It is proved by theory and practice that the peak wavelength λ of light is related to the band gap Eg of semiconductor materials in the light-emitting region, that is, λ≈ 1240/Eg(mm).
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. There are infrared, red, yellow, green and blue light emitting diodes, but blue light emitting diodes are expensive and not widely used. Limit parameter
(1) allowable power consumption pm: the maximum value of the product of the allowable forward DC voltage across the LED and the current flowing through it. If this value is exceeded, the LED will heat up and be damaged.
(2) Maximum forward DC current IFm: the maximum forward DC current that can be increased. Exceeding this value will damage the diode.
(3) Maximum reverse voltage VRm: Maximum allowable reverse voltage. If this value is exceeded, the LED may be damaged due to breakdown.
(4) Working environment topm: The ambient temperature range in which LEDs can work normally. Below or above this temperature range, LED will not work normally, and the efficiency will be greatly reduced.
Under the premise of not changing the material, the way to improve the brightness is to increase the current within the limit range of LED. With the increase of current, the calorific value of LED will increase sharply. Portable projectors with LED light sources, or friends in mini-projector, must have a deep understanding. Projectors with LED light sources are very hot and generally have obvious noise. On the one hand, these products are small in size, and the key is their high calorific value.
With the increase of power, the heat dissipation problem of LED becomes more and more prominent. A large number of practical applications show that the fundamental reason why LED can't improve the input power is that LED will release a lot of heat in the working process, which makes the junction temperature of the die rise rapidly and the thermal resistance increase. The higher the input power, the greater the heating effect. The increase of temperature will lead to the change and attenuation of device performance, the increase of non-radiation recombination, the increase of device leakage current, the increase of semiconductor material defects, the electromigration of metal electrodes, and the yellowing of epoxy resin for packaging. , will seriously affect the photoelectric parameters of LED. Even disable the power indicator. Therefore, it is more and more important for LED devices to reduce the thermal resistance and junction temperature and study the thermal characteristics of LED.
reliability
An important reason for the low life of LED driving power supply is that the life of aluminum electrolytic capacitor required for driving power supply is not enough. The primary reason is that the internal environment temperature of LED lamps is very high when they work for a long time, which leads to the rapid consumption of electrolyte of aluminum electrolytic capacitors and greatly shortens their service life, usually only working for about 5,000 hours. The life of LED light source is 50,000 hours, so the working life of aluminum electrolytic capacitor becomes the short board of LED driving power supply. Nowadays, in order to deal with this problem, some suppliers have created LED driving power supply schemes without aluminum electrolytic capacitors. But not all LED driving power suppliers support this practice. Chen Rong pointed out: "At present, none of the mass-produced LED driving power supplies choose a driving scheme without electrolytic capacitor. Without it, many test specifications will fail, such as EMI test and flicker-free test. " However, the LED driving power supply scheme with aluminum electrolytic capacitor is very simple. What happens when pcb is replaced by thin film capacitor, ceramic capacitor and tantalum capacitor? If the film capacitor reaches the same capacitance (usually 100-220uF), it will be very large and the cost will be too high, while the ceramic capacitor is usually too small. If multiple ceramic capacitors are used to complete such a large capacity, it will occupy too much circuit board area and cost. Tantalum capacitors should have such a large capacity. First, it is too expensive, but the pressure resistance is too low to meet the demand. Therefore, any other type of capacitor will not be too large. If these defects are replaced by capacitors with smaller capacity, the effect of eliminating ripple will not be so good, and many export products will not pass the strict certification test indicators, so the current high-quality LED driving power supply is mainly aluminum electrolytic capacitors. Many suppliers claim that the scheme of LED driving power supply without electrolytic capacitor is likely to only remove the aluminum electrolytic capacitor at AC input, and it should be difficult to remove or replace the aluminum electrolytic capacitor at constant current output. A company introduced a 13WLED bulb with built-in driving power supply, which can be dimmed by triac. The driving power supply scheme based on SSL2 102 adopts aluminum electrolytic capacitor. When the author asks about the working life of the light bulb, the working life of the aluminum electrolytic capacitor will definitely affect the life of the whole LED light bulb, but some physical methods can be used to alleviate this problem, such as positioning the aluminum electrolytic capacitor on the PCB near the lamp tail. Generally speaking, the temperature closest to the LED light source is the highest, which can reach 100-200℃, followed by the heat dissipation metal shell, usually 100℃. Our experiments show that its service life can reach about 10000 hours, which is equivalent to 10 years. For ordinary household users, it is basically affordable to replace LED bulbs every ten years.
Going green and environmental protection
Heavy metals that pollute the environment, such as mercury and lead, will generate ultraviolet rays when they emit light, so LED lighting will not have many mosquitoes around the light source like traditional lamps, making the environment cleaner and cleaner; The innovative design of constant current drive of metal shell makes the efficiency of converting electric energy into light very high. (1) Energy saving: the spectrum of LED is almost completely concentrated in the visible light band, and its luminous efficiency can reach 80-90%. The author also compares LED lamps with ordinary incandescent lamps, spiral energy-saving lamps and T5 tricolor fluorescent lamps. The results show that the light efficiency of ordinary incandescent lamp is 12lm/w, and its service life is less than 2000 hours. Spiral energy-saving lamp 60lm/w, service life less than 8000 hours; T5 fluorescent lamp is 96 aluminum/watt, and its service life is about 10000 hours. It is also predicted that the upper limit of LED life in the future is infinite.
It is generally believed that energy-saving lamps can save 4/5 energy, which is a great pioneering work, but LED is more energy-saving than energy-saving lamps 1/4, which is a greater reform of solid light sources. In addition, LED has other advantages, such as high light quality, basically no radiation, and it is a typical green lighting source. Reliable and durable, extremely low maintenance cost and so on. It is precisely because of the incomparable characteristics of LED with other solid-state light sources that LED will be the mainstream light source in the lighting industry after 10 years.
(2) Safety and environmental protection: the working voltage of LED is low, mostly1.4-3v; The working current of ordinary LED is only 10mA, and the ultra-high brightness is only1a.. In the process of LED production, there is no need to add "mercury", inflation or glass shell, which has good impact resistance, is not easy to break, is convenient to transport and is very environmentally friendly, and is called "green energy".
(3) Long service life: LED is small in size and light in weight, and the outer shell is encapsulated by epoxy resin, which not only protects the internal chip, but also has the ability of light transmission and light concentration. The service life of LED is generally between 50,000-65,438+10,000 hours, because LED is a semiconductor device, even if it is frequently switched on and off, it will not affect the service life. Nowadays, incandescent lamps, fluorescent lamps and energy-saving fluorescent lamps are mainly used in household lighting.
(4) Fast response: The response frequency fτ of LED is related to the lifetime τmc of injected minority carriers. For example, the response frequency of an LED made of GaAs is about 1- 10 ns, that is, about 16- 160 MHz. Such a high response frequency is enough to display a video signal of 6.5MHz.
The minimum response time of LED reaches 1 microsecond, usually a few milliseconds, which is about1100 of the response time of ordinary light sources. Therefore, it can be used in many high-frequency environments, such as car brake lights or status lights, which can shorten the braking time of vehicles behind the car and thus improve safety.
(5) High luminous efficiency: the luminous efficiency of incandescent lamps and tungsten halogen lamps is 65,438+02-24 lm/w (lumen/watt), that of fluorescent lamps is 50-70 lm/w, and that of sodium lamps is 90-65,438+040 lm/w, and most of the power consumption becomes heat loss. The improved LED has a light efficiency of 50-200 lm/w, good monochromaticity and narrow spectrum, and can directly emit colored visible light without filtering.
(VI) Small size of LED elements: it is more convenient for the layout and design of various devices, and can better realize the effect of "seeing only the light but not the light source" in night lighting.
(VII) High concentration of LED light energy: concentrated in a small wavelength window, with high purity.
(8) LED has strong directivity: the brightness attenuation is much lower than that of traditional light sources.
(9) LED can be driven by low-voltage direct current: it has the advantages of small load and weak interference, and has low requirements for the use environment.
(10) can well control the composition of luminescence spectrum, so it can be well used in museums and exhibition halls or local accent lighting.
(1 1) can control the band gap size of the semiconductor light-emitting layer and the semiconductor material, so as to emit light of various colors with high chromaticity.
(12) High color rendering: no harm to human eyes. With the introduction of LED into indoor and outdoor lighting market, the price of LED lighting has also dropped significantly, but the uneven quality has also caused many problems. It is pointed out that LED lighting standards are formulated one after another in various places, and a number of mandatory lighting standards will be on the road from 20 12, which is expected to accelerate the elimination of vicious competition of inferior products, thus driving the reshuffle effect of LED lighting market.
Due to the increasingly extensive and diversified application of LED lighting, the Bureau of Standards of the Ministry of Economic Affairs of Taiwan Province Province has successively formulated a number of LED lighting standards. Following the global release of LED street lamp specifications, at the end of 20 10, three common LED indoor lighting standards were issued, including LED T8 straight tube lamp, LED projection lamp and light steel frame lamp (including flat lamp). Insiders pointed out that although relevant standards and specifications have been formulated, LED street lamps have been promoted for many years. It is expected that the installation will not be expanded before 20 12. Due to the slow introduction of the government and budget operation, it is estimated that the procurement bidding will not be released until 20 13~20 14.
Although the development of LED industry in Chinese mainland is in full swing, due to the great differences in climate and environment in different regions, the industry standards have not yet entered the national standard project, and the provinces will take the lead in promoting the formulation of regional LED lighting standards. However, it is pointed out that the inland development and reform commission will adopt the technical specifications of LED lighting and give priority to indoor LED downlights. It is expected that after the technical specifications are finalized, the financial subsidy scheme for LED indoor lighting announced by the central government will be extended in 20 12 under the impetus of the Twelfth Five-Year Plan of the Mainland.
As for LED lighting products imported into the EU, they must pass CE certification, and considering that electromagnetic radiation may cause harm to human body, the EU also announced that the safety standard for human electromagnetic radiation will be enforced from 20 13.
With the release of the new version of the listing specification for solid-state lighting lamps and light sources by the US Environmental Protection Agency in October, 20 10/0, the industry pointed out that the LM-80 light decay test, which has attracted much attention, is expected to be enforced from April, 20 12. After the new specification is put on the road, lamp manufacturers in North America market will give priority to purchasing LED products that have passed LM-80 verification.
As the light decay verification of LM-80 takes 6,000 hours, which takes as long as 6-8 months, some LED industries think that the specifications and technologies of LED products are changing with each passing day, and in a few months, the verification standard of LED lighting may change again, so there are still doubts about this certification, but Gong Yan = said that LM-80 is regarded as a ticket to enter the American market. The manufacturer must issue the report certificate of LM-80(LED lumen maintenance rate) test to obtain the Energy Star seal. The North American Institute of Lighting Engineering (IES) has also formulated LM-80 light attenuation detection standard, which not only provides measurement standards for LED application products, but also provides quality assurance for consumers, and will become an international common detection standard.