Key issues affecting the life of LED street lamps

With the rapid development of LED lighting technology, in many cities in China, there are many LED street lights in road lighting. Especially after the luminous efficacy of high-power LED devices exceeds 100 lm/W, LED street lamps have been widely recognized as the development trend of road lighting. In 2009, several cities in China organized the evaluation and testing of LED road lighting products. Most of the LED street lamp products made significant progress in key technical indicators such as light distribution curve and system light efficiency, and the compliance and energy saving effects of road lighting standards. Other aspects have met the requirements of relevant standards and specifications.
Despite the rapid development of LED street light technology, most LED street lamp manufacturers neglect the special technical requirements in the cold application environment during product development. A common misconception is that LEDs in the cold area are good for heat dissipation and are not prone to failure. As everyone knows, in the cold application environment, there are more stringent technical requirements for LED street lamps, mainly in the following two aspects: First, the temperature in the cold regions is low, and the temperature difference is large, the sudden change of thermal shock and long-term low temperature work for the device Have more stringent quality requirements. Second, LED street lights used in cold areas must take precautions against ice condensation.
In view of the above reasons, the promotion and application of LED street lamps in cold regions need to solve the following key technical problems.
First, the temperature change of thermal shock may cause the failure of LED devices
Due to the solid components of the solid state after the LED chip is packaged, there is a mismatch between the expansion coefficients of the chip, the silicone (or resin), the metal bracket and the lead, and the influence of the thermal shock of the cold region is large. During the temperature change of silica gel, the expansion and contraction are intensified, and the internal stress of the device is too large, which will cause the displacement of the LED wire bonding point to increase, and the lead wire to be fatigued and damaged prematurely. At the same time, the solder joints with poor bonding state may be de-soldered, which may cause the solder balls and the chip electrodes to be desoldered, and even the LED chip may be delaminated.
As for LED streetlights, there are hundreds of LED devices, which are usually connected in series. If one LED fails, it will cause multiple LEDs to fail. Therefore, LEDs used in cold regions are used. Street lamps must first set reasonable process parameters such as ultrasonic power, bonding pressure, bonding time and bonding temperature according to the temperature variation characteristics of the lamps in a specific use environment to ensure the reliability of each LED device under low temperature operating conditions. .
Second, the reliability of LED drive device low temperature operation
Another key technical challenge is the reliability of LED street light drives in low temperature operating environments. At present, most LED street lamp driving power sources exhibit different types of soil and water dissatisfaction in the cold and low temperature environment. The more prominent characteristics are the low temperature failure and the high failure rate in the long-term low temperature operation environment. The main reason for the above problems lies in the fact that the device selection in the design stage does not take into account the reliability of the low-temperature operation state, and the characteristics of some key components in the low-temperature environment change, causing the drive device to fail to start or operate normally, causing malfunction. There are four main reasons for this:
First, the density and activity of the switching tube at the low temperature condition will decrease, and the starting point of the overload protection will also decrease. Second, the electrolytic capacitor electrolyte freezes at low temperature, losing the capacitance effect (ion in the solution) Only ion polarization), no load capacity; third, some types of optical devices can not work normally under low temperature; Fourth, the thermistor of the input terminal to prevent inrush current, the resistance becomes larger at low temperature (It is 3 to 5 times of normal temperature), and it will cause the low temperature to fail to start normally.
The solution to the above problems is mainly to select devices with better temperature characteristics. For example, not all electrolytic capacitors can't work at low temperatures. Generally speaking, electrolytic capacitors above 200V have poor low temperature resistance, while electrolytic capacitors below 160V can basically work normally at minus 40 °C. Low pressure electrolysis can be used in series to solve the problem. Modification of the device selection may slightly increase the cost of the drive, but it is negligible from the overall cost of the LED street light. This design adjustment is necessary and cost-effective considering the factors of improving reliability and reducing maintenance costs.
Third, LED street lights ice condensate protection measures
One problem that street lamps are popularized in cold regions is that they are easily overlooked because of the accumulation of ice and snow on the surface of the lamps and the formation of ice after melting. Once ice formations are formed, they will pose a great safety hazard to vehicles and pedestrians. Especially for LED street lamps, since the lamp housing is generally made of aluminum, the surface of the alumina is a hydrophilic material, which is more likely to cause ice condensation. In 2008, a large-scale snowstorm in the south of China caused a large number of ice-cold condensation on the surface of aluminum, which collapsed and destroyed a large number of power facilities. The consequences of this disaster are not only caused by the climate, but also the high-pressure cable on the alumina surface is a hydrophilic material. Because it affects the safety of road traffic, preventing ice condensation is a safety measure that LED street lights must consider in cold areas.
In order to solve this key technology, we analyzed the surface of the ice body by microscopic imaging analysis, and analyzed the reason why water freezes on the surface of the object and the ice can adhere firmly on the surface of the object. The experimental results show that the use of materials with excellent hydrophobic properties to prevent ice condensate is not ideal, and even water can be frozen very well on the surface of some 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 the freezing. The process does not stop the formation of ice.
In order to more accurately compare the ability of different materials to prevent ice condensation, we have studied the surface structure, surface properties and the intrinsic relationship of ice coating from different angles, including: structural shape and ice coating, structural material and ice coating. , surface finish and ice coating, surface flexibility and ice coating. The results show that the components with simple structure, tight structure, excellent surface waterproofing and hydrophobic properties are not prone to ice condensation.
Based on this test data, we have set up LED street lights with different appearances and shell materials in the winter for several years. Through experiments, we found that all the lamps are smooth, smooth and flat; there is no ice or snow or water storage structure; LED street lights with excellent hydrophobic properties do not produce ice condensation.
Most of the cold regions in China are in high latitudes, with cold climate and large temperature difference, which brings new challenges to the application of LED road lighting products. On the other hand, the average annual lighting time in most cold regions is longer than the domestic average, so the lighting power consumption is also higher than the domestic average. Therefore, the short-night regional characteristics bring more LED lighting application needs. Overall, the application of LED lighting products has both challenges and opportunities.

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