More and more people realize that estimating the specific temperature of LED chips by measuring LED lamps covers temperature is inaccurate. However, how to get the exact temperature of LED PN junction? At present, there are three commonly used PN junction temperature measuring methods: thermal resistance method, voltage method and thermal imagine method.
Thermal imagine method requires very expensive instruments which cannot be afforded by ordinary people. However, using thermal imagine method to measure PN junction temperature cannot guarantee the accuracy of results. Thermal imagines can only display the general heat distribution. There are many other things need to be taken into account to get the accurate temperature value. Surface emissivity is the main factor. In order to measure accurate temperatures though thermal image, we need to set a surface emissivity value at first. However, in the surface of LED Street Light chip, there are several materials. Different materials have different surface emissivity values. Even if every exact surface emissivity value was known by us, which one should be chosen as the reference value? Meanwhile, if we successfully obtain the accurate temperature by thermal images, it is a surface temperature, what about the temperature of LED PN junction? No matter what kind of optimization is taken, thermal imagine method can only get an approximate value. Therefore, it is a not commonly used method to measure temperatures of PN junctions. In the next step, we are going to do some introductions about thermal resistance method and voltage method.
According to semiconductor theory, the voltage of PN junction is not only a function of temperature, but also a function of current. And the relation between junction voltage and junction current is not linear, which means voltage-temperature relations are different under in different current. Thus, if we intend to measure the temperature of LED PN junction, the working current needs to be known at first. Before measuring PN junction temperature using voltage method, we need to set a fixed constant working current for LED High Bay. Then put them into a thermostatic equipment to offer a constant working temperature. The last step is reading the measuring voltage value. Change into another temperature, repeat this operation once again. Normally, 3-5 measuring results can give out a relatively accurate relation between voltage and working temperature of LED PN junction, namely voltage-temperature factor.