A tremendous change in the worldwide lighting industry occurred after the development of blue and white light emitting diodes (LEDs), thanks to a drastic drop in costs. Solid-state lighting, especially high power/brightness LEDs, with increasing energy conversion efficiency are now competing with traditional light sources. This development trend of LEDs (also known as Haitz’s law), which is measured in lumens per package, is exponential -- the emitted luminous flux per package doubles every year.
The continuous price decrease measured in dollar/lumen (cost of generating 1 lumen of luminous flux) dropped by an order of magnitude in 10 years. This has facilitated application of LEDs in the automotive industry, consumer electronics, and indoor and outdoor lighting applications such as street lighting.
Figure 1. Example of measured junction temperature transient of a device.
Figure 2. Example of luminous flux vs. junction temperature at different current settings.
Electrical, thermal, and optical characteristics of LEDs influence one another. The most often discussed aspect is how temperature influences an LED’s light output -- energy conversion efficiency drops with increasing temperature. Lighting designers refer to this by estimating the “hot lumens” of LEDs at actual operating temperatures, using information provided on vendors’ LED datasheets.
The LEDs’ light output depends on operating conditions that are critical for proper illumination and color, key factors in the success and failure of an LED device manufacturer. Current and temperature variances determine the LED's efficiency, thus thermal analysis and physical test are important approaches to designing better products. Engineers can use combined thermal and radiometric measurements to accurately test their products for improved performance and reliability, and these results can be quickly and easily converted into compact models to use in computational fluid dynamics (CFD) for simulating not only the thermal behavior but also to predict light output properties of LEDs under different operating conditions.
Measuring and evaluating LED performance
The first step is to measure the LEDs that are generally suitable for the lighting application and to evaluate them by thermal and radiometric characterization. The LED must be measured as it transitions from a hot to a cold state of operation to be able to thermally characterize it using the so-called 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 are used in this approach to identify the real thermal resistance and the real thermal impedance of the LED package. Also, not only is radiant power measured and used in the thermal resistance-impedance calculations, but the temperature dependence of other light output properties such as luminous flux or color coordinates can be measured. Hence, 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 characterization, 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 recommend by the JESD 51-51 and JESD 51-52 standards.
The measured junction temperature transient is turned into thermal impedance if it’s divided by the applied heating power. In the case of LEDs, this is the supplied electrical power (forward voltage x forward current) less the emitted optical power, also known as total radiant flux. The LED under test must be characterized optically to account for this.