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# The measurement of LED junction temperature

LED thermal performance test，first should to test LED junction temperature, that is the temperature of LED chip under working status. About the LED chip temperature test, there are many ways in theory, such as infrared spectroscopy, wavelength analysis method and the method of voltage and so on. At present actually use the voltage method.

Voltage method to measure the main ideas of the LED junction temperature is: Under the specific current, LED positive pressure VF with the temperature of the LED chip become a linear relationship, so as long as test two or more temperature VF value, can determine the relationship slope between voltage and temperature of LED, coefficient K value, the unit is the mV/°C. K values can be obtained by formula K =?Vf /?Tj. K value can be measured by real-time Vf value, calculate the temperature of the chip (junction temperature) Tj. In order to reduce voltage measurement error, when coefficient K is greater than the standard measurement, two temperature point temperature difference should be greater than or equal to 50 degrees. With voltage method to measure the instrument of junction temperature has a few basic requirements:

A. The basis of voltage method to measure the junction temperature is under specific current measurement, and the LED chip voltage change caused by the temperature change is millivolt level, so require testing instrument for voltage measurement must be high enough, stability continuous measurement of the volatility should be less than 1 mv.

B. The test current should be small enough so as not to cause the chip temperature change in the process of testing, but too small will cause the voltage measurement is not stable, some LEDs exsit turn fluid effect will affect the stability of the Vf test, so require the test current no less than the current value of inflection point position of the IV curve.

C. Due to test the LED junction temperature is under the working conditions, from working current (or heating current) to test process must be fast enough and stable, Vf test time must be short enough to ensure that the testing process can’t be cause junction temperature to drop. Currently used in the LED thermal performance test equipment is reference to the requirements of the EIA/JESD51 standard design. The imported LED heat performance test equipment price expensive, using complex, at present inland only a few units equipped with imported equipment. Compared to inland equipment and imported equipment, comprehensive technical indicators have certain gap, but due to the progress of technology, most of the indicators have been able to fully meet the testing requirements. In terms of price of domestic equipment has a great competitive advantage, the design requirements is mainly for LED test, also easy to use, in favor of the widespread use of LED thermal performance test.

Thus, Lisun developed JDS200 light electric integrated test system, used to test the LED light parameters and color parameters, electrical parameters and thermal parameters, but also can record the curve of each parameter changing with time.