Analysis of Key Technical Problems of LED Street Lights in Cold Area Application Environment

With the rapid development of LED lighting technology, in many domestic cities, road lighting has no shortage of LED street lamps. Especially after the luminous efficiency of high-power LED devices exceeds 100lm/W, LED street lamps have been widely recognized as the development trend of road lighting. In 2009, several cities in China organized assessment and testing of LED road lighting products. Most LED street lamps have made significant progress in key technical indicators such as light distribution curve and system light efficiency, and the compliance and energy saving effect of road lighting standards. Other aspects have reached the requirements of relevant standards and norms.

Although the technical level of LED street lamps develops rapidly, most LED street lamp manufacturers ignore the special technical requirements in the cold environment in the product development process. A common misconception is that the use of LEDs in cold areas is good for heat dissipation and is not prone to failure. As everyone knows, under the cold application environment, there are more stringent technical requirements for LED street lamps, mainly in the following two aspects: First, the temperature in cold regions is low, and the temperature difference is large, the sudden change of cold and heat shock and long-term low temperature work for the device. There are more stringent quality requirements. Second, LED street lamps that are applied in cold areas must consider preventive measures against ice-cream condensation.

In view of the above reasons, the promotion and application of LED street lamps in cold regions needs to solve the following key technical problems.

First, the thermal shock temperature changes may cause failure of LED devices

Since LED chips are packaged as solid solid devices, there is a mismatch between the expansion coefficient of the chip, the silicone (or resin), the metal support, and the leads, and the impact of the cold and heat shocks with large temperature differences in cold regions will make In the process of temperature change, the silica gel expands and shrinks, and the internal stress of the device is too large. This will cause the displacement of the LED wire bonding point to increase, leading to premature fatigue and damage of the lead. At the same time, the solder joints with poor bonding status may also be de-welded, which may cause the solder balls and chip electrodes to be de-soldered, and even cause the failure of the LED chip.

There are up to hundreds of LED devices used in LED street lamps. Usually, hybrid-type configuration methods are mainly used in a series connection. If one LED fails, multiple LEDs will fail due to failure. Therefore, LEDs used in cold regions are used. In order to ensure the reliability of each LED device in a low-temperature operating environment, the street lamp must first set the packaging process parameters such as ultrasonic power, bonding pressure, bonding time and bonding temperature according to the temperature change characteristics of the lamp in a specific use environment. .

Second, the reliability of low-temperature operation of LED drive

Another key technical challenge lies in the reliability of LED street lamp drivers in low-temperature operating environments. At present, most LED street lamp driving power sources show different types of water and soil dissatisfaction under low-temperature environment in cold regions. The most prominent feature is the phenomenon of high temperature failure rate and high failure rate under long-term low temperature operation environment. The above-mentioned problems are mainly caused by the fact that the device selection of the driving power supply at the design stage does not take into account the reliability of the low-temperature operation state, and the characteristics of some key components change in a low-temperature environment, causing the drive device to fail to start at a low temperature or normal operation, causing a failure. The specific reasons are mainly the following four aspects:

First, the density and activity of the unloader at low temperatures will decrease, and the starting point for overload protection will also decrease. Second, the electrolytic capacitor electrolyte freezes at low temperatures and loses the capacitive effect (ions in the solution There is only ion polarization) and no load carrying capacity. Third, some types of diaphragm devices do not work well at low temperatures. Fourth, the resistance of thermistors at the input to prevent inrush currents increases at low temperatures. (3 to 5 times the normal temperature), it will cause the low temperature to not start normally.

The solution to the above problem is mainly based on the selection of devices with better temperature characteristics. For example, not all electrolytic capacitors can work at low temperatures. In general, electrolytic capacitors above 200V have poor resistance to low temperatures, and electrolytic capacitors below 160V can operate normally at minus 40°C, as long as two are used. The use of low-voltage electrolytic series can solve the problem. The adjustment of device selection may slightly increase the cost of the driving device, but it is negligible from the viewpoint of the overall cost of the LED street lamp. Considering the factors of improving the reliability and reducing the maintenance cost, such design adjustment is very necessary and cost-effective.

Third, the LED street lights ice cream condensation protection measures

One of the problems that street lamps are popularly used in cold regions is that ice and snow are accumulated on the surface of the lamps, and the ice that forms after the heat is melted. Once the ice cream is formed, it will cause great safety hazards for vehicles and pedestrians. Especially for LED street lamps, aluminum is commonly used in lamp housings, and the surface alumina is a hydrophilic material, which is more likely to produce ice-cream condensation. In 2008, a large-scale snow disaster in southern China caused a large amount of ice condensation on the surface of the aluminum high-voltage wire, which collapsed and damaged a large number of power facilities. The consequences of this disaster are not only caused by the climate, but the high-voltage cables on the surface of alumina are also an important incentive for hydrophilic materials. As the safety of road traffic is affected, the prevention of ice condensation is a safety measure that LED street lamps must take into account in cold regions.

In order to solve this key technology, we conducted a microscopic imaging analysis of the surface of the icy body to analyze the reason why water freezes on the surface of the object and ice can adhere firmly on the surface of the object. The experimental results show that using only materials with excellent hydrophobic properties, the effect of preventing ice-cream condensation is not ideal, and even water can freeze very strongly on the surface of certain materials. The test results show that ice can adhere to the surface of any object; cracks and depressions on the surface of the object are another main reason why ice can adhere firmly to the surface of the object; materials with good hydrophobic properties can delay freezing. Process, but it can not stop the formation of ice.

In order to more accurately compare the ability of different materials to prevent ice-cream condensation, we studied the surface structure of different objects, the relationship between surface properties and the ice-covering intrinsic relationship from different perspectives, including: structural shape and ice-covering, structural material and ice-covering , surface finish and ice coating, surface rigidity and ice coating. The results show that the components with simple structure, compact structure, waterproof surface and hydrophobic properties are not prone to ice condensation.

Based on this test data, we have set up LED street lamps of different appearances and housing materials in the winter for two consecutive years. Through experiments, we have found that all lamps are sleek, smooth and smooth; there is no ice and water storage bearing structure; surface materials are available The LED lamp with excellent hydrophobic properties will not produce ice cream condensation.

Most of the cold regions in China are located in high latitudes, where the climate is cold and the temperature difference is large. Therefore, the application of LED road lighting products has brought new challenges. On the other hand, in most cold regions, the annual average daily lighting time is longer than the domestic average, so the lighting power consumption is also higher than the domestic average, so the short-term and long-term geographical characteristics have brought more demand for LED lighting applications. Overall, the application of LED lighting products has both challenges and opportunities.