This article was published in the September 2012 issue of LEDs Magazine.
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A light emitting diode, or LED, per the ANSI/IES RP-16 definition, is a “p-n junction semiconductor device that emits incoherent optical radiation when forward biased.” A p-n junction is formed at the boundary between a p-type and an n-type semiconductor, created in a single crystal of the semiconductor by doping or by epitaxy. Junction temperature, usually denoted as Tj, refers to the temperature at the p-n junction of a semiconductor device. Performance characteristics of LEDs, such as light output, color and lumen maintenance are a function of the junction temperature experienced by the LED during its operation. That fact means that the solid-state lighting (SSL) industry needs a standardized way to characterize junction temperature and other thermal specifications so that LEDs from different manufactures can be fairly compared and product designers can develop quality lamps and luminaires with appropriate thermal management systems.
For a given electrical power input to an LED, the junction temperature is dependent on the LED’s thermal resistance. In reality junction temperatures of the LEDs are not directly measured. Recently, measurement standards known as JESD 51-51 have been established by the Joint Electron Device Engineering Council’s (JEDEC) JC15 Committee to provide a consistent and reliable approach for the LED industry to obtain the Tj and the thermal resistance of an LED. The JEDEC is part of the Electronic Industries Association (EIA), and the JC15 Committee focuses on thermal characterization for electronic devices, including LEDs.
Thermal characterization of LEDs is a special case because unlike other semiconductor diodes, LEDs convert electric energy into energies in the form of light (optical energy) and heat (thermal energy). The electric energy to optical energy conversion efficiency is determined by the operating conditions of an LED – its junction temperature and the applied forward current.
Obviously, not all electric energy that’s input to the LED is converted to light since heat is also generated by the LED. The thermal energy dissipated at the p-n junction of the LED is determined by the difference of the total electrical energy applied and the total energy emitted as optical radiation. In other words, the total thermal energy or heat generated in the LED can be determined by subtracting the optical energy from the input power. Because the total optical energy can be measured by a photometry system, such as an integrating sphere, and total input electric energy can be determined using an electrical measurement instrument, thermal energy can then be calculated based on direct measurements of electrical and optical characteristics.
The thermal power and thermal resistance of the LED determine the junction temperature. There are several approaches to obtaining LED junction temperature. The method most widely practiced by LED manufacturers and test laboratories is the one recommended by the JESD 51-51 Standard. The standard recommends measuring LEDs with both the “heating” current, or LED operating current, as well as the “measurement” current, which is a very small reference current.
The first step is to obtain the LED’s thermal power. You begin by measuring the LED’s total optical power with the heating current and corresponding heating forward voltage after the LED has reached a steady-state. You then calculate the thermal power by subtracting the optical power from the input electric power, although the methodology requires additional steps for accuracy.
The second step is to switch from the heating current to the much smaller measurement current and quickly measure the corresponding forward voltage. If the forward voltage changes from the equivalent current level when using heating current and the optical power generated by the measurement current is ignored, then the power difference or corrected thermal power between driving the LED at heating (high) current and measurement (very low) current can be determined. This corrected thermal power value is the input electric power with heating current at steady-state, minus input electric power at the measurement current level, and minus the optical power at steady-state.
After the corrected thermal power is obtained, the LED’s thermal resistance can be determined. The JEDEC standard names the thermal resistance obtained from the measured data as “real” thermal resistance and provides the calculations for obtaining it based on measurement current, voltage and corrected thermal power. Once the thermal resistance is determined, the LED’s junction temperature can be calculated. The detailed measurement and calculation procedures are provided in JESD 51-51.
It is very important to have a consistent approach in determining LED junction temperature. In most LED manufacturers’ datasheets, LED performance characteristics are based on or referenced to the junction temperature. Because this value is not directly measured, and because it is a critical parameter when conducting LED lighting product design and testing, often LED users are concerned with how junction temperatures are determined and reported.
Furthermore, the thermal resistance of a given LED package is an important parameter in designing thermal management systems in LED lamps or luminaires. The JEDEC recommendation benefits LED users by helping them to understand measurement and calculation procedures regarding junction temperatures and LED thermal resistances, and how they should be obtained by LED manufacturers. In the process of standardizing the LED datasheets provided by LED manufacturers, it will be encouraged or specified that all LEDs’ thermal characteristics, such as thermal resistance and junction temperature, shall be obtained using the above JEDEC standard. Stay tuned for further LED standards development updates.