With the rapid development of LED technology, many urban road lighting has no shortage of LED street lights. Especially after the luminous efficiency of high-power LED devices exceeds 100lm / W, the development trend of LED street lamps as road lighting has been generally recognized.

More authoritative companies have organized the evaluation and testing of LED road lighting products respectively. Most LED street light products have made significant progress in key technical indicators such as light distribution curve and system light efficiency, and they are in compliance with road lighting standards and energy-saving effects. The aspect has reached the requirements of relevant standards and specifications.

Although the technical level of LED street lamps has developed rapidly, most LED street lamp manufacturers have ignored the special technical requirements in cold areas during product development. A common misconception is that the application of LEDs in cold areas is conducive to heat dissipation and is not prone to failure. As everyone knows, under the cold application environment, there are stricter 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 thermal shock and long-term low temperature work have stricter quality requirements for the device.

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

In view of the above reasons, the popularization and application of LED street lights in cold areas needs to solve the following key technical issues.

Temperature changes caused by thermal shock may cause failure of LED devices

After the LED chip is packaged, it is a solid solid device. There is a mismatch of expansion coefficients between the chip, silicone (or resin), metal bracket and lead, plus the impact of cold and hot shocks with large temperature differences in cold areas. The expansion and contraction of silica gel in the process of temperature change is intensified, and the internal stress of the device is too large, which will lead to increased displacement of the LED wire bonding point, premature fatigue and damage of the wire. At the same time, the solder joints with poor bonding state may be de-soldered, which may cause the solder balls and chip electrodes to be de-soldered, and even cause the LED chip to delaminate and fail.

There are hundreds of LED devices used in one LED street lamp, usually in a serial configuration configuration. If one LED fails, it will cause multiple LEDs to fail together. Therefore, LEDs used in cold areas The street lamp must first set reasonable packaging technology parameters such as ultrasonic power, bonding pressure, bonding time and bonding temperature according to the temperature change characteristics of the lamp under a specific use environment to ensure the reliability of each LED device in a low-temperature operating environment .

Reliability guarantee for low temperature operation of LED drive

Another key technical challenge lies in the reliability of LED street light driving devices under low-temperature operating conditions. At present, most LED street lamp driving power shows different types of soil and water dissatisfaction in cold and low temperature environments. The more prominent characteristics are the failure 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 drive power supply in the design stage does not consider the reliability of the low-temperature working state, and the characteristics of some key components under low-temperature environment have changed, resulting in the failure of the low-temperature startup or normal operation of the drive device, causing failure The specific reasons are as follows:

(1) The density and activity of the switching tube under low temperature conditions will be reduced, and the starting point of overload protection will be reduced accordingly.

(2) The electrolytic solution of the electrolytic capacitor freezes at a low temperature, and loses the capacitive effect (the ions in the solution only have ion polarization at this time), and has no loading capacity.

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

(4) The thermistor that prevents surge current at the input end becomes larger at low temperature (3 to 5 times the normal temperature), which also causes the low temperature to fail to start normally.

The solution to the above problems is to start with the selection of 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 at -40 ℃, as long as two are selected. The use of low-voltage electrolysis in series can solve the problem. The adjustment of device selection may slightly increase the cost of the driving device, but from the point of view of the overall cost of the LED street lamp, this design adjustment is very necessary and cost-effective considering the factors of improving reliability and reducing maintenance costs.

Protective measures of LED street lamp ice icing hazard

A problem that is easily overlooked when the street lamps are popularized and applied in cold areas is that ice and snow accumulate on the surface of the lamps, and the ice icicles formed after being melted by heat. Once the ice icicles are formed, it will cause a very big safety hazard to vehicles and pedestrians. Especially for LED street lamps, aluminum is generally used for the lamp housing, and the alumina on the surface is a hydrophilic material, which is more likely to cause ice condensation.

In order to solve this key technology, we carried out microscopic imaging analysis on the surface of ice-covered body to analyze the reason why water freezes on the surface of the object and the ice can be firmly attached to the surface of the object. The experimental results found that only using materials with excellent hydrophobic properties, the effect of preventing ice condensation is not ideal, and even water can be frozen very strongly 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 for the ice to firmly adhere to the surface of the object; simple materials with good hydrophobic properties can delay icing Process, but can not stop the formation of ice.

In order to more accurately compare the ability of different materials to prevent ice condensation, we have studied in depth the internal relationship between the surface structure, surface properties and ice coating of different objects, including: structural shape and ice coating, structural material and ice coating , Surface finish and ice coating, surface rigidity and flexibility and ice coating. The results of the study indicate that the components with simple shape, compact structure, excellent surface waterproof and hydrophobic properties are less prone to ice condensation.

Based on the test data, we set up LED street lamps with different appearances and shell materials in winter. Through experiments, we found that all lamps have a smooth, smooth and smooth appearance; there is no ice or snow or water storage load-bearing structure; surface materials have excellent hydrophobic properties LED street lights will not produce ice icing condensation.