LED road lighting reliability in low temperature operating environment

With the rapid development of LED lighting technology, in many domestic cities, road lighting has no shortage of LED street lamps. Especially after the luminous efficiency of high-power LED devices exceeds 100lm/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 LED street lamps have made significant progress in key technical indicators such as light distribution curve and system light efficiency, and the compliance and energy saving in road lighting standards. The effects and other aspects have reached the requirements of relevant standards and specifications.

Although the technical level of LED street lamps develops rapidly, most LED street lamp manufacturers ignore the special technical requirements in the cold environment in the product development process. A common misconception is that the use of LEDs in cold areas is good for heat dissipation and is not prone to failure. As everyone knows, under 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 change of cold and heat shocks and long-term low-temperature work have stricter quality requirements for the devices.

(2) LED street lights applied in cold areas must take precautions to prevent 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.

Change in temperature due to thermal shock may cause LED device failure

After the LED chip is encapsulated, it is a solid solid-state device. There is a mismatch between the coefficient of expansion of the chip, the silicone (or resin), the metal support, and the lead, and the impact of the cold and heat shocks in the cold region is large. The expansion and contraction of the silica gel during the temperature change 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 premature fatigue and damage of the lead. At the same time, the solder joints with poor bonding status may also be de-welded, which may cause the solder balls and chip electrodes to be de-soldered, and even cause the failure of the LED chip.

There are up to hundreds of LED devices used in LED street lamps. Usually, hybrid-type configuration methods are mainly used in a series connection. If one LED fails, multiple LEDs will fail due to failure. Therefore, LEDs used in cold regions are used. In order to ensure the reliability of each LED device in a low-temperature operating environment, the street lamp must first set the packaging process parameters such as ultrasonic power, bonding pressure, bonding time and bonding temperature according to the temperature change characteristics of the lamp in a specific use environment. .

The reliability of low temperature operation of LED drive

Another key technical challenge lies in the reliability of LED street lamp drivers in low-temperature operating environments. At present, most LED street lamp driving power sources show different types of water and soil dissatisfaction under low-temperature environment in cold regions. The most prominent feature is the phenomenon of high temperature failure rate and high failure rate under long-term low temperature operation environment. The above-mentioned problems are mainly caused by the fact that the device selection of the driving power supply at the design stage does not take into account the reliability of the low-temperature operation state, and the characteristics of some key components change in the low-temperature environment, causing the drive device to fail to start at a low temperature or normal operation, causing a failure. The specific reasons are mainly the following:

(1) Both the density and the activity of the unloader in the low-temperature condition of the switch tube will be reduced, and the starting point of the overload protection will be reduced accordingly.

(2) The electrolytic capacitor electrolyte freezes at a low temperature and loses the capacitance effect (ions in the solution exist only ion polarization at this time), with no carrying capacity.

(3) Some types of aperture devices do not function properly at low temperatures.

(4) The thermistor at the input end to prevent inrush current will increase the resistance at low temperature (3~5 times of normal temperature), and it will cause the low temperature to fail to start normally.