An Active Approach to Cooling High-Power Applications

An optimal thermal management solution for any device — be it a set-top box or an LED lighting system — should limit interference with the end design and be as quiet as possible. However, as designers develop higher-powered systems for lighting, medical, and home-entertainment markets, thermal management increasingly becomes a design obstacle.

When deciding on whether to use an active or passive cooling solution, designers have several options. The most common passive device, for instance, is the heat sink.

A new active cooling technique — dubbed synthetic jet technology (SynJet®) — offers high reliability, low power consumption, quiet operation, and highly directed airflow. SynJet technology enables innovative product design breakthroughs by allowing unique thermal management design approaches.

The following three elements should be evaluated when deciding which cooling solution is best suited for a design:

• Size — Active cooling devices can significantly reduce the size of thermal-management solutions by a factor of two or three compared with passive solutions, giving designers the flexibility to create the most attractive designs and fit into tight enclosures or other unobtrusive applications.
• Weight — The weight of a luminaire heat sink can be cut by 50% or greater in some applications using an actively cooled solution. This attribute pays dividends in many ways, from lower shipping costs to more flexible designs. The Table outlines the measurable improvements in volume and weight derived from using active cooling (SynJet) over passive alone.
• Orientation — Orientation is a more important characteristic when evaluating passive heat sinks rather than active cooling solutions. For example, the best passive solution possible is a heat sink with vertically oriented fins. However, if the system being designed can be installed or used with different orientations, such as on its side or bottom, it measurably changes the heat-sink efficiency. Conversely, the orientation has a nominal impact on the thermal effectiveness with an active cooling solution.

Active cooling is often necessary for applications that have a small volume or surface area where natural convection is inadequate, or located where the ambient temperature is high. Active cooling can ensure a lower temperature over a range of ambient temperatures or operating conditions without the repercussion of thermal damage or lower life expectancies, an important point since the number one cause of electronic component failure is excessive heat.

ACTIVE COOLING

The most commonly used active cooling solution is the industry-standard fan. However, fans present some limitations, including:

• Lifetime — Some applications — such as telecom or LED lighting — have stringent lifetime requirements in excess of 100,000 hours that require higher reliability than most fans can offer.
• Noise — Consumers are unlikely to accept a solution that increases environmental noise. Fan noise is a leading cause of product returns in consumer products, and is unacceptable in LED lighting.
• Power consumption — A power-hungry cooling solution negates the advantage of an LED's high lumen-per-watt performance, and lower-power servers and other products are always desired.
• Flexibility of design — Designing around a specific fan size, or having to consider the fan's unidirectional air flow can result in a design that pushes out excessive amounts of air.

In many applications, the inadequate reliability and acoustics from fans are considerable disadvantages. In some applications, such as cooling light bulbs, high airflow can suck in large amounts of dust and contaminates, or will simply be wasted. However, fan technology is not the only active-cooling solution available.

THE SynJet SOLUTION

Fig. 1 is a graphic representation of the SynJet, showing its turbulent air flow. SynJets are formed by the periodic suction and ejection of fluid out of an opening, bounding a cavity by the motion of a diaphragm built into the cavity's walls.

During the ejection phase, a coherent vortex, accompanied by a jet, is created and moved downstream from the jet exit. Once the vortex flow has propagated well downstream, ambient fluid from the vicinity of the opening is entrained as illustrated in Fig. 2.

The bulk of high-speed air moves away from the opening, avoiding re-entrainment, while ambient air from around the SynJet opening is sucked into the orifice. Thus, a synthetic jet is a “zero-mass flux” jet comprised entirely of ambient fluid and it can be conveniently integrated with the surfaces that require cooling without the need for complex plumbing. The far-field characteristics (e.g. rate of lateral spreading and stream-wise decay of center-line velocity) are similar to conventional turbulent jets.

This patented synthetic jet cooling technology is a radical approach to thermal management. Because the module creates turbulent, pulsated air-jets that can be directed precisely to locations where thermal management is needed, SynJet technology can meet cooling challenges requiring high precision, high reliability, and flexible form-factor implementations.

SynJet VS. FANS

Several intrinsic qualities of SynJet modules result in much greater thermal efficiency than 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 SynJet airflow increases mixing between the boundary layer and mean flow. This self-induced entrained flow results in the ability to move heated air efficiently out of the system.

SynJet modules can be tailored to the airflow needs of any system. Multiple hot spots can be cooled without heat sinks, as a SynJet module places the cooling directly where it is needed without complicated ducting.

Heat sinks can be cooled much more effectively by providing uniform flow across the entire device. The hub of a fan can often create problems and dead spots within a chassis, but SynJet's airflow spans the entire heat sink and cools all channels equally.

By the same count, SynJet 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 also becomes a thing of the past as the low pressure created at the heat sink inlet by a SynJet module actually causes more air to be drawn through the flow channels for a given energy input.

As described earlier, SynJet modules produce airflow that is much more thermally efficient, therefore the amount of airflow needed to cool the same heat load is reduced. Lower flow rates translate directly to lower acoustic emissions. In addition, by not having any bearings, brushes, or other frictional parts, the SynJet module eliminates the acoustic problems associated with these interfaces.

Acousticians know there is more to sound than just the SPL measurement. SynJet airflow can often be tailored to accommodate psychoacoustic perceptions as well.

Eliminating the frictional parts common to fans and blowers greatly reduces the potential for failures, the need to evaluate forced air vs. natural convection, and the mean-time-between-failures (MTBF) of even the most reliable fan is exceeded. Fig. 3 illustrates the superior lifetime of SynJets over traditional fans.

For applications in extreme environments, the device can be constructed with robust materials. In applications that previously required natural convection due to failure intolerances, the forced-air vs. convection tradeoffs are no longer applicable. Through the development of very efficient actuators, SynJet modules require very low power to operate.

SynJet AND POWER ELECTRONICS

As consumer devices become faster, smaller, and hotter, a reliable cooling solution is a necessity. For electronics that currently rely on a noisy, bulky, unreliable fan, SynJet technology may offer designers a better thermal-management choice in the near future. SynJet has proven itself as the best cooling solution for LEDs, but with other power electronics, the possibilities are truly endless.

Laptops, desktop computers, and data centers are all being pushed to their limit with the use of fans, and could benefit from a cooling solution that is efficient and virtually noise-free. From a noisy data center to an overheated laptop, the limitations in electronics brought on by excessive heat can be eliminated with the use of SynJet.

SynJet AND THE FUTURE OF COOLING

In the future, the SynJet is poised to be able to bridge the gap from LEDs to power electronics and, as the technology advances, it will become a viable alternative for most consumer applications. Fig. 4 shows an airflow demonstration of the SynJet PAR20 LED Cooler, while Fig. 5 is a cross-section showing entrainment in free air for the SynJet PAR38 LED Cooler.