With the rapid development of LED lighting technology, LED street lights in road lighting can be found in many cities in China. Especially after the luminous efficiency of high-power LED devices exceeds 100lm/W, the development trend of LED street lights as road lighting has been generally recognized.
Although the technological level of LED street lights has developed rapidly, most LED street light manufacturers neglect the special technical requirements in the cold application environment during the product development process. A common misconception is that the application of LEDs in cold areas is conducive to heat dissipation and is not prone to go wrong. As everyone knows, under the cold application environment, there are more stringent technical requirements for LED street lights, mainly in the following two aspects:
(1) The temperature in cold areas is low, and the temperature difference is large. The sudden change of cold and heat and long-term low temperature work have stricter quality requirements for the device.
(2) LED street lights used in cold areas must consider preventive measures to prevent ice condensation.
In view of the above reasons, the popularization and application of LED street lights in cold areas need to solve the following key technical problems.
1. The temperature change of hot and cold impact may cause the failure of the LED device
After the LED chip, a solid-state solid device, is packaged, there is a mismatch in the expansion coefficient among the chip, silica gel (or resin), metal bracket and lead. Coupled with the impact of the large temperature difference in cold areas, the expansion and shrinkage of silica gel will increase during the temperature change, and the internal stress of the device will be too large, which will lead to an increase in the displacement of the LED wire bonding point, causing premature fatigue and damage to the wire. At the same time, the solder joints with poor bonding state may occur loose weld, causing desoldering of the solder balls and chip electrodes, and even cause the failure of delamination of the LED chip.
There are hundreds of LED devices used in an LED street light, which usually adopt a series-based multiple-series configuration. If one LED fails, it will cause multiple LEDs to fail. Therefore, in order to ensure the reliability of each LED device in low temperature operation environment, the packaging process parameters such as ultrasonic power, bonding pressure, bonding time and bonding temperature should be set according to the temperature variation characteristics of LED lights in the specific use environment.
2. Reliability guarantee for low temperature operation of LED driving device
Another key technical challenge lies in the reliability of LED street light driving devices in low-temperature operating environments. At present, most LED street light driving power supplies show different types of inadaptability in low temperature environments, whose prominent characteristics includes the failure of normal start at low temperature and the high failure rate under long-term low temperature operation environment. The main reason for the above problems is that the reliability of low-temperature working state is not considered in the device selection of the driving power supply in the design stage, and the characteristics of some key components change under low temperature environment, which makes the driving device unable to start or operate normally at low temperature. The specific reasons for that failure are as follows:
(1) The density and activity of the carrier of the switch tube will be reduced under low temperature conditions, and the starting point of the overload protection will also be reduced accordingly.
(2) The electrolyte of electrolytic capacitor freezes at low temperature and loses the capacitance effect (the ions in the solution only have ion polarization at this time), and there is no load capacity.
(3) Some types of optical coupling devices cannot work normally at low temperature.
(4) The thermistor at the input to prevent inrush current will increase its resistance at low temperature (3 to 5 times that of normal temperature), which will also cause the failure to start normally at low temperature.
The solution to the above-mentioned problems is mainly to select devices with better temperature characteristics. For example, not all electrolytic capacitors cannot work at low temperature. Generally speaking, electrolytic capacitors above 200V have poor low temperature resistance, while electrolytic capacitors below 160V can basically work normally at -40°C. As long as two low voltage electrolyzers are used in series, the problem can be solved. The adjustment of the device selection may slightly increase the cost of the driving device. However, it can be ignored from the perspective of the overall cost of the LED street light. Considering the factors of improving reliability and reducing maintenance costs, this design adjustment is very necessary and cost-effective.
3. Protective measures against the hazards of ice condensation on LED street lights
One problem that is easily overlooked in the popularization and application of street lights in cold areas is the ice that is formed after being melted by heat due to the accumulation of ice and snow on the surface of the light. Once the ice is formed, it will cause a very big safety hazard to vehicles and pedestrians. Especially for LED street lights, since the light housing is generally made of aluminum, and the aluminum oxide on the surface is a hydrophilic material, which is more prone to ice condensation.
In order to solve this key technology, we analyzed the surface of the ice-coated body by microscopic imaging analysis, and analyzed the reasons why water freezes on the surface of the object and the reasons why the ice can adhere firmly on the surface of the object. The results of the experiment found that the result of only using materials with excellent hydrophobic properties to prevent ice condensation from being is not ideal, and water can even freeze very hard on the surface of some materials. The test results show that: ice can adhere to the surface of any object; cracks and sinking 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 process of freezing, but it cannot prevent 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 of different objects, the internal relationship between surface physical properties and ice coating from different angles, including structural shape and icing, structural material and icing, surface finish and icing, surface rigidity and flexibility and icing. The research results show that: components with simple appearance structure, compact structure, surface waterproof, and excellent hydrophobic properties are not prone to ice condensation.
Based on this test data, we have set up LED street lights with different appearances and housing materials in winter for two consecutive years. Through the experiment, it is found that the LED street lights with smooth appearance, without ice and snow or water accumulation bearing structure, with excellent hydrophobic performance, will not produce ice condensation.