In the LED industry, light failure is the most concerned topic. The light failure of LED is related to its junction temperature. The so-called junction temperature is the temperature of semiconductor PN junction. The higher the junction temperature, the earlier the light failure occurs, that is, the shorter the life. If the junction temperature is 105 ° C, the lifetime of 70% brightness is only 10000 hours, and that of 95 ° C is 20000 hours. If the junction temperature is 75 ° C, the lifetime is 50000 hours, and that of 65 ° C can be extended to 90000 hours. Therefore, the key to extending the lifetime is to reduce the junction temperature.

How to prolong the life of LED: The key to prolong its life is to reduce its junction temperature. The key to reducing junction temperature is to have a good radiator, which can timely emit the heat generated by LED. If we can measure the junction temperature that any kind of radiator can reach, we can not only compare the heat dissipation effect of various radiators, but also know the LED life that can be achieved after using this kind of radiator.

How to measure junction temperature: junction temperature seems to be a temperature measurement problem, but the junction temperature to be measured is inside the LED, and you can never put a thermometer or thermocouple into the PN junction to measure its temperature. Of course, its shell temperature can be measured by thermocouple, and then its junction temperature can be calculated according to the given thermal resistance Rjc (junction to shell).

But after the radiator is installed, the problem becomes complicated again. Because LED is usually welded to the aluminum substrate, and the aluminum substrate is installed on the radiator. If only the temperature of the radiator shell can be measured, many thermal resistance values must be known to calculate the junction temperature. It includes Rjc (junction to shell), Rcm (shell to aluminum substrate, in fact, it should also include the thermal resistance of film printed plate), Rms (aluminum substrate to radiator), Rsa (radiator to air). Any inaccuracy of one data will affect the accuracy of the test.

Temperature coefficient of LED volt ampere characteristic: We know that LED is a semiconductor diode, which has a volt ampere characteristic like all diodes, and also has a temperature characteristic like all semiconductor diodes. Its characteristic is that when the temperature rises, the volt ampere characteristic shifts to the left. Assume that the LED is powered by Io constant current. When the junction temperature is T1, the voltage is V1. When the junction temperature rises to T2, the entire volt ampere characteristic shifts to the left, the current Io remains unchanged, and the voltage becomes V2.

These two voltage differences can be removed by temperature to obtain the temperature coefficient, which is expressed in mV/oC. For ordinary silicon diodes, this temperature coefficient is about - 2mV/oC. But most LEDs are not made of silicon, so its temperature coefficient should be measured separately. The LED manufacturer's data sheet mostly gives its temperature coefficient.