Light Emitting Diode or LED has begun to be applied as light source replacing other source light such as incandescent or fluorescent lamp. LED has many advantages such as high efficiency, shock resistance, and long life time. However, LED performance depends on the temperature thus thermal management in LED technology is very important.
Talking about thermal management, the important parameter in LED technology is Junction temperature, but the problem is how to measure it. LED junction is very small and inside the encapsulation, therefore thermocouple or other temperature sensor couldn’t be used to measure it.
There are several methods to measure LED junction temperature, those are forward voltage and shift wavelength method. Both of them based on the dependence temperature, it means the difference temperature causes change of forward voltage and wavelength. LED junction measurement by these methods consists of calibration and actual measurement.
In calibration measurement, LED (including Junction) is heated until certain temperature then injected by pulse current with very low duty cycle. LED junction temperature is assumed same with that certain temperature because heat dissipated from LED is very small due to low duty cycle. Then forward voltage or wavelength is measured. Next, this measurement is repeated at different temperature. By this measurement the relation between forward voltage/wavelength and temperature is got and can be used as reference in actual measurement
In actual measurement, LED is operated in normal condition (in ambient temperature) and current (constant). Junction temperature can be determined by measuring the forward voltage/Wave length. Then data from calibration measurement is used as reference to determined junction temperature.
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