Analysis of the Causes Affecting LED Light Decay

Light fade is a common problem in the LED industry. I have been thinking about how to intuitively tell you the relationship between light decay and LED lamp life. Finally found a better way, is to look at the picture to speak. The way to reduce light decay in advance is to dissipate heat, heat, and heat... important things to say a few times.

Here is the light decay curve of Cree:

It can be seen from the figure that the light decay of the LED is related to its junction temperature. The so-called junction temperature is the temperature of the semiconductor PN junction. The higher the junction temperature, the earlier the light decay, that is, the shorter the lifetime. As can be seen from the figure, if the junction temperature is 105 degrees, the lifetime of brightness reduction to 70% is only 10,000 hours, there are 20,000 hours at 95 degrees, and the junction temperature is reduced to 75 degrees, and the life expectancy is 50,000 hours. It can be extended to 90,000 hours at 65 degrees. So the key to extending life is to lower the junction temperature, but these data are only suitable for Cree's LEDs, not for other companies' LEDs.

How to extend the life of LEDs

Therefore, when we buy LED lamps (all of which do not specifically refer to LED spotlights), we must look at its heat dissipation design.

It can be concluded from the figure that the key to extending its life is to lower its junction temperature. The key to lowering the junction temperature is to have a good heat sink that can dissipate the heat generated by the LED in time.

Here we are not going to discuss how to design a heat sink, but to discuss which heat sink has a relatively good heat dissipation effect. In fact, this is a junction temperature measurement problem. If we can measure the junction temperature that any heat sink can achieve, we can not only compare the heat dissipation effects of various heat sinks, but also know that after using this heat sink. The LED life that can be achieved.

How to measure junction temperature

The junction temperature appears to be a temperature measurement problem, but the junction temperature to be measured is inside the LED. It is not possible to put a thermometer or thermocouple into the PN junction to measure its temperature. Of course, its case temperature can still be measured with a thermocouple, and then based on the given thermal resistance Rjc (junction to the case), its junction temperature can be derived.

But after installing the heatsink, the problem became complicated again. Since the LED is usually soldered to an aluminum substrate, and the aluminum substrate is mounted on the heat sink, if only the temperature of the heat sink case can be measured, it is necessary to know the value of many thermal resistances in order to calculate the junction temperature. Including Rjc (junction to case), Rcm (shell to aluminum substrate, which should also include the thermal resistance of the film printed version), Rms (aluminum substrate to heat sink), Rsa (heat sink to air), as long as there is a Inaccurate data can affect the accuracy of the test.

The figure below shows a schematic diagram of the individual thermal resistance of the LED to the heat sink. It incorporates a lot of thermal resistance, making its accuracy more limited. That is to say, it is even worse to estimate the junction temperature from the measured surface temperature of the radiator.

Schematic diagram of the thermal resistance of the LED to the heat sink

Fortunately, there is a way to measure temperature indirectly, that is, to measure the voltage. So which voltage is the junction temperature related to? What is the relationship like? We must first start with the volt-ampere characteristics of LEDs.