Transient thermal test: focus on power LED thermal design

[Source: Gaogong LED's "LED Good Products" magazine February issue / Long Zonghui] As we all know, the increase of LED operating current will generate a lot of heat, causing significant changes in the LED chip PN junction temperature, resulting in LED performance. The important influences are the change of forward pressure drop, color temperature change, wavelength red shift, and low photoelectric conversion efficiency, which affect its luminosity, chromaticity and electrical parameters.

The thermal resistance of LED is an important parameter to determine the photoelectric characteristics and life of LED. The thermal resistance is too high, which directly leads to high junction temperature of LED and accelerates aging of chip.

The junction temperature and thermal resistance are the most important indicators to measure the thermal characteristics of LEDs. Therefore, fast and accurate testing of the junction temperature and thermal resistance of LEDs is an important prerequisite for effective thermal management.

At present, the main methods of LED thermal testing are infrared thermal imaging, electrical parameter method, transient thermal test method, spectrum and optical power method, and pin temperature method. These methods measure or derive the junction temperature and thermal resistance of the LED based on different principles.

Mentor Graphics' accurate transient thermal testing can help R&D personnel speed up R&D, reduce R&D investment, and timely and accurately determine problems in previous R&D, and then make targeted improvements.

T3Ster (pronounced "Trister") is a semiconductor device package thermal test instrument that provides thermal data for a wide range of packages in minutes, designed for semiconductor, electronics and LED industries, as well as R&D lab applications.

The system includes an easy-to-use software and hardware component. T3Ster is used to measure the transient thermal characteristics of packaged semiconductor devices and other electronic devices. The measured devices include discrete or integrated bipolar transistors, MOS transistors, common triodes, and LEDs. Packages and thyristors, devices of various package types and some components of MEMS. It also tests the thermal characteristics of PWB, MCPCB and other substrates, thermal interface materials or cooling components due to its specialized equipment and software.

“Because LED power is getting higher and higher, we should pay more attention to its thermal management, which is very important to ensure LED performance and life. This is why OSRAM gives great attention to thermal design. T3Ster is accurate and repeatable. Sex enables us to validate our thermal design and confirm the stability and reliability of our products. Through a lot of testing, our confidence in the measurement results continues to increase. The built-in structure function of T3Ster software is very powerful, in our large number of reliability tests, Identify the thermal issues of the different layers.” This is the evaluation of T3Ster by Dr. Thomas Zahner, Quality Manager of OSRAM Opto Semiconductors Germany.

According to reports, in the LED trial production stage, T3Ster transient thermal tester can evaluate the packaging level of LED products through transient thermal test, can detect the defects of LED product packaging, and after the reliability test of LED products, the package The level of assessment is carried out.

In addition, excellent thermal management also comes from a good thermal design.

Generally, in the design of LED luminaires, component designers use thermal analysis software and test instruments to analyze the material and structure of components to ensure that the junction temperature can be easily dissipated through the LED component layer.

The subsystem designer will arrange the LED components into an array, add heat sinks and other cooling devices, and then analyze the product again. They may adjust the separation distance between LED components or add additional cooling to ensure that the LEDs do not exceed the critical temperature.

The final step is usually done by a mechanical design engineer using a mechanical calculation aided design (MCAD) system that measures the LEDs and uses advanced computational fluid dynamics (CFD) software for thermal analysis.

Nevertheless, in the design of LED luminaires, the heat loss deviation in the component input model may still reach 20%, the material property deviation may reach 10%, and the error similar to the double thermal resistance model can reach 20%, because the components are solved. The thermal management problem does not mean that the thermal management of the subsystem is also solved.

At present, the process of thermal analysis only depends on the analysis of the product and the company's design process. There are no strict standards to follow, so LED luminaire designers are likely to be biased during the design process.

The T3Ster Transient Thermal Tester measures the thermal characteristics of LED devices, resulting in transient thermal impedance, steady-state thermal resistance, junction temperature, and thermal and thermal characteristics of the layers of the material in the heat flow path. , presented in a very detailed way through the structure function.

"T3Ster transient thermal tester can not only perform real-time transient thermal test on LED lights, but also test LED bulbs, T8 tubes and other commercial lighting LED lamps," said Wang Gang, senior engineer of Mingdao International MAD.

Unfinished: For more information, please check the February issue of Gaogong LED's "LED Good Products" magazine.

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