How To... Get Hot Lumens in situ from FloEFD’s unique LED Module
By Joe Proulx, Application Engineer, Mentor Graphics
Just occasionally a software product or module comes along that stimulates interest in a new technical area and can even prove to be a game changer. The LED Module for the 3D CFD product, FloEFD, is looking like such a capability.
It emerged two years ago from two separate technical strands within the Mechanical Analysis Division; the MicReD T3Ster & TeraLED hardware for thermal and optical/radiometric characterization of LED lights, along with the 3D CFD FloEFD software that permits accurate thermal simulation of LEDs within complex geometries easily.
The big problem with simulating LEDs in a traditional CFD approach, however, is that as temperature increases inside an LED, its forward voltage changes leading to heat dissipation changes and ultimately the quality of its light output changing. Hence, modeling an LED as a simple heat source is just not accurate enough. This can be a bit like a dog chasing its tail! Figure 2 illustrates this complex interconnectivity beautifully.
So how does the LED module help solve this problem? Let’s consider that we have an LED geometry (Figure 3) with a datasheet of manufacturer’s properties (boundary conditions) and we have access to Mentor’s tried and proven combination of T3Ster & TeraLED to characterize the LED (using JEDEC JESD51-1 and CIE 127-2007 compliant techniques). We could measure the LED under a range of conditions to produce a set of data for the FloEFD LED Module. The LED geometry itself could be input into FloEFD via your favorite MCAD tool (FloEFD is embedded within most of the popular CAD tools available today) and the luminaire it is attached to can also be added so that the LED is modeled in situ.
T3Ster/TeraLED measurements will yield for a given LED, the current and temperature dependence by way of R-C Structure Functions, Diode Characteristics, Optical Power (mW), Radiant Efficiency (Popt/Pelect), Luminuous Flux (lm), Efficacy (lm/W), Scotopic Flux (lm), and Optical Color Coordinates (X,Y,Z tristimulus values). As a result of the combined thermal/optical measurement, this data can be displayed based on the LED junction temperature.
This process is illustrated in Figure 4, where the Temperature Sensitive Parameter (TSP), i.e. the diode forward voltage in the case of an LED, is measured to calibrate the LED. It is then powered up to steady state, allowing the temperature, light output, and power draw to stabilize. Measurement is made by suddenly powering down the LED to a very small measurement current that is used to record the TSP as the part cools. The T3Ster software is used to convert the temperature vs. time response into a cumulative thermal structure function – a graph of thermal capacitance vs. thermal resistance to reveal the thermal structure of the LED package.
FloEFD’s LED Module has a standard, premeasured set of typical LEDs in its Engineering Database. Alternatively, you can add a characterized LED yourself from your own hardware measurements (Figure 5).
An LED Package Cumulative Thermal Structure Function can also be used to create a Resistance-Capacitance (RC Ladder) Model as shown in Figure 4 step 4, and then imported into FloEFD’s LED module (Figure 6). These models determine junction temperature accurately, without the overhead of requiring the LED package internal geometry to be modeled, and can be used in transient situations. Less accurate, 2-Resistor (2-R) models can also be used but have the limitations of being steady-state only, use an assumed heating power as input, and give no information about the light output.
The LED Module user can select Native CAD faces and bodies to apply LED boundary conditions within their favorite MCAD packages; PTC Pro/ENGINEER and Creo Parametric, CATIA V5, Siemens NX or standalone (Figure 7).
The net effect of this LED Module approach is to allow the user to use FloEFD’s accurate CFD solver to calculate the actual thermal impact of the LED inside its luminaire geometry, yielding real-world useful data for the LED Junction Temperature, the LED Heat Generation Rate and, uniquely, the LEDs light output, or ‘hot lumens’ in situ (Figure 8). This allows for the most accurate predictions of an LED’s operational thermal performance in any given product and application environment.