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

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

　　Many companies may disagree with this result, because they think their heat sinks are well designed. This may be the case, but the test results are beyond doubt. What is the problem?

　　I think that the heat sink may not be designed so badly, but it may be the result of some street lamps using constant voltage power supply. But why does constant voltage power supply cause light decay? This sounds a bit like a fantasy. But in fact it is really that serious. Let's start from the beginning!

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

　　1. The volt-ampere characteristics of LED

　　As we all know, LED is a diode, and the most important electrical characteristic of a diode is its volt-ampere characteristic. Figure 2 shows the volt-ampere characteristic of Cree's XLamp7090XR-E.

　　Figure 2. XLamp7090XR-E volt-ampere characteristics

　　2. Temperature characteristics of LED volt-ampere characteristics

　　Although it looks the same as a normal diode, the biggest difference is its temperature characteristics. In fact, all diodes have temperature characteristics, but LEDs require special attention. This is because:

　　2.1 The working current of high-power LED is relatively large, 1W is 0.35A, 3-5W is 0.7A, 20W is 1.05A, 30W is 1.75A, 50W is 3.5A. However, some people may think that the forward current of the rectifier diode may also reach such a large value.

　　2.2 Since the current luminous efficiency of LED is still relatively low, most of the input electrical power is converted into heat, so its heat generation is very high. If the heat sink is not well made, the junction temperature will rise very high.

　　2.3 LED is different from rectifier diodes. It is not made of ordinary silicon materials, but special materials (such as gallium nitride). Therefore, the temperature characteristics of its volt-ampere characteristics are also different from those of ordinary diodes, but are significantly greater than those of ordinary diodes. For example, the temperature characteristics of the volt-ampere characteristics of ordinary diodes are -2mV/°C, but the temperature characteristics of the volt-ampere characteristics of Cree's XLamp7090XR-E are as high as -4mV/°C, which is twice as large as ordinary diodes.

　　3. Problems caused by increased junction temperature

　　3.1 When the LED junction temperature increases, the first thing that comes into play is a decrease in light output.

　　Figure 3. XLamp7090XR-E relative light output decreases as junction temperature increases.

　　3.2 The left shift of the V-I characteristic caused by the increase of junction temperature

　　Because the temperature coefficient of the volt-ampere characteristic is negative, it means that as the temperature rises, the characteristic shifts to the left. For example, if the junction temperature rises by 50 degrees, the volt-ampere characteristic will shift to the left by 200mV.

　　3.3 Using a constant voltage power supply will cause the LED forward current to increase as the temperature rises.

　　Because the power supply voltage is constant, but the volt-ampere characteristic shifts to the left, the result is an increase in the forward current. From the volt-ampere characteristic in Figure 2, it can be seen that if a 3.3V constant voltage power supply is used at room temperature, the forward current is 350mA; after the junction temperature rises by 50 degrees, the volt-ampere characteristic shifts to the left by 0.2V, which is equivalent to the power supply voltage rising to 3.5V. At this time, the forward current will increase to 600mA.

　　3.4 Using a constant voltage power supply will cause a vicious cycle of increasing temperature rise

　　After the forward current increases, because the power supply voltage does not change, the input power of the LED increases to 3.3Vx0.6A=1.98W.

　　It almost doubled. However, as can be seen from Figure 3, after the junction temperature rises, the light output will decrease, which means that more input power is converted into heat energy, that is, if the forward current is increased at this time, its light output will not increase, but decrease. Therefore, the increase in forward current at this time will only cause the junction temperature to increase, but will not increase the light output.

　　Therefore, after the junction temperature increases, the forward current increases, and when the junction temperature increases again, the forward current increases again, which causes a vicious cycle of rising junction temperature.

　　Conclusion: Using a constant voltage power supply will increase the junction temperature, increase the light decay, and shorten the life.

　　Therefore, from the previous analysis, we can draw the following conclusion: using a constant voltage power supply will increase the junction temperature, and the result of the increased junction temperature is increased light decay and shortened life. Assuming that the LED is turned on at a normal temperature of 25 degrees, the junction temperature will rise after turning on. Assuming that the heat sink is designed to rise to 75 degrees, that is, the junction temperature increases by 50 degrees, then the forward current will increase to 600mA. The total power increases from 1.155W to 1.98W, an increase of 0.825W. And this part of the increased power is almost all converted into heat. Assuming that the original LED luminous efficiency is 30%, that is, 70% of the input power (0.8W) is converted into heat energy. Now there is twice as much heat energy that needs to be dissipated from the heat sink. Obviously, this is not taken into account in the original heat sink design. This makes the junction temperature of the LED rise by another 50 degrees to 125 degrees. Let's go back to Figure 1 and look at the light decay curve. At 125 degrees, the light decay is 14%, and the life span is about 1200 hours. This can also explain why a carefully designed heat sink, if powered by a constant voltage power supply, still results in a large light decay and a short life span!

　　Therefore, when powering LEDs, a constant current power supply must be used. After the current is constant, no matter how the temperature changes or how the volt-ampere characteristic shifts to the left, the current will not change! The junction temperature will not go into a vicious cycle!