Reliability assurance of LED road lighting in low temperature operating environment

With the rapid development of LED lighting technology, LED street lights are now widely used in road lighting in many cities in China. Especially after the luminous efficiency of high-power LED devices exceeds 100lm/W, LED street lights 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 light products have made significant progress in key technical indicators such as light distribution curve and system luminous efficiency, and have met the requirements of relevant standards and specifications in terms of compliance with road lighting standards and energy-saving effects.

Although the technical level of LED street lights has developed rapidly, most LED street light manufacturers have ignored the special technical requirements for cold-region application environments during product development. A common misconception is that LEDs are good for heat dissipation and are not prone to failure in cold regions. However, in cold application environments, LED street lights have more stringent technical requirements, mainly in the following two aspects:

(1) The temperature in cold regions is low and the temperature difference is large. The sudden change of temperature and long-term low-temperature operation have more stringent quality requirements for devices.

(2) LED street lights used in cold regions must consider preventive measures to prevent ice condensation.

In view of the above reasons, the promotion and application of LED street lights in cold areas need to solve the following key technical problems.

Temperature changes caused by hot and cold shocks may cause failure of LED devices

After LED chips are packaged, they are solid devices. There is a mismatch in the expansion coefficients between the chip, silicone (or resin), metal bracket and lead wires. In addition, the hot and cold shocks with large temperature differences in cold areas will cause the silicone to expand and contract more during temperature changes, and the internal stress of the device will be too large, which will lead to increased displacement of the LED lead bonding points, premature fatigue and damage of the leads. At the same time, it will also cause the solder joints with poor bonding conditions to desolder, causing the solder balls and chip electrodes to desolder, and even cause the LED chips to fail due to delamination.

An LED street lamp uses hundreds of LED devices, which are usually connected in series. If one LED fails, multiple LEDs will fail. Therefore, LED street lamps used in cold areas must first set reasonable packaging process parameters such as ultrasonic power, bonding pressure, bonding time and bonding temperature according to the temperature change characteristics of the lamps in a specific use environment to ensure the reliability of each LED device in a low-temperature operating environment. Reliability guarantee of low-temperature

operation of LED driver devices

Another key technical challenge is the reliability of LED street lamp driver devices in low-temperature operating environments. At present, most LED street lamp driver power supplies show different types of acclimatization in cold and low-temperature environments. The more prominent manifestations are the inability to start normally at low temperatures and the high failure rate in long-term low-temperature operating environments. The main reason for the above problems is that the device selection of the driver power supply during the design stage did not take into account the reliability of the low-temperature working state. The characteristics of some key components in low-temperature environments change, causing the driver device to be unable to start or operate normally at low temperatures. The specific reasons for the failure are mainly the following:

(1) The density and activity of carriers in the switch tube will decrease under low-temperature conditions, and the starting point of the overload protection will also be reduced.

(2) The electrolyte of the electrolytic capacitor freezes at low temperatures, loses the capacitance effect (the ions in the solution only exist as ion polarization at this time), and has no load capacity.

(3) Some types of optical coupler devices cannot work normally at low temperatures.

(4) The thermistor at the input end that prevents surge current increases its resistance at low temperatures (3 to 5 times that of normal temperature), which can also cause the device to fail to start normally at low temperatures.

The solution to the above problems is mainly to start with devices with better temperature characteristics. For example, not all electrolytic capacitors cannot work at low temperatures. Generally speaking, electrolytic capacitors above 200V have poor low temperature resistance, while electrolytic capacitors below 160V can basically work normally in a -40℃ environment. The problem can be solved by selecting two low-voltage electrolytic capacitors in series. The adjustment of device selection may slightly increase the cost of the driver, but it can be ignored from the perspective of the overall cost of LED street lights. Considering the factors of improving reliability and reducing maintenance costs, this design adjustment is very necessary and cost-effective.

Protective measures against the hazards of icicle condensation on LED street lamps

One problem that is easily overlooked in the promotion and application of street lamps in cold areas is the accumulation of ice and snow on the surface of the lamps, which forms icicles after melting due to heat. Once icicles 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 aluminum oxide on the surface is a hydrophilic material, which is more likely to produce icicle condensation. In 2008, a large-scale snowstorm in southern China caused a large amount of icicles to condense on the high-voltage wires on the aluminum surface, which collapsed and damaged a large number of power facilities. This disaster is not only caused by the climate, but also by the fact that the high-voltage cables on the surface of aluminum oxide are hydrophilic materials. Because it affects the safety of road traffic, preventing icicle condensation is a safety measure that must be considered when LED street lamps are used in cold areas.

In order to solve this key technology, we analyzed the reasons why water freezes on the surface of objects and why ice can firmly adhere to the surface of objects by performing microscopic imaging analysis on the surface of ice-covered objects. The experimental results show that the effect of preventing ice condensation by only using materials with excellent hydrophobic properties is not ideal, and water can even freeze very solid 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 objects are another main reason why ice can adhere firmly to the surface of objects; materials with good hydrophobic properties alone can delay the freezing process, but 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 intrinsic relationship between the surface structure, surface properties and ice coating of different objects from different angles, including: structural shape and ice coating, structural material and ice coating, surface finish and ice coating, surface rigidity and flexibility and ice coating. The research results show that components with simple appearance structure, compact organizational structure, waterproof surface 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 shell materials in winter for two consecutive years. Through experiments, we found that LED street lights with smooth, bright and flat appearance, no ice or water accumulation bearing structure, and excellent hydrophobic surface materials will not produce icicles.

Most of China's cold regions are located in high latitudes, with cold climates and large temperature differences, which brings new challenges to the application of LED road lighting products. On the other hand, the average daily 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 regional characteristics of short days and long nights have brought more demand for LED lighting applications. In short, there are both challenges and opportunities.