Thursday July 24, 2014

CFD Simulation Reduces the Complexities of LED Design

Manufacturers are striving to drive down costs by reducing material use, while at the same time, consumers are demanding smaller and smaller form factors packed with more and more functions. In many applications—from electronics to lighting—thermal management is challenging since the integrated circuits (ICs) that control devices generate more heat trapped in ever decreasing spaces. Manufacturers are traveling down two parallel innovation paths: one involves the reduction of heat produced in ICs, the other is based on improved cooling methods. This article closely examines the latter in the context of LED light manufacturing.

LEDs – Smaller Lights with a Big Impact

Light emitting diodes (LEDs) are taking the lighting industry by storm. They are lighter, brighter, draw less power to provide more lumens per watt and can be designed in small form factors. They are appearing in almost every application where power efficiency is paramount from household and marine lighting, Christmas lights, gas bar lighting, to car headlights and more.

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Though LED lighting consumes less power that traditional lighting, its design is more complex due to numerous safety and power efficiency regulations as well as limited circuit board space.

Despite their growing ubiquity and popularity, today’s LED lighting applications present a complex design challenge. Requirements such as current and voltage parameters, safety and power efficiency regulations, thermal management for improved reliability and longevity, limited circuit board space and the need to meet time-to-market deadlines must all be addressed simultaneously.

LEDs Heating Up? Throw a Sink at it

Passive ventilation techniques are the name of the game in solid-state lighting. Maintaining temperatures below the LED’s maximum junction temperature (MJT) provides better light quality, increases brightness, reduces color shifting and enhances bulb longevity. The typical method for cooling an IC in a small space is to use a heat sink.

Heat sinks work by efficiently transferring thermal energy from one space to another. There are many kinds of heat sinks. LEDs typically employ heat sinks that promote convection-based heat transfer, and because the heat sink design must fit to the light housing, most of them must be custom designed. Customization does not allow designers to spec an off-the-shelf heat sink so each manufacturer must design their own, depending on the form factor.

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Heat sinks are used to transfer heat from one space to another. For LED design, each heat sink must be custom designed in order to fit to each light’s housing.

Prototyping Then

Manufacturers have a range of options they can access to design effective heat sinks. Up until about five or six years ago, nearly everyone used a combination of rule of thumb and physical testing. In the absence of any other way to model thermal performance, designers would use mathematical rules of thumb. A rule of thumb might be that for every X lumens of desired light output, Y watts of dissipation would be produced based on the number of LEDs required. The tough part then becomes effectively dissipating that wattage to maintain junction temperatures below the maximum specification.

Often heat sinks would be over-designed since designers wanted to make sure they would pass UL tests. For manufacturers, over-designing can lead to poor materials use and eliminate margin optimization based on increased materials costs. Lighting design is part art, part science, so using rules of thumb to design them can lead to the development of less innovative designs, and a competitor using a more precise modeling method could gain an advantage combining superior performance with improved aesthetics.

Once rules of thumb were applied, designers would make physical prototypes for testing. Problems with physical testing include lead times for key components, which slow down prototyping and leads to long design phases. This time-consuming prototyping process draws out time to manufacture and delay to market. In addition, designers are working with real parts in real time leads to mounting costs. Creating a prototype and taking it from an internal testing lab to a UL testing lab only to find it does not pass UL specifications can be a costly iterative cycle, taking into account UL testing fees.

Prototyping Now

Today, there is another way to model thermal performance. Computational Fluid Dynamics (CFD) software leverages computing power to build and test more precise mathematical models. CFD provides a means to test different approaches without physical prototyping, allowing designers to work in the virtual world to iteratively explore options for heat sinks and other ways to improve heat transfer. While the underlying model is math-based, CFD programs provide sophisticated visuals of airflow and thermal gradients to make it easier to see problem areas and alter variables for different outcomes.

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In LED lights, heat sinks encourage convection currents, allowing warm air to escape the light compartment from the top and be replaced by cooler air drawn in from below.

In this example, we designed a heat sink for an LED architectural luminaire, working with their in-house designer. We modeled the LEDs, housing and heat sink in their original design and found that the heat sink was suboptimal, resulting in a relatively high MJT. The problem was that as the hot air from the LEDs rose around the bottom face of the luminaire, there was no efficient way to direct it into the heat sink fins above.

Working back and forth with the manufacturer, we modeled several different heat sinks and ended up changing the actual housing design as well as the heat sink to significantly improve thermal performance. The central insight was that if we punched holes in the face of the light housing, it would allow air from below to flow in through the fins for greater surface area flow cover. We also changed the profile of the fins to remove material and open up the channels between the fins. By curving the fins, we achieved greater velocities with reduced material. Modeling the design, we reduced the MJT by 50% in a week. We also subjected the design to virtual UL testing parameters to ensure it would pass in the real world.

Innovation through Collaboration

Many lighting manufacturers do not have in-house CFD capabilities, but could benefit from the insight derived from simulating designs using CFD. A CFD consultant can work collaboratively with any design team to improve thermal performance. The key is collaboration, because simulations can model an excellent thermal convection system, but the resulting design might not meet the design aesthetic specification, or might use the wrong material, or break other specification rules like weight or size.

If a manufacturer can articulate their constraints upfront, collaborating with a CFD consultant can come up with a design that meets technical specifications and exceeds design and material cost expectations. Simulation specialists have years of experience in extracting the design insight from diverse lighting applications to conceive of more innovative solutions that address even the most challenging performance and form factor goals.

By Jason Pfeiffer, director of CFD Consulting, IMAGINIiT Technologies