What is the main cause of LED light decay?

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

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

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

I think that maybe the radiator is not so badly designed, but it may be because some street lights are powered by a constant voltage power supply.

But why does the use of constant voltage power supply cause light decay? This sounds like a fantasy. But in fact it is so serious. Let's start from the beginning!

1. LED volt-ampere characteristics

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

Figure 2. The volt-ampere characteristics of XLamp7090XR-E

2. Temperature characteristics of LED volt-ampere characteristics

Although its appearance is no different from ordinary diodes, the biggest difference lies in its temperature characteristics. In fact, the volt-ampere characteristics of all diodes have temperature characteristics, but LEDs need 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, and 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 The current luminous efficiency of LED is still relatively low, so most of the input electric power is converted into heat, so its heat is very high. If the heat sink is not done well, the junction temperature will rise very high.

2.3 LED is different from rectifier diode, it is not made of general silicon material, but made of special material (such as gallium nitride). Therefore, its volt-ampere characteristics and temperature characteristics are also different from ordinary diodes, but are significantly larger than ordinary diodes. For example, the temperature characteristic of the volt-ampere characteristic of a general diode is -2mV/°C, but the temperature characteristic of the volt-ampere characteristic of Cree's XLamp7090XR-E is as high as -4mV/°C, which is twice as large as that of a general diode.

3. Problems caused by rising junction temperature

3.1 After the LED junction temperature rises, the first thing brought about is the decrease in light output.

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

3.2 The rise in junction temperature causes a left shift of the volt-ampere characteristics

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

4. Using a constant voltage power supply will cause the LED forward current to increase with the increase in temperature.

Because the power supply voltage is constant, but the volt-ampere characteristic has shifted to the left, the result is an increase in the forward current. As can be seen from the volt-ampere characteristics of Figure 2, if a 3.3V constant voltage power supply is used at room temperature, its forward current is 350mA; after the junction temperature rises by 50 degrees, the volt-ampere characteristics shift to the left by 0.2V, which is equivalent to the power supply The voltage rises to 3.5V, at this time, the forward current will increase to 600mA.

5. The use of constant voltage power supply will cause a vicious circle of increased 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,

Almost doubled. However, it can be seen from Figure 3 that after the junction temperature increases, the light output will decrease, which means that more input power is converted into heat energy, which means that if the forward current is increased at this time, its light output will not increase. , But reduced. Therefore, the increase in the 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, the junction temperature increases again, and the forward current increases again...

This causes a vicious circle of rising junction temperature.

Conclusion: The use of constant voltage power supply will increase the junction temperature, increase the light attenuation, and shorten the lifespan

Therefore, from the previous analysis, it can be concluded that the use of a constant voltage power supply will increase the junction temperature, and the increase in the junction temperature results in increased light attenuation 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 increased from 1.155W to 1.98W, an increase of 0.825W. Almost all of the increased power in this part is converted into heat. Assuming that the original LED's luminous efficiency is 30%, that is, 70% of the input power (0.8W) is converted into heat. Now twice as much heat energy needs to be dissipated from the radiator. Obviously, this is not considered in the original radiator design. This causes the junction temperature of the LED to increase by 50 degrees to 125 degrees. Let’s go back to Figure 1 to look at the light decay curve. The lifetime of 14% at 125°C is almost 1200 hours, which can explain why a well-designed radiator is powered by a constant voltage power supply. The light decay is still great, and the life span is very short!

Therefore, to supply power to the LED, a constant current power supply must be used. After the current is constant, no matter how the temperature changes, how the volt-ampere characteristics shift to the left, the current will not change! The junction temperature will not be a vicious circle!