Hot Lumens of Street Lighting LEDs
How physical testing and CFD analysis helped develop new LED-based street lighting luminaires
By Andras Poppe, Product Manager, Mentor Graphics
Nowadays in most lighting applications, Light-Emitting Diodes (LEDs) are by far the most efficient light source, since they turn supplied electrical energy into useful light, producing much less energy loss in the form of heat than any other conventional light source. Compact fluorescent light bulbs were designed to take over the role of incandescent bulbs but in recent days white LEDs, with about 40% energy conversion efficiency, are the favored choice to replace old bulbs.
Having said this, even the small amount of heat generated by LEDs can cause problems if not properly conducted away and dissipated to the environment. A significant temperature elevation of the LED chip occurs, resulting in reduced light output and a shortening of shelf life. Therefore proper cooling of LEDs is key to maintaining the high lumen output and long life of these light sources.
In applications such as the headlights of cars or street lighting, where safety is paramount, lighting standards are very strict. In addition to the prescribed spatial distribution patterns that are required, illumination levels also need to be provided consistently; for example, even on hot summer nights, luminous flux of LED-based luminaires must meet the strict lighting standards. This necessitates having the appropriate knowledge about the thermal and light-output properties of LEDs.
With this knowledge available, solid-state lighting (SSL) designers who consider thermal properties in their LED based design are more likely to produce luminaires with long-term consistent light output that have a longer operating life.
The latest testing standards
From a semiconductor standpoint, LEDs are simple pn-junctions, so it would seem that they should be easier to measure, when in actuality they are not. Light emission must be considered when measuring the LED’s thermal resistance, as a significant proportion (30 - 40%) of the supplied energy is converted into light.
Based on these efficiency figures, if the supplied electrical power rather than the correct (heating) power is used to calculate the package’s thermal resistance, the thermal resistance value would be significantly lower, suggesting that the package (of a less efficient LED) would be far better at dissipating the heat generated in the LED than it actually is. Therefore JESD51-51, one of a series of the latest LED thermal testing standards, requires that consideration be made to the emitted optical power when measuring LEDs’ thermal resistances. The emitted optical power along with other light output characteristics of LED (like luminous flux, color, color temperature) can be precisely measured in a CIE 127-2007-compliant total-flux measurement environment. Using the optical power figure obtained this way allows for the calculation of the real thermal resistance if thermal testing of the LED in question is performed in a combined thermal and radiometric/photometric test setup such as that of the Mentor Graphics T3Ster® Transient Thermal Characterization and the TeraLED® total flux measurement solution.
In the TeraLED system the temperature of the LED under test can be precisely set to a desired value by a temperature-controlled cold plate. Such a measurement setup is also suggested by one of the recent LED thermal testing standards, JESD51-52, which provides guidelines on methods to measure LED light output in connection with LED thermal measurements.
As for the thermal characteristics of LED components, the junction-to-case resistance is the most appropriate metric for packaged LEDs. This is because it characterizes the heat flow path from the point of heat generation at the pn-junction down to the bottom of the case — exactly how LED packages are designed to be cooled. A relatively new standard, JEDEC JESD51-14, for junction-to-case thermal resistance measurement, is based on the latest thermal-transient measurement techniques.
This method uses a dual-interface approach in which the thermal resistance of the part is measured against a cold plate with and without thermal grease. The junction-to-case resistance is determined by examining where the two measurements differ. Very high measurement repeatability is required because the thermal impedance curves for the two measurements must be identical up to the point where the heat starts to leave the package and enters the thermal interface between the package and the cold plate. This ensures that the point where the curves deviate is clear. This method, combined with the LED thermal testing standards provides the real junction-to-case thermal resistance for LED packages.
Comprehensive solutions for LED characterization
The Mentor Graphics T3ster Transient Thermal tester uses a smart implementation of the static test version of the JEDEC JESD51-1 electrical test method that allows for continuous measurement junction temperature transients. , This also forms the basis of the JESD51-14 test method for the junction-to-case thermal resistance measurements which is also the preferred test method in the LED-specific thermal measurement guidelines provided in the JESD51-51 standard. The combination of T3Ster and TeraLED systmes provide a comprehensive solution for LED testing which meets the requirements of all the mentioned standards. See Figure 1.
The T3Ster Master Program is post-processing software that fully supports the JESD51-14 standard for junction-to-case thermal resistance measurement, allowing the temperature versus time curve obtained directly from the measurement to be re-cast as “structure functions” (described in JESD51-14 Annex A ), and then automatically determine the junction-to-case thermal resistance value.
Because the JESD51-14 methodology yields the junction-to-case thermal resistance, as a “side product”, the step-wise approximation of the structure function up to this thermal resistance value provides the dynamic compact thermal model of the LED package automatically. The identified junction-to-case thermal resistance values may be published on the product datasheet, and the automatically generated dynamic compact thermal model of the LED package can be applied directly in CFD analysis software such as Mentor Graphics FloTHERM® and FloEFD™ tools as shown in Figure 1.
The combination of the light output measurement (performed with equipment such as TeraLED), and thermal transient testing allows measurement of the light-output characteristics as a function of the temperature. Providing this data as a function of the reference temperature of the cold-plate is useful information for SSL designers to correlate light output to temperatures of test points of the luminaires they develop. But the same data is also available as a function of the LEDs’ junction temperature, which is required for the correct physical modeling of the light output of LEDs. In other words, the obtained luminous flux – junction temperature relationship is the basis of hot lumen calculations.
The latest quantum leap in test based modeling of LEDs’ characteristics was realized recently by the introduction of the hot lumen modeling feature in the LED module of the FloEFD simulation tool. In the usual forward current and junction temperature operating ranges of today’s high power LEDs, when the LEDs are driven by a constant forward current the temperature dependence of the luminous flux can be well modeled by a linear relationship in the temperature range of interest. The parameters of the required formula are automatically calculated by the TeraLED View program, the results post processing software of the TeraLED system.
The thermal model and the light output model together form a multi-domain LED model. Such models collected in libraries are handled by FloEFD. When a LED luminaire is designed, the LEDs themselves can be referred to from this library. This way, “placing” such virtual LED components taken from the model library into the MCAD design of a luminaire, FloEFD delivers the first hot lumen estimates along with key thermal data about the effectiveness of cooling. In other words, the hot lumens of the cool LEDs are obtained early in the design phase.
Success in application
This component level physical characterization and system level simulation flow was successfully applied in the KÖZLED project  in Hungary. The project targeted the development of LED based street lighting luminaires. T3Ster and TeraLED measurements were used to verify LED data sheet information. Measurement results were also turned into FloEFD LED models, so FloEFD simulations could be used to prove quality of the design of the largest member of an LED based street lighting luminaire family.None of the applied LEDs exceeded the critical value of junction temperature and the total output lumens of the LEDs reached the required level even under the hottest foreseen environmental conditions. Figures 2 and 3 show FloEFD simulation results for the largest luminaire designed in the project.
The success of the integrated T3Ster TeraLED FloEFD solution from Mentor Graphics is proved by the success of the KÖZLED project. The developed LED luminaire family can be found in the recently renovated streets of Budapest and the developed LED street ligting luminaire family received the “Product of the Year 2012” by Industorg award in Hungary .
- JEDEC Standard JESD51-14, "Transient Dual Interface Test Method for the Measurement of Thermal Resistance Junction-to-Case of Semiconductor Devices with Heat Flow through a Single Path"
- MCAD model by courtesy of Optimal Optik Ltd, Hungary, physical characterization by Budapest University of Technology and Economics, Hungary.