Light-emitting diodes (LEDs) don't like heat. Heat shortens their lifetimes. It also damages brightness, ruins efficiency and diminishes color.
"Heat is death to an LED," says Joe DeNicholas, Lighting Business Unit director for National Semiconductor. "There's widely published data suggesting that if you keep the junction temperature of an LED at about 100 degrees Celsius, it will last for 80,000 hours. But if you let it go to, say, 135 degrees Celsius, the lifetime drops down to about 20,000 hours. That's why we have to keep them cool."
Keeping them cool, however, is no simple task. In the past decade, LED current levels have soared. Whereas 20 mA was typical a few years back, 1A is commonplace today. Similarly, power levels have skyrocketed from milliwatt levels a few years ago to more than a watt today. In some industries power levels are said to be doubling every three to six months.
"The old LEDs never required cooling," says Barry Dagan chief technical officer of Cool Innovations, a maker of heat sinks. "But when you've got two watts of dissipated power, you have to have cooling. And now we're starting to hear talk of power levels at 10 watts, 15 watts and more."
So how's an engineer supposed to cool a little LED that may need to dissipate a couple of watts? Answers are many. Electronics manufacturers have developed drivers that can back off the current when the junction temperature rises too high or when the LED is illuminating an unoccupied room. Materials experts have created metal-clad substrates that draw heat away from the LED. And heat sink manufacturers are finding new and innovative ways to move heat into the surrounding air.
"The engineer has to figure out the path that takes the heat from the LED junction out to the ambient air," says Joe Jablonski, applications engineering manager for Osram Opto Semiconductors, an LED manufacturer. "That path is your thermal system."
Go to the Source
So if LEDs require such care, why go to the trouble? Why not use an incandescent bulb? Engineers say there's good reason. LEDs offer exceptionally long life, making them ideal for applications where it's difficult to change a bulb. Then there's efficiency. LEDs deliver a lot of light for little power. Although reliable figures for lumens-per-watt are difficult to pin down, the difference between an incandescent and an LED is vast. By some accounts, LEDs offer about 120 lm/W, while incandescents hover between 10-15 lm/W.
"There's no doubt about it," DeNicholas says. "The LED is the most efficient controllable light source ever created." To be sure, though, LEDs still carry higher costs. "There has to be an overwhelming need, other than efficiency, to use an LED," says Tom Morris, applications engineering manager for TT Electronics' IRC Div. "The price you pay per lumen of output is still much more than what you'd pay with a conventional light bulb."
Still, the number of applications is growing rapidly. Between 1999 and 2009, prices dropped to about one-third of what they had been, making it possible to use LEDs in a wide variety of special projects, including such notable applications as the driver-customizable Ford Mustang interior and the massive 12-million-light Walgreens sign in New York's Times Square. Experts say LED use also continues to increase in medical equipment, handheld electronics, architectural lighting, parking garages, street lights, television backlighting, signage, vehicle interiors and even automotive head lamps.
The key to making it happen is thermal management, say experts. "With the development of high-brightness LEDs in one-watt packages, there's enough heat being produced so that you now have to get rid of it," Morris says. "That's the only way you're going to attain the luminosity and reliability that you need from your LED light source."
The place to start managing heat is at the source, engineers say. Electronics manufacturers such as Texas Instruments (TI) and National Semiconductor are doing it with the development of smart LED drivers. TI, for example, has rolled out a product known as UCC28810SIMPLEDrive, capable of controlling current and dimming LED illumination. By integrating the intelligence of TI microcontrollers, the driver can monitor junction temperature or employ a feature known as intelligent occupancy sensing.
"It senses if there are occupants in the room, and it turns on or off the lights or it dims the lights," says Peter Di Maso, product marketing manager with TI's Power Management Business Unit. The company also enables users to program the LED's operation to keep temperatures at a desirable level. Engineers can set the driver to gradually cut back on electrical current until the unit reaches a prescribed shut-off temperature.
"If we start to detect it's getting hot, we decrease the current," says DeNicholas of National Semiconductor. "When we decrease the current, it lowers the light output, but it also decreases the power dissipation, so we can keep the LEDs in a safe operating region." (National Semiconductor also offers help for designers with its WEBENCH® LED designers tool suite.)
Spreading the Heat
Still, heat happens. And when it does, the entire LED system has to be able to dissipate it. Typically, heat travels a circuitous path, gradually exiting the LED through its electronic packaging, moving into a printed circuit board and across a dielectric layer, and then onto a heat sink where it is transmitted into the surrounding atmosphere. In some cases, applications engineers also employ blowers or other active devices to help push the heat away from the LED assembly.
"It's very similar to water flow," DeNicholas says. "You don't want to resist it. You want the heat to travel from its generated source to where it is being dissipated, as smoothly and fluidly as possible."
Materials experts, such as The Bergquist Co. and TT electronics' IRC Advanced Film Div., create substrates that can quickly transfer heat away from the LED's base. Bergquist's Thermal Clad Insulated Metal Substrates (IMS) provide an alternative to the commonly used and inexpensive FR-4 material, which consists of copper laminated to a glass-epoxy board. In contrast to FR-4, Thermal Clad IMS is a thin, thermally conductive layer bonded to an aluminum-copper substrate for the expressed purpose of heat dissipation. The layer combines electrical isolation with high thermal conductivity, while bonding the base metal and circuit foil together. The bottom line for users is that the metal-clad substrate quickly transfers heat to the aluminum base plate, thus improving the LED's performance.
"The LEDs are placed on top of the substrate using regular surface mount technology," says Justin Kolbe, a senior research and development engineers for The Bergquist Co. "Conduction to, and through, the thick metal layer is very good."
Similarly, TT electronics' IRC Advanced Film Div. has rolled out a substrate technology known as Anotherm, which consists of an aluminum alloy substrate, a thin dielectric layer and a screen-printed conductor atop the dielectric. "Because of the high thermal conductivity that the base aluminum substrate offers, in many cases the Anotherm board eliminates the need for additional heat sinks," says Morris of TT Electronics' IRC Div.
Creating Air Flow
In some cases, however, heat sinks, fans and other devices may be needed. In those cases, engineers typically try to employ natural convention first, and forced convection (fans and other active devices) second. "In the natural convection mode, if you just move the air slightly, you can increase cooling by a factor of as much as ten," says Dagan of Cool Innovations. "If you run a fan at low speeds, you can improve your cooling even more, but many people don't want fan noise in their LED applications."
Cool Innovations has developed an innovative way to boost the cooling of naturally convected systems. The company's flared pin heat sinks depart from the conventional by employing thermally conductive pins that are splayed outward. Because the flared pins incorporate significantly wider spacing than conventional straight pins, they feature low friction between the air and the heat sink.
"By nature, there's a limit to how much heat you can dissipate by the natural convection mode," Dagan says. "But with the flared design, we're tricking the heat sink. By flaring the pins outward, we're using free space and the distance to adjacent pins is greater. So we're optimizing the surface area and the distance between the pins."
Dagan says the flared pin design dramatically aids heat dissipation. He adds, however, that some systems still need a fan or other type of active device when power generation reaches high levels. For those applications, Cool Innovation also offers "pin-fin fansinks" that embed a fan in the heat sink's pin array. The fansinks, designed for applications that are restricted in height but need substantial cooling power, generate 6.1 cubic ft of air flow per minute.
Similarly, Nuventix Inc. markets a product know as the SynJet - a synthetic jet that uses a diaphragm to cool the region around the LED. The device works by activating an electromagnetic driver, which causes the diaphragm to oscillate, thus pulling the surrounding air into the device's housing. Rapid cycling of air in and out of the housing creates turbulent pulsating air jets that can be directed to precise locations where cooling is needed.
Experts say forced convection will continue to be a necessity in some applications. "There is a class of LEDs where the users don't care about noise," Dagan says. "In applications where replacement of bulbs is costly, such as in skyscrapers, or where LEDs are hard to reach, it makes sense to have a small fan on top. That way, you increase the life of the device."
More Heat to Come
With average LED power levels doubling every few months, engineers say the number of potential LED applications will balloon, as well. New electronic products can now drive 40 LEDs at once, and experts predict future chips will soon be able to drive as many as 80 LEDs at a time. Moreover, new research suggests that future LEDs of 180-200 lm/W may be only two years away. That's why engineers in the auto and consumer electronics industries are talking about more widespread use of LEDs.
"LEDs have moved from cheap indicator lights to much more sophisticated applications over the last few years," says Kolbe of Bergquist. "You're going to see them in architectural and artistic applications and in automotive head lamps and streetlights more than ever before."
But as power levels rise to 10W and possibly even 20W, engineers know there will be more heat and they'll need new ways to manage it.
"It's not like an incandescent light," says Di Maso of ?TI. "Incandescent bulbs eliminate their heat through radiation, whereas with LEDs it's more of a mechanical issue. But with the efficiency of LED systems increasing as they are, that's a problem worth solving."
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