All things happen, develop, and die. LED is no exceptio […]
All things happen, develop, and die. LED is no exception and has a certain life span. The early LED was just a gift like a flashlight or a table lamp. The time it took was not long and the problem of life span was not prominent. But now LED has begun to be widely used in outdoor and indoor lighting, especially high-power LED street lamps, its power, high heat, long working hours, the problem of life is very prominent. In the past, the myth that LED life must be 100,000 hours seems to have completely shattered. So where is the problem?
If you do not consider the failure of the power supply and drive, the LED's life is reflected in its light decay, that is, a long time, the brightness will be more and more dark, until the final extinguishment. The time to decay by 30% is usually defined as its lifetime. Most white LEDs are obtained by irradiating blue phosphors with blue LEDs. There are two main reasons that cause the LED to fail. One is the light failure of the blue LED itself. The blue LED's light decay is far faster than the red, yellow, and green LEDs. There is also a light decay of the phosphor, and the attenuation of the phosphor at a high temperature is very serious. Various brands of LEDs have different light decays. The 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 fades, that is, the shorter the lifetime. So the key to prolonging life is to reduce the junction temperature.
How to measure junction temperature
Junction temperature appears to be a temperature measurement problem, but the junction temperature to be measured is inside the LED. It is not always possible to measure the temperature of a PN junction with a thermometer or thermocouple. Of course, its case temperature can still be measured with a thermocouple, and according to the given thermal resistance Rjc (junction to case), its junction temperature can be calculated. However, after installing the radiator, the problem has become complicated again. Because usually the LED is soldered to an aluminum substrate and the aluminum substrate is mounted on the heat sink, if only the temperature of the heat sink housing can be measured, many values of thermal resistance must be known to calculate the junction temperature. Including Rjc (junction to case), Rcm (shelf to aluminum substrate, in fact, it should also include the thermal resistance of thin-film printed board), Rms (aluminum substrate to heat sink), Rsa (radiator to air), where there is only one Inaccurate data can affect the accuracy of the test. Figure 3 shows a schematic of each thermal resistance from LED to heat sink. This incorporates many thermal resistances, making its accuracy even more limited. That is, the accuracy of the junction temperature is estimated to be worse from the measured surface temperature of the heat sink. Fortunately, there is a method of indirectly measuring the temperature, which is to measure the voltage. As long as you know the temperature coefficient of the LED, you can easily calculate the junction temperature of the LED from the measured forward voltage of the LED.