Helping LEDs Keep Their Cool

Mick Wilcox, Nuventix Inc.

In the general illumination market, LEDs are winning acceptance and experiencing phenomenal growth. However, despite the enthusiasm, there still are significant barriers to their widespread use in other potentially hot markets.

As designers develop higher-lumen LEDs for the lighting, medical and home entertainment markets, thermal management increasingly becomes a design limitation. Passive cooling, the historic choice of the lighting industry, physically cannot cool LEDs to a point where they can produce enough lumens to be used in these general illumination applications.

New thermal management techniques will help move LEDs closer to becoming affordable, long-lasting replacements for incandescent and compact fluorescent lighting.

The hope is that, by embracing active cooling solutions, it will be possible to leapfrog current LED adoption for those areas by two years while maintaining the promised efficiency and reliability of LEDs.

Efficiency declines

Without effective thermal management, the junction temperature of an LED rises, causing it to lose efficiency and diminishing light output. Thermal stress also can lead to failure of the wire bond, to delamination, to detached internal solder joints and to die-bond epoxy damage. For every 17 °C above 90 °C, an LED’s life expectancy is halved.

Passive cooling somewhat limits lumen output in LED lighting solutions. This low-power output is sufficient for accent lighting but not effective for general illumination. Active cooling is poised to double the lumen output while maintaining the same acceptable low temperature.

An active cooling solution – such as a fan – has serious drawbacks for the lighting industry. High acoustic levels, excessive airflow output and limited reliability make it a less than attractive option. Liquid cooling is suggested as a possible active cooling solution for LEDs, but it requires significant power and severely restricts the form factor. Also, the costs do not outweigh the proposed benefits.

Synthetic jet approach

A new active cooling approach, synthetic jet technology, was developed to cool consumer electronic products. The jets effectively cool with no form factor constraints, with near-silent acoustic levels and with high reliability. The technique offers a new option for thermal management, addressing the needs of the LED market. Using synthetic jets to create a turbulent pulsating flow of air, the process cools with greater efficiency and effectiveness than fans – and without the noise.

Synthetic jet cooling is produced by the oscillation of a diaphragm and the pulling in of air. The air is then expelled in a turbulent fashion, setting up a secondary flow.

The technology has been implemented in SynJet modules from Nuventix Inc. The modules use an oscillating diaphragm mounted within a cavity with precision-molded nozzles. An electromagnetic driver oscillates the diaphragm, pulling surrounding air into the housing and then expelling it in a directed, unsteady, turbulent fashion. The air jets’ turbulent nature enhances the mixing of the thermal boundary layer and the global secondary flow. This produces a much higher heat transfer coefficient when compared with typical laminar airflow, cooling higher-powered LEDs with less work.

The lower temperature enables designers to put twice as much power into the same form factor. So an average LED lighting solution that emits 500 lm could emit 1000 lm, a 100 percent improvement. Active cooling through synthetic jet technology can double or triple the light output of a single fixture.

The velocity vectors are shown for the flow as the jet is ejected.

The diaphragm design makes it completely scalable and allows for cooling solutions that can be shaped into nearly any form. Also provided are spot cooling and chip cooling in form factors that fit those of LEDs.

Less noise

The lower flow rate, which allows for increased efficiency, also leads to lower acoustic emission. Because less airflow is needed to cool the same heat load, the associated audible noise automatically is lower as well. Additionally, by not using bearings, brushes, or other frictional parts, the synthetic jet module eliminates the acoustic problems commonly associated with fans and other cooling solutions as they wear. This also leads to increased reliability because there are no motors or bearings that can wear out and cause a malfunction.

One thing seems certain: LEDs are the next step in revolutionizing energy-efficient lighting for consumers in a multitude of applications. By embracing active cooling, the market for effective general illumination stands to exceed expectations. Mass market acceptance of active cooling solutions can truly help to move the LED industry forward.

Meet the author

Mick Wilcox is director of marketing at Nuventix Inc. in Austin, Texas; e-mail: mwilcox@nuventix.com.