In-depth analysis of the important reasons for LED light decay

Author: Mao Yu Hai

Now everyone is beginning to realize that light decay is the main reason why high-power LED street lights can't work for a long time. It is also beginning to realize that an important way to reduce light decay is to improve its heat dissipation. Despite this, judging from the test results of various street lamps by the Shenzhen Lighting Environmental Management Center, the light decay of most street lamps is still unable to meet the requirements for use. The light decay after 1200 hours of lighting, the best is 8%, the worst is 26%, the average is 14%. According to Cree's test results, when the junction temperature is 105 degrees, 14% of the light decay should be after 6000 hours of operation, and the junction temperature of most street lamps is above 105 degrees.

Many companies may not agree with this result because they think their radiators are carefully designed. The actual situation may be the same, but the results of the test are not doubtful. What's the problem?

I think that the heat sink may not be designed so badly, but it may be because some street lights are powered by a constant voltage power supply. But why does the use of a constant voltage power supply cause light decay? It sounds like a bit of a fantasy. But it is actually so serious. Let's start from the beginning!

Figure 1. Cree's LED junction temperature and light decay life test results

1. Volt-ampere characteristics of LED
We all know that an LED is a diode, and the most important electrical characteristic of a diode is its volt-ampere characteristics. Figure 2 shows the volt-ampere characteristics of Cree's XLamp 7090XR-E.

Figure 2. Voltammetric characteristics of the XLamp7090XR-E