LED thermal characteristics and lifetime detection technology

In recent years, LED lighting technology has developed rapidly. At the same time, LED's light color, color temperature, color rendering and other light color indicators have attracted much attention. At the same time, the thermal characteristics and life of LED have been paid more and more attention, especially thermal characteristics. It has a significant impact on the performance and lifetime of LED light color. However, the detection of thermal properties and lifetime is challenging.

The thermal characteristics of LEDs mainly include LED junction temperature, thermal resistance, transient curve (heating curve, cooling curve) and so on. The junction temperature refers to the PN junction temperature of the LED. The thermal resistance refers to the ratio of the temperature difference between the LED heat dissipation channel and the dissipation power on the channel. It is used to characterize the heat dissipation capability of the LED. Research shows that the lower the thermal resistance of the LED The better the heat dissipation performance, the higher the corresponding LED light efficiency and the longer the life. The key to the detection of thermal characteristics is the accurate measurement of the junction temperature of the LED. The existing test for the junction temperature of the LED generally has two methods: one is to measure the temperature of the surface of the LED chip by infrared temperature measurement and regard it as an LED. Junction temperature, but the accuracy is not enough; the other is to obtain the PN junction temperature by temperature-sensitive parameter (TSP), which is the most common LED junction temperature test method. The technical difficulty lies in the test equipment. The requirements are higher.

The life of an LED is mainly manifested by its light decay. The LED light output is usually attenuated to 70% or 50% of the initial light output as an indicator of life failure, that is, the luminous flux maintains its lifetime. However, since LEDs are highly reliable devices, the lifetime generally exceeds several thousand hours or even more than 10,000 hours. It is very difficult to directly measure the waiting for light decay to a specified value in industrial applications.

1 LED thermal characteristics test

2.1 LED junction temperature and thermal resistance measurement

US EIA/JESD51 "Methodology for the Thermal Measurement of Component Packages" series of standards and national standards SJ20788-2000 "Semiconductor diode thermal impedance test method", GB/T4023-1997 "Semiconductor device discrete devices and integrated current Part 2: rectifier diode 》, QB/T 4057-2010 "Performance Requirements for LEDs for General Lighting" and other international and domestic standards have introduced the method of measuring junction temperature and thermal resistance through temperature sensitive parameter TSP. For LEDs, TSP is the forward voltage across the PN junction. Under the determined current, the forward bias of the LED is approximately inversely proportional to the junction temperature, and the change in junction temperature is obtained as follows:

In equation (2), VL is the forward voltage of the LED under the measurement current IM (small current) when the low junction temperature TL (such as 25 ° C), and the LED is measuring the current IM when the VH is the high junction temperature TH (such as 100 ° C) The forward voltage underneath.

The timing of the LED junction temperature measurement is shown in Figure 1:

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