Predicting and Controlling Heat in LED Lighting

    “The incandescent bulb in your desk lamp is actually an electric space heater that produces a small amount of light as a by-product.”

That statement by an expert on thermal issues explains why LED lighting is becoming more popular every day. LED fixtures can approach 40% efficiency in their conversion from electrical current to usable light, while the common household incandescent delivers only 5% or so. It’s no wonder that more and more designers are being asked to create LED-based lighting solutions for homes, vehicles, businesses, and more. Even the ubiquitous traffic light is making the transition.

In spite of the LED’s vaunted efficiency, heat is still a threat. As an LED’s junction temperature increases beyond specified limits, its intensity, color, and life expectancy can suffer. Therefore it is the designer’s responsibility to analyze and predict the flow of heat from the junction all the way out to the ambient environment, and to ensure that the junction temperature, (TJ) stays within tolerable limits. And that means transporting heat away from the device and its junction via heat sinks and housings.

The fluid flow analysis in an LED lighting design project has an entirely different mission than the same procedure performed on a jet fuel valve, for example. To design a desk lamp it’s necessary to consider the aesthetics that will help sell the product; the fire safety and UL issues; and of course the cost. One misjudgment about cost, for example, might make the lamp less attractive to a whole chain of retailers. So your task when evaluating heat flow within the desk lamp is to optimize, optimize, optimize.

This is where a simulation approach shows its true colors. A hardware prototyping process means physically building and testing every concept and variation. The money and time costs of this approach were once unavoidable but now can be drastically reduced by simulating the same variables in virtual form.

The graph illustrates just one of many variations that might be analyzed as part of a desk lamp design project. Two variables are involved:  the number of heat sink fins and the heat sink material.

Working with the MCAD data for analysis, the designer created a baseline model with 11 heat sink fins as depicted in the inset image. “Cloning” this model, he then iterated through variants with 22 through 88 fins and materials ranging from aluminum oxide to gold, and ran the analysis for each.

The results are enlightening. All thoughts of using expensive heat sink materials can be dismissed; common aluminum performed well, and almost identically to gold and silver. Conversely, the cheapest heat sink material, aluminum oxide, does not deliver satisfactory heat transfer for this application.

Equally important, the number of fins reaches a point of diminishing returns with 66 fins; anything more would be unnecessarily complex, adding cost.

All of these findings illustrate the importance of optimization and “what-if” testing in the simulation environment. Now the lamp designer can go forward with confidence that his heat sink choices are as cost-effective as possible.

Of course, even a simple desk lamp is a “system” and other CFD analyses on the LED and the housing will be required. CFD software can provide cut plots, 3D plots, Iso-surfaces, flow trajectories, particle studies, and more.

A comprehensive webinar on the subject of LED and lamp design using Concurrent CFD is available. Industry experts offer their best tips on “Demystifying LED Design for Everyday Applications with Concurrent CFD”.  The webinar explains Concurrent CFD technology and goes on to discuss projects including a table lamp, a ceiling lamp, and more.  Other LED and heat sink design webinars are also available at http://www.mentor.com/products/mechanical/multimedia/ .

 

This Quick Technical Quide has been provided by Mentor Graphics (Mechanical)

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