Analysis of technical points of LED street lights

"Ten Cities, Ten Thousand Lights" is a demonstration project for my country's high-tech industry to promote development through application. Its purpose is to select a group of cities with good foundations, adopt a model of joint investment by the state, local governments and enterprises, take the lead in carrying out pilot applications of LED municipal lighting, explore experience for the comprehensive promotion of semiconductor lighting in my country, and enhance the industry's independent innovation level through application, enhance international competitiveness, and promote the growth and strengthening of China's semiconductor lighting industry.

This article selects three aspects, street lamps and tunnel lamps with more demonstration projects, as well as driving power supplies that have a greater impact on products, to analyze their technological progress, providing a reference for the smooth implementation of the "Ten Cities and Ten Thousand Lamps" campaign.

At present, the traditional light source used for road lighting is mainly high-pressure sodium lamps, and LED street lamps are mainly used in pilot applications on some branch roads, but the two have shown obvious differences in quality. In terms of light efficiency, high-pressure sodium lamps can reach up to 140 lm/W, which is higher than the current commercial high-power LED light efficiency of 100 lm/W, but the color rendering index of LED (about 80) is much higher than that of high-pressure sodium lamps (about 25), and under the same illumination, white light LEDs are more helpful for drivers or pedestrians to identify targets, and their road lighting effect and comfort are much higher than high-pressure sodium lamps. In terms of lamp efficiency, high-pressure sodium lamps use a spherical light-emitting design. Taking into account the reflector efficiency, the lamp efficiency of high-pressure sodium lamps is generally only 70%. However, LEDs emit light in a directional manner. If the appropriate light distribution design is adopted, most of the light will be projected directly onto the road surface, and the lamp efficiency can reach more than 85%.

Therefore, from the perspective of light effect and lamp efficiency alone, we can see the huge potential of LED street lamps to replace traditional street lamp light sources. To this end, this article will focus on the research results of the three key technologies of light distribution, power supply and heat dissipation in the development of LED lighting applications by the Semiconductor Lighting System Research Center of Sun Yat-sen University to explain the technical route and technical support of "LED street lamps" which are closely related to the "Ten Cities and Ten Thousand Lamps" demonstration application project.

Light Distribution

Obtaining bat-wing-shaped light intensity distribution through optical design

Currently, the LED street lights on the market mainly have two types of light sources: single 1W high-power white light LED array and high-power integrated package light source module. Although the national standard for LED street lights has not yet been issued, the light distribution of LED street lights should achieve the following goals when referring to the standard requirements of traditional light source road lighting: appropriate average road brightness; high total illumination uniformity and longitudinal illumination uniformity; appropriate ambient ratio; glare control, etc.

From the light distribution curve, the above goals can be achieved mainly through appropriate optical design to obtain bat-wing-shaped light intensity distribution, so as to obtain rectangular light spot distribution on the road surface. However, the packaging lens (i.e. primary optical lens) of ordinary high-power white light LED is not suitable for direct application in LED street lamps, so a secondary optical lens must be added to the primary optical lens of each high-power white light LED. At present, the "peanut" type 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 separate secondary optical lens, but to directly design a wavy optical lens mask outside the light source of the primary packaged LED, and use the lens mask to achieve the function of a secondary optical lens for the entire LED street lamp light source.

As the packaging industry moves closer to the needs of downstream application industries, the Semiconductor Lighting System Research Center of Sun Yat-sen University has also developed and designed a special shape of primary optical lens, which is directly installed when the LED chip is packaged. It has the characteristics of small size and low cost, and fully meets the requirements of LED street lamps and road lighting (Figure 1).

With the advancement of packaging technology, the packaging method of white light LEDs has gradually shifted from single 1W high-power LED devices to high-power integrated package light source modules. The current high-power integrated package light source modules have a maximum power of more than 100W, but this type of light source has a large luminous area, which brings difficulties to optical light distribution design.

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 of more than 90 and an adjustable correlated color temperature of 2500~8000K (Figure 2). This technology uses the integrated photoelectric conversion chip on the packaging substrate to monitor the white light chromaticity parameters of the light source module in real time. The photoelectric conversion chip feeds back the changes in the detected white light chromaticity parameters to the intelligent control system. The system ensures that the lamp can output white light with optimal chromaticity performance after calculation, and can ensure that the output of the light source module maintains the set correlated color temperature range and color rendering index; the packaging substrate also integrates a temperature sensor chip to detect the temperature of the packaging substrate in real time, and realizes indirect monitoring of 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 1 W high-power white light LED array or a high-power integrated package light source module, and has been used in LED street lights.

power supply

Enhance the reliability of the driving power supply design to match the LED life

At present, the mainstream LED street lights are powered by AC. There is a common problem with AC LED street lights, which is that it is difficult to ensure that the life of the driving power supply matches the life of the LED. Because AC must be rectified and filtered by the switching power supply to become DC, and electrolytic capacitors must be used in the switching power supply for filtering. The life of a general electrolytic capacitor is only 8,000 hours, which is far less than the theoretical life of the LED of 50,000 hours. Moreover, for every 10°C increase in ambient temperature, the life of the electrolytic capacitor is reduced by half, so that the life of the entire LED street light system will inevitably be dragged down by the electrolytic capacitor. Therefore, an important factor restricting the life of LED street lights is the reliability design of the driving power supply. In order to ensure the reliable operation of the power supply of LED street lights in outdoor environments, it is generally necessary to consider several aspects such as high efficiency, high power, long life, overvoltage and overcurrent, isolation, surge, overtemperature, protection, and compliance with safety regulations and electromagnetic compatibility requirements.

For high-power LED street lamps, no matter the light source adopts a single 1 W high-power white light LED array or a high-power integrated package light source module, the mainstream power drive mode is constant current drive. The commonly used circuit structure is a constant voltage source providing several constant current sources, each constant current source drives a series of LEDs and the mains directly converts into constant current, and the LEDs operate in series and parallel combinations.

For this method of using a single 1 W high-power white light LED array, the constant voltage source is a relatively mature traditional switching power supply architecture; and the matching constant current source part is a DC step-down type, with an efficiency of more than 95%. In addition, it occupies a small circuit space and can be combined with the constant voltage source part or integrated with the LED, which has greater flexibility. Each LED current can be controlled independently to ensure the overall light emission of the lamp is consistent, but the cost will be slightly higher.

For high-power integrated packaged light source modules, they are divided into two categories: isolated and non-isolated. The former has advantages in cost and efficiency, but because it is non-isolated, the power supply is unstable, especially at night when the voltage is high or surges generated during thunderstorms, which can easily cause the LED light source and the power supply to be damaged. Although the latter has lower efficiency and higher circuit complexity, its reliability is guaranteed. Whether it is an isolated or non-isolated AC-DC constant current source, since the number of LEDs on the street lamp ranges from dozens to hundreds, the back-end LEDs must consider a combination of series and parallel connections, which inevitably makes the currents of the parallel currents inconsistent. At present, these two types of power supplies coexist. The multi-channel constant current output method has better performance and reliability, and will be the mainstream development direction of LED street lamp power drivers in the future.

Exploiting the potential of batteries to extend the life of solar street lights

With the development of solar energy, a new energy source, solar LED street lights are gradually emerging in various places. The low-voltage direct current and long life characteristics of solar cells are just right for LEDs. However, there is still a life bottleneck in the solar LED street light system, which is the lead-acid battery. The life of a general lead-acid battery is 500 charging cycles, about 2 years, or about 5,000 hours. The intelligent charging and discharging intelligent controller developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University can extend the life of the lead-acid battery to 1,500 cycles.

In traditional solar street light charging systems, solar panels are usually directly connected to batteries through current backflow protection diodes, which will cause the working point of the solar panel to deviate from the maximum power point (MPP) and fail to effectively utilize the output power of the solar panel. At the same time, it is easy for the battery to be in an under-charged state for a long time due to insufficient energy supply, resulting in a shortened lifespan. The solar LED street light system developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University uses sun tracking (Sun Tracking) and maximum power point tracking (MPP Tracking, i.e. MPPT) technology to stabilize the output of solar cells near the MPP, thereby effectively utilizing the maximum power output of the solar panel.

Intelligent dimming system flexibly adjusts light output to reduce energy consumption

Traditional high-pressure sodium street lamps can only achieve small-scale dimming control, such as turning off one side or intervals, which inevitably changes the lighting pattern and easily causes safety hazards. LED street lamps can achieve 0-100% continuous dimming, and can flexibly adjust light output according to ambient light and traffic conditions, while ensuring lighting quality and reducing unnecessary power consumption. The intelligent dimming system of LED street lamps developed by the Semiconductor Lighting System Research Center of Sun Yat-sen University can easily control the working state of LEDs and control their brightness by changing the driving current. For example, after entering the second half of the night, low-power operation can be achieved by reducing the entire lamp current or turning off some LED light-emitting components in the lamp to achieve energy-saving effects.

The Semiconductor Lighting System Research Center of Sun Yat-sen University also applies Zigbee wireless communication control technology to LED street light systems. The starting point of the Zigbee wireless control system is to develop a low-cost wireless network that is easy to deploy, with simple and compact protocol stack, power saving, reliability, short delay, and large network capacity (Figure 3). The Zigbee transceiver module is integrated in each LED street light, and through the relay method, all information is collected on the terminal, so that the operation of each LED street light can be effectively monitored at the terminal, giving full play to the functions of system control, troubleshooting and anti-theft.

Heat dissipation

Optimize heat dissipation and thermal management control systems

When LED is under forward voltage, electrons overcome the electric field of the pn junction under the drive of the electric field, jump from the n region to the p region and recombine with the holes in the p region. Since the free electrons jumping to the p region have higher energy than the valence electrons in the p region, the electrons return to a low energy state during recombination, 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.

Taking into account factors such as current injection efficiency, radiant luminescence quantum efficiency, and chip external light extraction efficiency, for a 100 lm/W LED, only about 30% of the electrical energy is converted into light energy, and the rest of the energy is converted into heat energy, which increases the temperature of the LED chip. For LED chips, if the heat cannot be effectively dissipated, the temperature of the chip will increase, causing non-uniform distribution of thermal stress, and the chip luminous efficiency and phosphor efficiency will decrease.

As the temperature of the pn junction rises, the emission wavelength of the LED chip will redshift, resulting in a decrease in the excitation efficiency of the YAG phosphor, a decrease in the total luminous intensity, and a shift in the chromaticity of the white light. When the temperature exceeds a certain value, the failure rate of the device will rise exponentially. For every 2°C increase in device temperature, the reliability will decrease by 10%. In order to ensure the life of the device, the junction temperature of the pn junction is generally required to be below 90°C. When multiple LEDs are densely arrayed or integrated, the system heat dissipation problem is more serious. Therefore, solving the heat dissipation problem has become a prerequisite for LED street lights.

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 problem of LED street lamps is divided into three levels: chip pn junction to epitaxial layer; epitaxial layer to packaging substrate; packaging substrate to the external environment. These three links constitute the channel for heat conduction. In response to the heat dissipation problem of LEDs, the Semiconductor Lighting System Research Center of Sun Yat-sen University has optimized the design of heat dissipation and thermal management systems at the following levels.

Heat dissipation from the chip pn junction to the epitaxial layer: During the growth process of gallium nitride materials, the material structure is improved, the growth parameters are optimized, high-quality epitaxial wafers are obtained, the quantum efficiency of the device is improved, the heat generation is fundamentally reduced, and the heat conduction from the chip pn junction to the epitaxial layer is accelerated.

Heat dissipation from the epitaxial layer to the package substrate: In chip packaging, flip chip structure, eutectic solder package (Figure 4), and metal circuit board structure are used. In device packaging, choose appropriate substrate materials, such as metal printed circuit boards (MC-PCB), ceramics, composite metal substrates and other packaging substrates with good thermal conductivity to accelerate the heat dissipation from the epitaxial layer to the package substrate.

Heat dissipation from the package substrate to the external environment: Current LED street lamps generally use reflow soldering to solder high-power white light LEDs in an array on a metal package substrate, and then tightly mount the metal package substrate on a large-volume heat dissipation fin made of aluminum or copper. The heat generated by the high-power white light LED is transferred to the heat dissipation fin through the metal package substrate, and the heat dissipation is achieved by natural convection or artificial forced convection.

In view of the large and concentrated heat characteristics of high-power integrated packaged light source modules, the Semiconductor Lighting System Research Center of Sun Yat-sen University installed the high-power integrated packaged light source modules on a temperature-averaging board, and used the rapid heat diffusion performance of the temperature-averaging board to quickly diffuse the heat generated by the LED laterally; heat pipes (straight heat pipes, loop heat pipes and pulse heat pipes) were also used in the heat dissipation fins to reduce and enhance heat conduction and reduce thermal resistance (Figure 5); artificial forced convection was generated in the cavity of the LED street lamp to enhance convective heat dissipation.

In summary, the technical support guiding the development of LED street lamps will be reflected in the high-power integrated packaging light source module based on eutectic welding technology; primary optical lens; white light intelligent control system with high color rendering index and adjustable color temperature; long-life driving power supply; LED street lamp junction temperature intelligent control system; Zigbee wireless communication control technology; system heat dissipation and thermal management control system based on heat pipe technology, etc.

With the rising energy prices, the intensification of the energy crisis and the improvement of human environmental awareness, LED lighting has attracted more and more attention due to its energy-saving and environmentally friendly characteristics. At present, the application of LED in ordinary lighting fields such as street lighting and indoor lighting is just in its infancy. Driven by the 2008 Beijing Olympic Games and the 2010 Shanghai World Expo, the luminous efficiency of high-power white light LEDs will soon exceed 150 lm/W, which will be the perfect time for LED to enter ordinary lighting. With the reduction of the price per lumen, LED street lights will completely replace the existing traditional street lights. By then, the global demand for LED street lights will reach hundreds of millions, and the demand in China alone will reach tens of millions, with an output value of hundreds of billions of yuan.