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Optoelectronics/Displays??

Myths about cooling high-power LEDs

Posted: 07 Nov 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Active cooling? DC ball-bearing brushless fans? LED? luminaires?

In high-power lighting, there are emerging requirements for LED luminaires rated to 100, 200 or even 600 watts (over 60,000 lumens) to cover wide areas. The higher-power heat sinks increase in size and weight exponentially. The consequence is escalation of costs for tooling, assembly, shipping, and installation.

Active cooling (i.e., the use of fan-cooled heat sinks) can exhibit striking size, weight and cost reductions at those elevated power levels. A 400-watt, passive-cooled high-bay fixture might weigh 75 pounds and be the size of a two-drawer file cabinet. An active-cooled version might weigh only 15 pounds and be no larger than a pizza box. Figure 1 shows the rule-of-thumb relationship.

Figure 1: The rule-of-thumb relationship.

Figure 2: This shows a 2.3 X 2.4 X 1.4 inch fine-pitch extruded heat sink, with a 50 watt Bridgelux LED array. With no air flow, this has a thermal resistance of 4 C/W, meaning it can handle only about 10 watts. When a low-speed fan is added to it as shown in the upper view, it exhibits a thermal resistance of only 0.8 C/W, increasing its power capability to over 50 wattsa five times improvement! (Photo courtesy of Nutron Mfg Inc.).

Why would anybody use passive cooling? You ask. Often it makes sense. Often it does not.

Imagine if, in 1905, the auto industry said ..."We don't want to actively cool the engine (i.e., use a radiator and fan). A V-8 engine block would now be the size of a refrigerator.

While the industry is inexorably embracing active cooling at power levels above 50 watts, there is still resistance in many "old school" quarters.

Let's review the oft-repeated myths used by those folks.

Myth #1: Fans are noisy
Yessiree... that fan in your microwave oven makes a racket, as does the fan in your PC or even that window fan. But take a standard, low-speed (under 2000-RPM) DC ball-bearing, brushless fan made by one of the top fan makers, such as Delta, NMB, EBM-Papst or others, and operate it at the low end of its voltage range... and guess what? Its sound level can be less than 16-18 dBcompletely inaudible in a dead-quiet room when in a fixture seven feet or more above the floor. In fact you will be hard pressed to hear it from one meter away in a quiet room. 16 dB is way below the 24-dB ANSI standard for quietest fluorescent lighting ballasts.

Scratch Myth No.1.

Myth #2: Fans are unreliable
DC brushless ball-bearing fans have one of the most visible and enviable field histories of any electromechanical component in the electronic industry. Millions of mission-critical computers powering the Internet and wireless communications systems have been operating 24/7 for well over a decade with fan-cooled power supplies using such fans. What does that say about their proven reliability and user confidence?

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