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How to solve the light decay problem of LED lamps?
Feb 18,2019
After LED lamps have been used for a period of time, we will find that their light is not as bright as it was at the beginning. This is because the lifespan of the LED lamps is coming to an end, which leads to the phenomenon of light decay in LED lamps. So, what causes the light decay in LED lamps? How can we solve the light decay problem of LED lamps?
What is light decay? Currently, there is no defined standard for light decay in our country, but generally speaking, light decay refers to the reduction in light intensity of LED lamps after being lit for a period of time, which cannot be restored. The reduced part is considered the light decay of the LED lamp. (Different types of lamps have different causes for light decay.)
In addition, the U.S. Energy Star LM-80 specifies that after 6000 hours, a luminous flux maintenance rate of ≥94.1% is considered a failure.
Causes of light decay in LED lamp light sources.
There is still considerable debate regarding the causes of light decay, and the microscopic mechanisms causing the decline are not yet conclusive. However, generally speaking, the light decay of LEDs is mainly due to heat dissipation issues.
It is well known in the industry that LEDs are sensitive to heat. The ideal operating temperature for LEDs is between -5° and 0°, but this is basically impossible. Heat affects the light decay and lifespan of LED lamps. When LEDs operate, about 80% of the electrical energy is converted into light energy. LED heat sinks are used for LED cooling because the ambient temperature of the LED chip is inversely proportional to the light output rate. The higher the temperature, the lower the light output rate. When the temperature reaches the maximum operating temperature of the LED chip, the lamp will fail. In addition, the thermal resistance of the LED chip itself, the influence of silver glue, the heat dissipation effect of the substrate, as well as the colloid and gold wire aspects are also related to light decay.
How to solve light decay in LED lamp light sources.
In fact, strictly speaking, light decay in LED lamps is unavoidable. It is a technical problem that the industry is collectively concerned about and urgently needs to solve. However, as long as we understand the heat dissipation methods of the lamps, we can significantly reduce the light decay problem.
However, before discussing the heat dissipation of LED lamps, we must first understand junction temperature. What is junction temperature? Junction temperature refers to the operating temperature of the PN junction of the semiconductor chip (wafer, bare chip). The higher the junction temperature, the earlier light decay will occur.
If the junction temperature is 105°, the lifespan at 70% brightness is only over 10,000 hours. At 95°, it is 20,000 hours, while reducing the junction temperature to 75° extends the lifespan to 50,000 hours, and at 65°, it can be extended to 90,000 hours. Therefore, the key to extending lifespan is to lower the junction temperature, and the key to lowering junction temperature is to have a good heat sink. So how can we systematically identify the quality of LED lamp heat dissipation?
Generally, as the junction temperature of LEDs increases, the luminous flux will decrease. Therefore, we can measure the change in illuminance at the same position of the lamp to infer the change in junction temperature. The specific method is as follows:
1. Choose a location that is not affected by external light, preferably at night, and turn off other lights.
2. Turn on the light in a cold state and immediately measure the illuminance at one position. The reading at this time is called "cold state illuminance."
3. Keep the position of the lamp and the illuminance meter unchanged, and let the lamp continue to operate.
4. After half an hour, read the illuminance value at this location again, and record the reading as "hot state illuminance."
5. If the two values are similar (10-15%), then the heat dissipation system of this lamp is basically good.
6. If the two values differ significantly (greater than 20%), then the heat dissipation system of this lamp is questionable. The "half-hour illuminance method" provides a simple measurement of junction temperature changes.
In addition, we cannot simply look at the cost to determine the quality of the heat sink. We should choose the one with the highest cost-performance ratio, rather than using the best for everything. Therefore, my summary is:
1. If the heat sink is very hot to the touch, it is definitely not good. If the heat sink is not hot to the touch, it does not necessarily mean it is good.
2. Under reasonable design, for the same weight, the one with a larger surface area is the better heat sink.
3. For the same surface area, the heavier one is the better heat sink. (Provided they are of the same material.)
4. The fins of a finned heat sink are not necessarily better if there are more of them; denser fins are better.
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