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Significance of LED thermal characterisation

Posted: 26 Feb 2014  Print Version  Bookmark and Share

Keywords:LED  computational fluid dynamics  thermal transient tester  CFD  Mentor Graphics 

Based on a June 2012 report from The Climate Group, many commercially available, outdoor light-emitting diode (LED) products offer high-quality light, durability, and significant electricity savings in the range of 50 to 70%. More European cities are adopting LED streetlights because LEDs can save energy costs, while exceeding local lighting standards. They last 50,000 to 100,000 hours, changing little in colour, and have a failure rate of around 1%, compared, for example, up to 10% for ceramic metal halide fixtures over a similar time period. LED streetlights are a "gateway" technology- when LED designers solve the current problems of reducing cost and thermal challenges, they'll be paving the way for wider adoption and the energy-saving potential of LEDs.

Thermal management is one of the more complex areas of LED system design. And until a couple of years ago, the methods and technology to scientifically characterise the thermal behaviour of the component, as well as its systems and sub-systems, was not available. Instead, most engineers calculate their thermal needs from data sheets published by component manufacturers. Understandably, having data available to engineers on the specific thermal mechanics of LED-based devices within the system in which they are being used could be a huge step forward for future lighting designs.

This article describes a method that combines hardware measurement (a thermal transient tester), and computational fluid dynamics (CFD) software to provide high measurement throughput, which enables systems integrators to verify a vendor's thermal resistance data during design and to test incoming commercial off-the-shelf parts before they are introduced into production. This data can be used during the design and product development phase to accurately capture the thermal response of an LED lighting system.

Figure 1: Example of measured junction temperature transient of a device.

Comprehensive LED characterisation
The first step is to measure the LEDs that are generally suitable for the lighting application and to evaluate them by thermal and radiometric characterisation. The LED must be measured as it transitions from a hot to a cold state of operation to be able to thermally characterise it using the electrical test method. The results of such measurements are LED package thermal metrics and descriptive functions that will help design engineers understand the structure. The proper thermal design of the cooling solution can be created when the latest JEDEC LED thermal testing standards [1, 2] are used in this approach to identify the real thermal resistance and the real thermal impedance of the LED package. Also, not only the radiant power is measured and used in the thermal resistance-impedance calculations, but the temperature dependence of other light output properties such as luminous flux or colour coordinates can also be measured. This way the best suitable LED from various LEDs of different vendors can be selected for the design of a particular lighting application.

These LED testing standards are fully implemented by the Mentor Graphics T3Ster and TeraLED measurement systems, providing comprehensive LED characterisation, including thermal transient measurements and measuring almost all light output properties of LEDs. Figure 1 shows an example of an LED junction temperature transient measured on a cold plate—as the JESD 51-51 and JESD 51-52 standards recommend.

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