LED thermal characteristics and lifetime detection technology

1 Overview

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.

2 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:

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

1) First, apply a test current IM to the LED, and measure the forward junction voltage VFI;
2) Replace the IM with the heating current IH to the two ends of the LED to be tested, and heat it for a certain period of time (tH) until the LED reaches a steady state.

Heat dissipation power (PH) on the LED heat dissipation channel;
3) Re-use IM to quickly replace IH to both ends of the LED to be tested, and measure the forward junction voltage drop (VFF);
4) Calculate the junction temperature and thermal resistance of the LED:

In the formula (3), the initial temperature, in the formula (4), is the temperature of a specified point on the heat dissipation channel, for example, the ambient temperature or the case temperature. For LEDs, part of the input electrical power is used for LED illumination, and the other part generates heat, and the heat dissipation power PH is often difficult to distinguish from the total input electric power, so for convenience and simplified measurement, QB/T 4057-2010 proposes " Refer to the concept of thermal resistance, which uses the total input power instead of the heat dissipation power in equation (4). The “reference thermal resistance” has been used more and more because of its convenient measurement and good reproducibility.

Under the action of heating current IH, it is also meaningful to monitor the junction temperature rise of the LED and obtain the heating curve. Figure 2 shows a typical heating curve, which can not only determine whether the LED reaches thermal equilibrium, but also the structure and heat dissipation capability of the LED. Analysis has a reference role.

The technical difficulty in obtaining the heating curve is that the test current and heating current switching time must be short enough, and the transient data must be collected very quickly. The switching time and data acquisition time generally reach several to ten microseconds. Otherwise, the actual change process of the LED junction temperature cannot be reflected, resulting in the thermal resistance of the final test being greatly underestimated.

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