In recent years, due to the increase in power efficiency and price reduction of power LEDs, the number of LEDs used in the field of lighting has grown rapidly, from a variety of landscape lighting, outdoor lighting to ordinary home lighting, the application is increasingly widespread. LED is used in lighting in addition to energy saving, long life is also a very important advantage. At present, LEDs and their lamps cannot achieve the ideal service life due to the thermal performance of LEDs; the junction temperature of LEDs in operation is directly related to their lifetime and efficacy; the thermal resistance directly affects the junction temperature of LEDs under the same conditions of use; Whether the design of the thermal conductivity system of the LED luminaire is reasonable also directly affects the life of the luminaire. Therefore, the thermal performance test of power LEDs and their lamps has a direct meaning for the production and application of LEDs. The following is a brief description of the main thermal performance indicators of LEDs and their lamps, the test principle of voltage temperature coefficient K, junction temperature and thermal resistance, test equipment, test content and test methods for LED R&D, production and application companies. First, the principle of measuring the junction temperature of LED by voltage method The LED thermal performance test first tests the junction temperature of the LED, ie the temperature of the LED chip under working conditions. Regarding the test of the temperature of the LED chip, there are various methods in theory, such as infrared spectroscopy, wavelength analysis, and voltage method. Currently, the voltage method is actually used. The standard issued by the Electronic Industry Federation/Electronic Engineering Design and Development Joint Conference in December 1995 is a detailed specification of the principles, methods, and requirements for voltage measurement of semiconductor junction temperatures. The main idea of ​​the voltage method to measure the junction temperature of the LED is that the forward voltage drop Vf of the LED at a specific current is linear with the temperature of the LED chip, so as long as the Vf value of two or more temperature points is tested, the LED voltage can be determined. The slope of the temperature relationship, that is, the temperature coefficient of voltage K, in mV/°C. The K value can be obtained from the formula K = ⊿Vf / ⊿Tj. With the K value, the temperature (junction temperature) Tj of the chip can be calculated by measuring the real-time Vf value. In order to reduce the error caused by the voltage measurement, > the standard specifies the measurement coefficient K, the temperature difference between the two temperature points should be greater than or equal to 50 degrees. There are several basic requirements for an instrument that measures the junction temperature by voltage: A. The voltage method measures the junction temperature based on the Vf measurement at a specific test current, and the voltage variation of the LED chip due to temperature changes is millivolts. Therefore, it is required that the stability of the voltage measurement of the test instrument must be sufficiently high, and the fluctuation of the continuous measurement should be less than 1 mV. B, this test current must be small enough to avoid the chip temperature change during the test; but too small will cause unstable voltage measurement, some LEDs have a åŒ fluid effect will affect the stability of the Vf test, so the test current is required to be not less than IV The current value at the inflection point of the curve.
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