What is junction temperature ?
A: A metric that is often listed on LED datasheets is a maximum junction temperature. The junction temperature of a diode is the temperature at the point where a diode connects to its base. There are a number of reasons why this junction temperature should be kept as low and stable as feasibly possible.
1. Lumen output decreases with increasing junction temperature, decreasing the fixtures efficiency.
2.The forward voltage of an LED can vary with junction temperature. This can be problematic for parallel strings, where strings with differing forward voltages can lead to current imbalances, and in worst case scenarios, thermal runaway.
3.The colour output of the light emitted from an LED can vary with changing junction temperature.
4.The junction temperature is also proportional to the lifetime of the diode. The lower the junction temperature, the longer the lifetime.
The three metrics affecting junction temperature are forward current, thermal resistance to ambient and ambient temperature itself. Therefore, the current that be driven through an LED while maintaining expected lifespan, light output and colour depends how much heat can be removed from the junction, which in turn depends on both ambient temperature and thermal resistance to output. Careful design is important to maximise the heat transfer to ambient.