With the rapid development of LED technology , in many cities in China, there are many LED street lights in the 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 evaluation and testing of LED road lighting products. Most LED street lighting products made significant progress in key technical indicators such as light distribution curve and system light efficiency. The effects and 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:
(1) In cold regions, the temperature is low, and the temperature difference is large. The sudden thermal shock and long-term low temperature work have stricter quality requirements for the device.
(2) 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 needs to solve the following key technical problems.
Temperature changes in thermal shock can cause 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 coefficient of the chip, the silicone (or resin), the metal bracket and the lead, and the influence of the thermal shock of the large temperature difference in the cold region. The expansion and contraction of the silica gel during the temperature change process is intensified, and the internal stress of the device is too large, which leads to an increase in the displacement of the LED wire bonding point, and the lead wire is 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. .
Reliability guarantee for low temperature operation of LED drive unit
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 consider 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 at low temperature, causing failure. The specific reasons are as follows:
(1) The density and activity of the downloading tube at the low temperature condition will be reduced, and the starting point of the overload protection will also be reduced.
(2) The electrolytic capacitor electrolyte freezes at low temperature, and loses the capacitive effect (the ions in the solution only have ion polarization at this time), and there is no load capacity.
(3) Some types of optical devices do not work properly at low temperatures.
(4) The thermistor that prevents the inrush current at the input end, the resistance value becomes larger at low temperature (3 to 5 times of the normal temperature), and the low temperature cannot be started 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 -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.
LED street light 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 excellent hydrophobic properties are not prone to ice condensation.
Based on this test data, we have set up LED street lamps with different appearances and shell materials in the winter for 2 consecutive years. Through experiments, we found that all the lamps are smooth, smooth and flat; there is no ice or snow accumulation structure; the surface materials are available. LED street lamps with excellent hydrophobic properties will 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. In short, there are both challenges and opportunities.
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