Related topics: LED technology
Time: 2009- 12-03 09:49 Source: LED Global Online
"Ten Cities and Ten Lights" is a demonstration project to promote the development of high-tech industries in China by application. Its purpose is to select a group of cities with good foundation, adopt the mode of joint investment by the state, local governments and enterprises, take the lead in launching the pilot application of LED municipal lighting, explore the experience for comprehensively popularizing semiconductor lighting in China, enhance the level of independent innovation in the industry through application, enhance the international competitiveness, and promote the semiconductor lighting industry in China to become bigger and stronger.
In this paper, street lamps, tunnel lamps and driving power supply which have great influence on products are selected, and their technical progress is analyzed, so as to provide reference for the smooth implementation of "Ten Lights in Ten Cities".
At present, the traditional light source of road lighting is mainly high-pressure sodium lamp, and LED street lamps are mainly used in some branches, but they have shown obvious differences. In terms of light efficiency, the highest light efficiency of high-pressure sodium lamp can reach 140lm/W, which is higher than the current commercial high-power LED, but the color rendering index (about 80) of LED is much higher than that of high-pressure sodium lamp (about 25), and under the same illumination, white LED is more helpful for drivers or pedestrians to identify targets, and its road lighting effect and comfort are much higher than that of high-pressure sodium lamp. In terms of lamp efficiency, the high-pressure sodium lamp adopts spherical luminous design. Considering the reflector efficiency, the lamp efficiency of high-pressure sodium lamp is generally only 70%. However, LEDs emit light in a directional manner. If proper light distribution design is adopted, most of the light will be directly projected onto the road surface, and the efficiency of lamps can reach over 85%.
Therefore, only from the perspective of light efficiency and lighting efficiency, we can see that LED street lamps have great potential to replace traditional street lamps. Therefore, starting from the research results of three key technologies, such as light distribution, power supply and heat dissipation, obtained by the Semiconductor Lighting System Research Center of Sun Yat-sen University, this paper will focus on the technical route and technical support of "LED street lamps" closely related to the demonstration and application project of "Ten Cities with Ten Lights".
opportunity
The light intensity distribution of bat airfoil is obtained by optical design.
At present, the light sources of LED street lamps in the market are mainly divided into two ways: single 1W high-power white LED array and high-power integrated packaging light source module. Although the national standard of LED street lamps has not yet been issued, when referring to the requirements of traditional road lighting standards, the light distribution of LED street lamps should achieve the following goals: appropriate average road brightness; The overall illumination uniformity and longitudinal illumination uniformity are high; Appropriate environmental ratio; Glare control, etc.
From the light distribution curve, to achieve the above goal, the light intensity distribution of bat airfoil is obtained mainly through appropriate optical design, so as to obtain the rectangular spot distribution on the road surface. However, the packaging lens (primary optical lens) of ordinary high-power white LED is not suitable for direct application in LED street lamps, so a secondary optical lens should be added to the primary optical lens of each high-power white LED. At present, peanut secondary optical lens can achieve better results.
The design idea developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University is not to use a single secondary optical lens, but to design a wavy optical lens mask directly outside the once packaged LED light source, and use the lens mask to realize the function of the secondary optical lens of the whole LED street light source.
With the demand of packaging industry approaching to the downstream application industry, Sun Yat-sen University Semiconductor Lighting System Research Center has also developed and designed a special-shaped primary optical lens, which can be directly installed when led chips are packaged. It has the characteristics of small size and low cost, and completely meets the requirements of LED street lamps and road lighting (Figure 1).
With the development of packaging technology, the packaging mode of white LED has gradually changed from a single 1W high-power LED device to a high-power integrated packaging light source module. At present, the maximum power of high-power integrated packaging light source module can reach more than 100W, but this light source brings difficulties to optical light distribution design because of its large luminous area.
The red light enhanced high-power white LED intelligent control system technology developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University can obtain a light source module with a color rendering index above 90 and an adjustable correlated color temperature of 2500~8000K (Figure 2). In this technology, the photoelectric conversion chip is integrated on the packaging substrate to monitor the white chromaticity parameters of the light source module in real time, and the photoelectric conversion chip feeds back the detected white chromaticity parameters to the intelligent control system. After calculation, the system can ensure that the lamp can output white light with the best chromaticity performance, and can ensure that the output of the light source module can maintain the set relevant color temperature range and color rendering index. A temperature sensing chip is also integrated on the package substrate to detect the temperature of the package substrate in real time, thus indirectly monitoring the junction temperature of the high-power LED chip. When the junction temperature exceeds the preset temperature of the system, the system can automatically adjust the heat dissipation path of the heat dissipation system or reduce the power of the LED. The light source module can be composed of a single 1W high-power white LED array or a high-power integrated packaging light source module, which has been used in LED street lamps.
Power?Supply?
Strengthen the reliability design of driving power supply to match the life of LED.
At present, the mainstream LED street lamps are powered by alternating current, and there is a * * * problem in alternating current LED street lamps, that is, it is difficult to ensure that the life of the driving power supply matches the life of the LED. Because alternating current must be rectified and filtered by switching power supply before it can become direct current, electrolytic capacitors must be used for filtering in switching power supply. Generally, the service life of electrolytic capacitor is only 8000 hours, which is far less than the theoretical service life of LED of 50000 hours. Moreover, when the ambient temperature rises by 10℃, the life of electrolytic capacitor will be reduced by half, which will inevitably drag down the life of the whole LED street lamp system. Therefore, an important factor that restricts the life of LED street lamps is the reliability design of driving power supply. In order to ensure the reliable operation of power supply in outdoor environment, LED street lamps generally need to be considered from several aspects, such as high efficiency, high power, long life, overvoltage and overcurrent, isolation, surge, overheating, protection, compliance with safety regulations and electromagnetic compatibility requirements.
For high-power LED street lamps, whether the light source part adopts single 1W high-power white LED array mode or high-power integrated packaging light source module mode, the mainstream power driving mode is constant current driving. The public circuit structure is provided with several constant current sources by a constant voltage source. Each constant current source drives a series-connected LED and the commercial power to directly become a constant current, and the LEDs work in a series-parallel combination mode.
For this way of using a single 1W high-power white LED array, the traditional constant-voltage switching power supply architecture has been relatively mature; The matching constant current source is DC buck type, the efficiency can reach above 95%, the circuit space is small, and it can be combined with constant voltage source or integrated with LED, which is very flexible. The current of each LED can be controlled independently, which ensures the overall lighting consistency of the lamp, but the cost will be slightly higher.
For high-power integrated packaging light source modules, there are two types: isolated and non-isolated ones. The former has advantages in cost and efficiency, but due to non-isolation, the power supply is unstable, especially when the voltage is high at night or surge occurs during thunderstorm, which is easy to cause LED light source and power supply to be damaged together. Although the latter has low efficiency and high circuit complexity, its reliability is guaranteed. Whether it is isolated or non-isolated AC DC constant current source, the number of LEDs on street lamps is from tens to hundreds, so the LEDs at the back end should be combined in series and parallel, which inevitably makes the parallel current different. At present, these two power supply modes coexist. Multi-channel constant current output mode has good performance and reliability, and will be the mainstream development direction of LED street lamp power drive in the future.
Tap the battery potential and prolong the life of solar street lamps
With the development of solar energy as a new energy source, solar LED street lamps are gradually emerging everywhere, and the characteristics of low voltage DC and long life of solar cells just match LED. However, the solar LED street lamp system still has a life bottleneck, that is, lead-acid batteries. The life of a typical lead-acid battery is 500 charging cycles, about 2 years, about 5,000 hours. The intelligent charge-discharge controller developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University can make the life of lead-acid battery reach 1500 cycles.
In the traditional solar street lamp charging system, the solar panel is usually directly connected with the storage battery through the anti-current backflow diode, which will cause the working point of the solar panel to deviate from MaximPowerPoint (MPP), and the output power of the solar panel cannot be effectively utilized. At the same time, it is easy to make the storage battery undercharged for a long time due to insufficient energy supply, which will lead to shortened service life. The solar LED street lamp system developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University adopts solar tracking and MPPT technology, which can stabilize the output of solar cells near MPP, thus effectively utilizing the maximum power that solar panels can output.
Intelligent dimming system flexibly adjusts light output to reduce energy consumption.
The traditional high-voltage sodium street lamp can only achieve a small range of dimming control, such as turning off one side or turning off the street lamp at intervals, which will inevitably bring about changes in the lighting mode and easily cause security risks. The LED street lamp can realize 0- 100% continuous dimming, and can flexibly adjust the light output according to the ambient lighting and traffic conditions to ensure the lighting quality and reduce unnecessary power consumption. The intelligent dimming system for LED street lamps developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University can easily control the working state and brightness of LED by changing the driving current. For example, after midnight, low-power operation can be realized by reducing the current of the whole lamp or turning off some LED light-emitting elements in the lamp, thus achieving the effect of energy saving.
The Semiconductor Lighting System Research Center of Sun Yat-sen University also applies Zigbee wireless communication control technology to LED street lamp system. The starting point of Zigbee wireless control system is to develop a low-cost wireless network that is easy to deploy. It has the characteristics of simple and compact protocol stack, reliable power saving, short time delay and large network capacity (Figure 3). Zigbee transceiver module is integrated on each LED street lamp, and all information is collected to the terminal through relay, so that the operation of each LED street lamp can be effectively monitored at the terminal, and the functions of system control, troubleshooting and anti-theft can be exerted.
dissipate heat
Optimize heat dissipation and thermal management control system
Under the DC voltage of LED, driven by electric field, electrons overcome the electric field of p-n junction, jump from N region to P region, and recombine with holes in P region. Because the free electrons that jump to the P region have higher energy than the valence electrons in the P region, when they recombine, the electrons return to the low energy state, and the excess energy is released in the form of photons. The radiated light still needs to pass through the semiconductor medium and packaging medium of the chip itself to reach the outside world.
Considering the current injection efficiency, radiation quantum efficiency, external light extraction efficiency and other factors, for the LED with 100lm/W, only about 30% of the electric energy is converted into light energy, and the rest is converted into heat energy, which makes the LED chip temperature rise. For LED chips, if they can't dissipate heat effectively, the temperature of the chip will rise, resulting in uneven distribution of thermal stress, and the luminous efficiency and phosphor efficiency of the chip will decrease.
With the increase of p-n junction temperature, the emission wavelength of LED chip will shift to red, which leads to the decrease of excitation efficiency, total luminous intensity and white light shift of YAG phosphor. When the temperature exceeds a certain value, the failure rate of the equipment will increase exponentially. The reliability will decrease by 10% when the device temperature increases by 2℃. In order to ensure the lifetime of devices, the junction temperature of pn junction is generally required to be below 90℃. When multiple LEDs are densely arranged or packaged, the heat dissipation problem of the system is more serious. Therefore, solving the heat dissipation problem has become a prerequisite for LED street lamps.
How to improve the heat dissipation capacity of LED street lamps is the core issue of LED packaging and LED street lamp design. The heat dissipation of LED street lamps is divided into chip pn junction and epitaxial layer. An epitaxial layer of the packaging substrate; The substrate is packaged into the external environment at three levels. These three links constitute the channel of heat conduction. Aiming at the heat dissipation of LED, the Semiconductor Lighting System Research Center of Sun Yat-sen University has optimized the heat dissipation and thermal management system from the following aspects.
Heat dissipation from the pn junction of the chip to the epitaxial layer: in the growth process of gallium nitride material, improve the material structure, optimize the growth parameters, obtain high-quality epitaxial wafer, improve the quantum efficiency in the device, fundamentally reduce the generation of heat, and accelerate the conduction of heat from the pn junction of the chip to the epitaxial layer.
Heat dissipation from epitaxial layer to package substrate: In chip package, flip chip structure, crystal bonding package (Figure 4) and metal circuit board structure are adopted. In the device packaging, select appropriate substrate materials, such as metal printed circuit board (MC-PCB), ceramics, composite metal substrates and other packaging substrates with good thermal conductivity, so as to accelerate the heat dissipation from epitaxial layers to the packaging substrates.
The packaging substrate radiates heat to the external environment: At present, high-power white LEDs are generally array-welded on the metal packaging substrate by reflow soldering, and then the metal packaging substrate is closely attached to the large-volume aluminum-copper fins. The heat generated by high-power white LED is transferred to the heat sink through the metal packaging substrate, and the heat dissipation is achieved through natural convection or artificial forced convection.
The Research Center of Semiconductor Lighting System of Sun Yat-sen University installed the high-power integrated packaged light source module on the temperature equalizing plate, aiming at the large and concentrated heat of the high-power integrated packaged light source module, and used the rapid heat dissipation performance of the temperature equalizing plate to quickly spread the heat generated by the led laterally. Heat pipes (straight heat pipe, loop heat pipe and pulse heat pipe) are also used in the heat sink part to reduce the enhanced heat conduction and reduce the thermal resistance (Figure 5); Artificial forced convection is generated in the cavity of LED street lamp to strengthen convection heat dissipation.
To sum up, the technical support to guide the development of LED street lamps will be reflected in the way of high-power integrated packaging of light source modules based on * * * crystal bonding technology; Primary optical lens; White light intelligent control system with high color rendering index and adjustable color temperature; Long-life driving power supply; Intelligent control system for junction temperature of LED street lamps: Zigbee wireless communication control technology; System heat dissipation and thermal management control system based on heat pipe technology.
With the high energy price, the intensification of energy crisis and the improvement of human environmental awareness, LED lighting has attracted more and more attention because of its energy-saving and environmental protection characteristics. At present, the application of LED in street lighting and indoor lighting has just started. Driven by the 2008 Beijing Olympic Games and the 20 10 Shanghai World Expo, the luminous efficiency of high-power white LED will soon exceed 150lm/W, which will be an excellent opportunity for LED to enter ordinary lighting. With the decrease of unit lumen price, LED street lamps will completely replace the existing traditional street lamps. By then, the global demand for LED street lamps will reach hundreds of millions, and the demand in China alone will reach tens of millions, with an output value of several hundred billion yuan.
It is worrying that it is difficult for citizens to accept local development.
Release date: 2009- 12-04 07:00 | Author:-| Source: HC Electronic Network | View times: 0 times.
Recently, Chancheng District of Foshan City, as the pilot area of LED energy-saving street lamps and lighting engineering in Guangdong Province, has completed the street lamp conversion of Hu Jing Road, Lv Jing Road, Heyuan Road, Cai Hong Road, Foshan Avenue Zhangcha Tunnel, Wenhua Road, Tongji East, Fenjiang South Road, Lingnan Avenue (short light), Shen Ning Road, Wufengsi Road, Dongping Road and Weiguo Road. However, nearby citizens are not satisfied with LED street lamps. Some citizens have reported that the road is obviously darkened after the installation of LED street lamps, which has great safety hazards.
In response to questions raised by the public, Chancheng Public Utilities Bureau responded that the white light emitted by LED is close to sunlight, and its color rendering index is high, which is better than that of high-pressure sodium lamp. In response to netizens' report that "the white light is too pale to uglify the city", he believes that this is only because citizens are used to the yellow light of high-pressure sodium lamps.
White light is a kind of composite light, which generally consists of two-wavelength light or three-wavelength light. At present, there are three main ways for LED to realize white light: one is to synthesize white light through multi-chip combination of red, green and blue primary colors; Secondly, the blue LED chip is used to excite the yellow phosphor, and the blue light of the LED and the yellow-green light emitted by the phosphor synthesize white light, and a proper amount of red and green phosphors can be added to improve the color rendering characteristics; Thirdly, the tricolor phosphor is excited by ultraviolet LED to synthesize white light. For a long time, high-pressure sodium lamps have been the main street lamps in China, and citizens have generally adapted to yellow lighting street lamps. It will take some time to accept white LED street lamps.
The Opinions on the Development of Semiconductor Lighting Energy Saving Industry jointly issued by the National Development and Reform Commission and other six ministries and commissions recently pointed out that in the future, it will focus on developing and promoting LED lighting products in general indoor lighting, parking lots, tunnels, roads, automobile lighting, medical care, agriculture and other special fields, and improve related service systems.
In 2009, the state vigorously promoted the construction project of "10 cities with 10,000 lamps", and planned to popularize more than 300,000 LED municipal lighting lamps in 65,438+00-20 cities by 2065,438+00. This move plays an important role in many fields such as energy conservation and environmental protection, stimulating domestic demand and developing local economy. However, from the current situation, there are still considerable difficulties in the popularization of LED localization.
First of all, the price of LED lighting products is still high. Although in the long run, the product has energy-saving characteristics, but most consumers and business users still hold a wait-and-see attitude towards it. At present, LED lighting products are mainly used in high-end application fields such as high-end room office lighting, automobile lighting and government procurement. The popularization trend is slow.
Secondly, the main patents of upstream and downstream products of LED lighting are still in the hands of foreign leading enterprises, and the technology of local enterprises is backward and the product quality is uneven. The domestic market is mainly in the hands of foreign brands such as Osram and Philips. These large enterprises have built a "patent encirclement circle" for local enterprises, which makes the development of local LED enterprises quite slow.
Although the domestic LED market is promising, the local development is still difficult, and the public's acceptance, price and technical factors are all high walls in front of LED manufacturers. The development of LED in China also needs the strong support of the government and the constant publicity of the media. With these two factors, coupled with the self-improvement and innovation of enterprises, I believe that the LED industry in China will soon step into the peak of the world.