In designing an LED lighting application, it is imperative for engineers to consider certain factors in their blueprint. Obvious considerations including lumen output, CRI, system efficacy, size, weight, and cost may be discussed at length, but a suitable cooling solution may be overlooked. For lighting designs, especially in LED applications, an appropriate cooling technique is crucial in allowing the luminaire to function in an optimal manner.
The selection of a cooling solution for a given design can be a difficult and time-consuming process. There are several forms of thermal management technology that are available on the market. The first step in choosing a cooling method is deciding between a passive or active solution. While passive cooling has historically been the go-to solution for the lighting industry when designs were little more than accent lighting; active cooling solutions are beginning to gain traction now that LED lighting has progressed into more mainstream applications such as down lights and spot lights.
There are many technologically advanced methods available for active cooling solutions. For example, California-based Cooligy, Inc., offers a liquid cooling solution by which the system employs a fluid pumped in a sealed cooling loop for high-heat flux semiconductors. However, a designer may feel uncomfortable about having liquid flowing so close to nearby electrical components.
Another solution from The Bergquist Company uses specialized circuit board materials in conjunction with high power LEDs. Bergquist’s Thermal Clad IMS is a thin, thermally conductive layer bonded to an aluminum or copper substrate for heat dissipation. However both of these solutions only help solve the source side heat issue. It still leaves you with the air aspect, getting the heat off of the heat exchanger and into the air.
The traditional active air cooling solution to which most designers are accustomed is the fan. This method, though typically hailed as the standard, has its drawbacks. For example, a fan has multiple moving parts which causes the device to become noisy over time and, therefore, have a more limited lifetime. For telecom or LED lighting uses which often require well in excess of 50,000 hours at elevated temperatures, a longer lifetime is a necessity.
Synthetic Jet Technology
An alternative to the traditional fan, and one that is much more suited to the demanding requirements of the LED industry, is synthetic jets. Synthetic jets are formed by periodic suction and ejection of air out of an opening by the motion of a diaphragm. During the ejection phase, a vortex accompanied by a jet is created and convected downstream from the jet exit. Once the vortex flow has propagated well downstream, ambient fluid from the vicinity of the opening is entrained.
During the ejection phase, the bulk of the high speed air moves away from the opening, avoiding re-entrainment, while ambient air from around the synthetic jet opening is sucked into the opening. Thus, a synthetic jet is comprised entirely of the ambient fluid and can be conveniently integrated with the surfaces that require cooling without the need for complex plumbing.
In comparison to other active cooling options, synthetic jet technology offers increased thermal efficiency, low audible noise, high reliability and low power consumption.
• Thermal Efficiency - Several intrinsic qualities of synthetic jet modules result in significantly more thermally efficient air flow than that created with conventional air movers. The turbulence of the flow results in more efficient heat transfer from the heat source to the air. The pulsating nature of a synthetic jet airflow increases mixing between the boundary layer and mean flow. This self-induced entrained flow results in the ability to move the heated air efficiently out of the system. Synthetic jet modules produce airflow that is much more thermally efficient, therefore the amount of air flow needed to cool the same heat load is reduced.
• Low Audible Noise - Lower flow rates translate directly to lower acoustic emissions. By not having any bearings, brushes, or other frictional parts, the synthetic jet module eliminates the acoustic problems associated with these interfaces. Acousticians know that there is more to sound than just the SPL measurement. Synthetic jet flow can often be tailored to accommodate psychoacoustic perceptions as well. Synthetic jets operate at frequencies below that of human hearing, 30-100Hz, so the only noise heard from synthetic jets is the movement of the air.
• High Reliability - With the elimination of frictional parts common to fans and blowers, the potential failure modes are greatly reduced, the need to evaluate forced air vs. natural convection is eliminated, and the MTBF (Mean-Time-Between-Failures) of even the most demanding systems are exceeded.
• Low Power Consumption - Through the development of very efficient actuators, synthetic jet modules require very low power to operate, thus helping system and luminaire designers maintain their green objectives.
This radical approach to thermal management can help designers solve cooling challenges requiring high reliability and flexible form-factor implementations and do so in a more thermally efficient and quieter manner than fans.
Synthetic Jet Technology and Flexible Design
Because this technology is extremely customizable, designers have freedom when it comes to their design implementations. Synthetic jets enable an engineer to open up the design envelope and think about how the technology can radically improve a desired product through new cooling architectures. Air doesn’t need to be just moved from left-to-right, it can be focused right where the design needs it.
Synthetic jet modules can be tailored to the air flow needs of any system. Multiple hot spots can be cooled as a synthetic jet module places the cooling directly without complicated ducting. Heat sinks can be cooled much more effectively by providing uniform flow across the entire heat sink. The hub of a fan can often create problems and dead spots within a heatsink, but a synthetic jet flow spans the entire heat sink and cools all channels equally. By the same count, the synthetic jet modules may be tailored to direct more flow across the center of the heat sink where the heat source is located. Heat sink flow bypass becomes a thing of the past as well - the low pressure created by a synthetic jet module at the heat sink inlet actually causes more air to be drawn through the flow channels for a given energy input.
Examples of various design applications:
Because of its design capabilities, synthetic jet technology offers a more flexible cooling option than fans or other thermal management competitive technologies.
Synthetic jet thermal solutions are applicable to a wide variety of industries, including consumer electronics, telecommunications, industrial and LED lighting. Designers will have more flexibility in product development and will be able to offer their customers smaller, higher light output and more reliable designs, thus increasing their value and differentiation in the marketplace.
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