Reliability evaluation criteria for LED lighting systems
LED lighting products have been used in general lighting for at least a decade, but how to evaluate the reliability of LED lighting products is still a topic of debate, sometimes even quite confusing. This article will discuss some of the evaluation criteria and procedures for the reliability of LED lighting systems.
The performance of incandescent or fluorescent lamps using existing lighting technology is usually measured by the average service life, which is the time it takes for half of the tested lamps to fail (no longer illuminate). The average life of an incandescent lamp is about 1000 hours, the average life of a compact fluorescent lamp is about 12,000 hours, and the average life of a linear fluorescent lamp is about 25,000 hours. In contrast, LEDs operating under suitable environmental conditions can have a luminous time of up to 100,000 hours or more, so their average lifetime cannot be measured.
The LED illumination lifetime is characterized by the L70 indicator, which is the time it takes for the output intensity measured in lumens (L) to drop to 70% of the initial brightness. From the perspective of reliability engineering, the disadvantage of the L70 is that the measurement method issued by the lighting engineering association “Lighting Engineering Association” only measures the lumen output of the LED package, without other system factors such as drive circuit, thermal management design and even optical path. Today's lighting manufacturers do not focus on the reliability of the entire system, because lights are almost always the first bad parts. But with the development of LED lighting, manufacturers and end users have come to realize that other parts of the lighting system may become a limiting factor in overall life, as described in the table below.
Reliability is the probability that a component or system will work as intended under a given working condition for a specified period of time. Product reliability assessments are basically based on statistical modeling and prediction methods for accelerated life testing. For LED lighting systems, rated light fading and other component failures can affect overall reliability, as shown in Figure 1. Statistical models that combine these two different factors of lighting system performance have proven to be less applicable, which is the main reason why the focus has been on robust testing.
In field operations, robustness is a non-quantitative measure of the ability of a product or system to work as intended under a particular environment by confirming that products made in accordance with a particular design and manufacturing process can withstand normal Greater stress under working conditions. For decades, the electronics industry has been using robust testing as part of the verification process. Until recently, this concept was accepted by more LED lighting organizations because they finally realized that LED-based lighting products are not much different from other electronic products.
Unlike reliability testing using design prototypes, robustness testing typically places production samples under various super-stress conditions, making them more effective as part of the overall quality certification system. Unlike the reliability test, which detects faults in order to understand the failure mode, the robustness test expects that all products will work properly at the end of the test.
The robustness test of LED lighting is very similar to the testing of other types of electronic equipment released by JEDEC, IEC and other organizations, but some adjustments have been made according to the particularity of LED products. The test plan for a given product will vary depending on the target application, such as a test board for an indoor office product that may not need to contain high temperature work or vibration. The table below lists the more common robustness test items recommended for LED lighting products.
Recently, some standards bodies have studied the robustness of LED lighting systems, and there is still much work to be done. Next time we will discuss the existing and planned standards and how they solve the reliability problems of LED lighting systems.