when walking, and 500 million LEDs are used in Wenzhou alone; Using light-emitting diode as indicator light of electric toothbrush, according to the introduction of domestic manufacturers that are putting into production, this

LED light (5) companies have a small number of healthy toothbrushes on the market, and it is estimated that mass production needs 300 million light-emitting lights every year; The popular LED Christmas lights are novel in shape, colorful, unbreakable and safe to use at low voltage. Recently, they have sold strongly in Southeast Asia, such as Hong Kong, and are widely welcomed by people, threatening and replacing the existing light bulb Christmas market.

(6) LED products used for home indoor lighting are becoming more and more popular. LED downlights, LED ceiling lamps, LED fluorescent lamps and LED fiber-optic lamps have quietly entered the family!

LED lighting color

LED (Lighting Lighting Diode) lighting is light-emitting diode lighting, which is a semiconductor solid-state light-emitting device. It uses a solid semiconductor chip as a luminescent material, which releases excess energy through the recombination of carriers in the semiconductor, causing photon emission and directly emitting red, yellow, blue, green, cyan, orange, purple and white light. LED lighting products are lighting appliances made of LED as light source.

Lighting principle

LED is made of III-IV compounds, such as semiconductors such as GaAs (gallium arsenide) and GAAP (gallium phosphide), and its core is PN junction. Therefore, it has the I-N characteristics of general pn junction, that is, forward conduction, reverse cutoff and breakdown characteristics. In addition, under certain conditions, it also has luminous characteristics. Under DC voltage, electrons are injected from N region to P region, and holes are injected from P region to N region. A part of minority carriers (minority carriers) entering the opposite area and majority carriers (multi-carriers) compound to emit light.

Advantages of LED

The inherent characteristics of LED determine that it has many advantages, such as:

First, the volume is small.

LED is basically a very small chip, encapsulated in epoxy resin, so it is very small and light.

Second, low power consumption.

LED power consumption is quite low, driven by DC, ultra-low power consumption (0.03-0.06 watt per tube), and electro-optical power conversion is close to 30%. Generally speaking, the working voltage of LED is 2-3.6V, and the working current is 0.02-0.03a; ; That is to say, it consumes no more than 0. 1W, and the same lighting effect saves nearly 80% energy compared with traditional light sources.

Third, the service life is long.

Some people call LED light source longevity lamp. It is a solid cold light source, encapsulated by epoxy resin, with no loose parts in the lamp body and no shortcomings such as easy burning, thermal deposition and light decay of the filament. Under proper current and voltage, the service life can reach 60,000-65,438+10,000 hours, which is 65.438+00 times longer than that of the traditional light source.

Fourth, high brightness and low fever.

LED adopts cold light emitting technology, and its calorific value is much lower than that of ordinary lighting.

Verb (abbreviation of verb) environmental protection

LED is made of non-toxic materials, unlike fluorescent lamps, which cause pollution and can be recycled. There are no ultraviolet and infrared rays in the spectrum, and there is no heat and radiation. It is a cold light source with small glare, so you can feel at ease. It belongs to a typical green lighting source.

Six, strong and durable

LED is completely encapsulated in epoxy resin, which is stronger than light bulbs and fluorescent tubes. There is no loose part in the lamp body, which makes the LED not easy to be damaged. Led light

Seven, more changes

LED light source can make use of the principle of three primary colors of red, green and blue, and under the control of computer technology, make the three colors have 256 gray levels and mix them at will, which can produce 256× 256× 256 =16777216 colors, form a combination of different light colors, and realize colorful dynamic change effects and various images.

Eight, advanced technology

Compared with the monotonous luminous effect of traditional light sources, LED light sources belong to low-voltage microelectronic products. It successfully combines computer technology, network communication technology, image processing technology and embedded control technology. , so it is also a digital information product, a "high-tech" technology of semiconductor photoelectric devices, with the characteristics of online programming, unlimited upgrade and flexibility.

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LED shortcomings

First of all, LED is considered as the most efficient artificial lighting technology in the world. Although there are still many people in China who think that LED is a display technology to replace LCD, in fact, this energy-saving lighting technology has been applied since the 1960s and 1970s, and now LED lighting is widely used from various indicator lights, street lights and holiday lights to notebooks and TV backlights. Because of its high energy efficiency, it is generally believed that replacing traditional light bulbs and fluorescent lamps with LED lamps is a very environmentally friendly practice.

However, a recent survey conducted by the University of California, Irvine shows that the environmental protection effect of using LED is likely to be offset by the toxic substances it contains. In this research jointly conducted by the Ministry of Social Ecology and the Public Health Project, they analyzed the red, yellow, green and blue LED lights in the common Christmas tree lantern groups in the market, including both high-brightness LEDs and low-brightness products.

The results show that these LED lamps contain antimony, arsenic, chromium, lead and other metal elements. Among them, the content of toxic elements in some LED lamps has exceeded the standards set by the regulatory authorities. For example, in low-brightness red LED lamps, researchers found that the lead content exceeded the standard by 8 times and the nickel content exceeded the standard by 2.5 times.

In fact, in California law, the vast majority of LED lamps have been clearly defined as toxic waste, which will pollute the soil and groundwater if treated by ordinary landfill. Moreover, if the LED light is broken, direct contact may also cause harm to human health. But so far, no matter governments or people all over the world know little about the environmental and health risks of LED lamps.

According to the report, arsenic, lead, nickel and copper in led have the most serious impact on human body and environment, and a more detailed and in-depth investigation should be conducted in the future to promote the government to formulate norms for the safe use and recycling of LED products. In short, it should be clear to everyone that although LED is very energy-saving, it is by no means a completely environmentally friendly choice, but the potential dangers it contains are different from other lighting technologies.

The second point is that LED needs to be integrated on the circuit board on a large scale because of its narrow single light emitting surface, which will cause a lot of heat accumulation and sometimes breakdown the circuit board. Therefore, the heat dissipation of LED lamps must be good.

The third point is that blue light and ultraviolet light (that is, ultraviolet light) are the most unacceptable to human eyes. The ability of blue light to kill human active cells is 10 times that of green light, and the ability of ultraviolet light to kill human active cells is 10 times that of blue light. Long-term exposure to a large number of low-wavelength blue light will kill a large number of human eye active cells, and eventually cancer will form plaques. The formation of LED white light mainly depends on blue light with wavelength of 450-455NM to excite phosphor, and the lower the wavelength, the stronger the excitation ability. Generally, the wavelength of LED is controlled within 500NM, that is, 450-455NM or 455-460NM, which is the most harmful section. If the wavelength becomes larger, the ability to excite phosphors will decrease and the efficiency will decrease. In order to pursue brightness, people usually strengthen the blue light intensity of LED. The longer the illumination time, the faster the phosphor decays, which leads to the stronger light in the blue band that human eyes contact, thus causing harm to human eyes.

Therefore, there are some disadvantages in the use of LED lights in road traffic, such as LED navigation instructions, LED street lamps and LED desk lamps. It is easy to make people feel dizzy and uncomfortable during use, and even long-term use will cause eye injuries and increase the probability of eye diseases.

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LED design concept

The appearance of LED breaks the traditional design methods and ideas of light source. At present, there are two latest design concepts.

1. Scene lighting: It was proposed by Philips in 2008 to design lamps according to the needs of the environment. Situational lighting takes the place as the starting point, aiming at creating a beautiful and gorgeous lighting environment to contrast the scene effect and make people feel a scene atmosphere.

2. Mood lighting: It was put forward by Casio in 2009, and the lamp is designed according to people's needs. Emotional lighting is based on people's emotions and creates an artistic lighting environment from the perspective of people. Emotional lighting is different from scene lighting. Emotional lighting is dynamic, which can meet people's spiritual needs and make people feel emotional. Scene lighting is static, which can only emphasize the needs of scene lighting and cannot express people's emotions. In a sense, emotional lighting covers scene lighting. Emotional lighting includes four aspects: environmental protection and energy saving, health, intelligence and humanization.

Mr. Wu Yulin, the general manager of Casio Company, compiled Emotional Lighting Book, which is the first book in China to lead the trend of led lighting design, breaking the situation that the design theory has been monopolized by foreign giants for a long time and making the application of LED easier to market. I will share the latest design concept of emotional lighting with you. I hope more experts, scholars and designers will participate in the discussion and make suggestions.

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The luminous principle of LED

The LED of LED flashlight is made of III-IV compounds, such as semiconductors such as GaAs (gallium phosphide) and GaAsP (gallium phosphide), and its core is PN junction. Therefore, it has the I-N characteristics of general pn junction, that is, forward conduction, reverse cutoff and breakdown characteristics. In addition, under certain conditions, it also has luminous characteristics. Under DC voltage, electrons are injected from N region to P region, and holes are injected from P region to N region. A part of minority carriers (minority carriers) entering the opposite area and majority carriers (multi-carriers) compound to emit light.

Assuming that luminescence occurs in the p region, the injected electrons directly combine with valence band holes to emit light, or are first captured by the luminescence center and then combined with holes to emit light. In addition to this luminescence recombination, some electrons are captured by the non-luminescence center (this center is near the middle of the conduction band and the dielectric band) and then recombine with holes, and the energy released each time is not large enough to form visible light. The greater the ratio of luminous recombination to non-luminous recombination, the higher the quantum efficiency of light. Because recombination emits light in the minority carrier diffusion region, light is generated only within a few microns near the surface of the PN junction.

Theory and practice have proved that the peak wavelength λ of light is related to the band gap width Eg of semiconductor materials in the light-emitting region, that is,

λ≈ 1240/Eg (nm)

Where the unit of Eg is electron volts (eV). If visible light (purple light with a wavelength of 380nm to red light with a wavelength of ~780nm) can be generated, the Eg of the semiconductor material should be between 3.26 ~ 1.63 EV. Light with a longer wavelength than red light is infrared light. At present, there are four kinds of light emitting diodes: infrared, red, yellow, green and blue, but the blue light emitting diode is expensive and not widely used.

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White LED for lighting

Relationship between visible light spectrum and LED white light. As we all know, the wavelength range of visible light spectrum is 380 nm ~ 760 nm, which is seven colors of light that human eyes can feel-red, orange, yellow, green, cyan, blue and purple, but these seven colors of light are monochromatic. For example, the peak wavelength of red light emitted by LED is 565 nm. There is no white light in the spectrum of visible light, because white light is not monochromatic light, but synthetic light composed of many monochromatic lights, just as sunlight is composed of seven monochromatic lights, and white light in color TV sets is also composed of three primary colors: red, green and blue. Therefore, in order to make LED emit white light, its spectral characteristics should include the whole visible spectral range. However, under the current process conditions, it is impossible to manufacture an LED with this performance. According to people's research on visible light, the white light that people can see needs at least a mixture of two kinds of light, that is, two-wavelength light (blue light+yellow light) or three-wavelength light (blue light+green light+red light). The above two modes of white light need blue light, so taking blue light has become the key technology to make white light, which is the "blue light technology" pursued by major LED manufacturing companies. At present, there are only a handful of manufacturers who have mastered the "blue light technology" in the world, so there is still a process for the promotion and application of white LEDs, especially the promotion of high-brightness white LEDs in China.

Technical structure of white LED and white light source. For general lighting, from the process structure, white LED is usually formed by two methods. The first method is to use "blue light technology" and phosphor to form white light; The second is a variety of monochromatic light mixing methods. These two methods have successfully manufactured white light devices. Application prospect of new light source for white LED lighting. In order to illustrate the characteristics of white LED, let's take a look at the current situation of lighting source. Incandescent lamps and tungsten halogen lamps have luminous efficiency of 12 ~ 24 lumens/watt; The luminous efficiency of fluorescent lamps and HID lamps is 50 ~ 120 lumens/watt. White LED: 1998, the light efficiency of white LED is only 5 lumens/watt, 1999, reaching 15 lumens/watt, which is similar to ordinary household incandescent lamps. In 2000, the luminous efficiency of white LED reached 25 lumens/watt, which was similar to that of halogen tungsten lamp. Some companies predict that by 2005, the light efficiency of LED will reach 50 lumens/watt, and by 20 15 years, the light efficiency of LED is expected to reach 150 ~ 200 lumens/watt. At that time, the working current of white LED can reach ampere level. It can be seen that the development of white LED lighting source for writers will become a possible reality.

Although incandescent lamps and tungsten halogen lamps for general lighting are cheap, they have low luminous efficiency (the thermal effect of lamps consumes electricity in vain), short service life and heavy maintenance workload. However, if white LEDs are used as lighting, they not only have high luminous efficiency, but also have a long service life (continuous working time is longer than 10000 hours) and hardly need maintenance. At present, German Hella Company has developed airplane reading lamp by using white LED. A street in Canberra, the capital of Australia, has used white LED as street lighting. China's urban traffic management lights are also replacing the early traffic order indicators with white LEDs. It can be predicted that in the near future, white LED will definitely enter the home to replace the existing lighting.

LED light source has the advantages of using low-voltage power supply, low energy consumption, strong applicability, high stability, short response time, no pollution to the environment, multi-color luminescence and so on. Although the price is more expensive than the existing lighting equipment, it is still considered that there will be existing lighting equipment.

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Dimming control of LED

Traditionally, the dimming of LED is realized by adjusting the forward current in LED with DC signal or filtered PWM. Reducing LED current will adjust the light output intensity of LED, however, the change of forward current will also change the color of LED, because the chromaticity of LED will change with the change of current. Many applications (such as automobile and LCD TV backlight lighting) do not allow any color drift of LED. In these applications, a wide range of dimming is necessary, because there are different light changes in the surrounding environment, and human eyes are sensitive to small changes in light intensity. Controlling the brightness of the LED by applying PWM signal allows the dimming of the LED to be completed without changing the color.

People often say that PWM dimming is to use a PWM signal to adjust the brightness of LED.

There are three common methods to adjust LED brightness:

(1) Using the set resistor, different conversion resistors are connected in parallel at both ends of the LED drive control IC pin RSET, and a DC voltage is used to set the current of the LED drive control IC pin RSET, thus changing the forward working current of the LED and adjusting the brightness of the LED.

(2) Using PWM technology and PWM control signal, the luminous brightness of LED is adjusted by controlling the duty ratio of LED forward working current.

(3) Linear adjustment, the simplest method is to use external setting resistor in LED drive control C to realize LED dimming control. Although this dimming control method is effective, it lacks flexibility and cannot allow users to change the light intensity. Linear adjustment will reduce the efficiency and lead to the color shift of white LED to yellow spectrum. This may be a slight deviation, but it can be detected in sensitive applications.

Using digital or PWM LED dimming control method, the pulse duty ratio of LED driving current is changed by pulse width modulation, and the switching frequency is greater than 100HZ. Switching dimming control frequency greater than 100HZ is mainly used to avoid human eyes from feeling dimming flicker. Under the PWM dimming control of LED, the luminous brightness of LED is directly proportional to the pulse duty ratio of PWM. Under this dimming control mode, the luminous color of LED can remain unchanged in the range of high brightness ratio, and the dimming ratio range of LED dimming control with PWM can reach 3000: 1.

The dimming control method of linear LED adopts analog dimming control method. Under the analog dimming control, the LED dimming control is realized by adjusting the LED forward working current, and the dimming control range can reach 10: 1.

If we want to further reduce the forward working current of LED, it will cause some problems, such as the color of LED will change, and the forward working current of LED cannot be accurately regulated.