LED lamp heat dissipation problem

LED lamp cooling technology introduction

LEDs offer exciting design opportunities, and all the arguments about this are often hindered by such words: thermal management. “When incandescent bulbs can dissipate heat through radiation, LEDs generate almost no infrared heat,” said Michael Gershowitz, director of technology marketing at Purui. Therefore, the LED must be dissipated by heat conduction, otherwise there is a risk that the brightness, life, and color stability will be weakened.

But time and market demand always have the means to push technology to such a node, that is, thermal management will soon become a late consideration in the design of luminaires, not an obstacle. As LEDs continue to iterate, light efficiency continues to grow exponentially. Today's LED efficiencies have exceeded 50%, ie they convert energy into light more than heat. Most OEMs have abandoned the original use of LEDs to simply replace traditional luminaires, and instead produce tailored and optimized luminaires based on the characteristics of LED sources. And now LEDs can "work at higher temperatures for longer, and the brightness does not decay," said Mark Hand, director of new product development and technology at Acuity Brands Lighting.

Advances in LED technology and luminaire design, and what manufacturers can boast about can be more than that. Thermal management strategies have also been progressing and flourishing. Although passive cooling and active cooling are still the main choices, other innovative technologies are also joining.

Passive cooling

More than 90% of LED luminaires use passive heat dissipation. Under this strategy, the heat of the LED package is conducted through direct physical contact via a highly thermally conductive material heat sink. “Aluminum – usually extruded or die-cast into a fin-shaped circle – is a standard material for heat sinks due to its light weight, low cost and ease of manufacture,” said Xicato Strategic Partner Manager Christopher Reed. . Currently, manufacturers are more willing to passive radiators because of their reliability. "Even if it's not exactly the same, their performance after 50,000 hours or 20 years is not far from the first day," Reed said.

Figure: Passive Heat Dissipation - Fin Radiator (Source: baidu)

But even a proven and proven solution can be improved. While designers have optimized the heat sink by cleverly handling fin thickness and spacing, the passive cooling strategy has also evolved into a new form – the fin radiator. Unlike extruded blades, finned radiators have a large number of legs associated with an inverted round table, and LED devices are attached to the surface of the table.

Figure: Passive heat dissipation - fin radiator (Source: baidu)

According to Reed, the column fins raise the heated air and then flow unimpeded around the fin. This trait is advantageous on some rotatable luminaires, such as track lights. “When you tilt a standard extruded finned radiator, air can't flow in the same direction or on the gravity axis because it always goes into the fins,” Reed said.

Lighting manufacturers have also begun to turn the lamp body into an integrated cooling solution. With this strategy, the rim of the luminaire installed in the insulated ceiling system can both modify and dissipate heat. In the same way, the sculpted fins of the CREE Aeroblades series of street lamps do not function as "flower vases".

Figure: CREE Aeroblades series street light

"As LEDs become more efficient, the required heat sinks are getting smaller and smaller," said Mark McClear, vice president of application engineering at CREE. As a result, material and transportation costs will also decrease. “When LEDs become more efficient, everything is moving in the right direction,” McClear said.

Active cooling

The convenience of passive thermal management is often plagued by the bulkiness of the heat sink. This volume can be a problem for small luminaires, not for sealed outdoor luminaires, and not for high-gloss downlights. Active cooling relies on forced circulation of air rather than natural convection, "making it emit more heat in a much smaller space," Hand said.

Active cooling itself requires the use of more moving parts and increases costs, so the progress accepted by OEMs is slower. However, recent advances have made active cooling solutions more efficient and reliable.

Even the actual active cooling solution, the fan, has made great progress. According to Xicato's Reed, the long-term configuration has either used plain bearings—typically made of plastic or high polymer materials, and is susceptible to dust accumulation and time—or more durable metal ball bearings. system.

Figure: Comparison of heat distribution when a product uses active cooling and no fan (Source: baidu)

In order to eliminate the friction between the fan bearing and the drive shaft, Sunon, a thermal design company based in Brea, Calif., introduced magnetic levitation technology. To cope with dust accumulation, the company has also developed a two-way revolving technique in which the machine reverses the fan for the first few seconds of operation, "to remove dust from the blades," Reed said.

Noise is another reason for active heat to cause complaints. For example, in a room with a track light, the average volume of each luminaire is 15-30 decibels, and the noise caused by active cooling is very obvious. “It sounds like a beehive,” Reed said. Ebm-Papst, based in Farmington, Conn., offers a fan that runs at only 7 decibels, the quietest fan product Reed has seen so far.

For manufacturers who still have doubts about fans, Nuventix, Austin, Texas, offers a pulsed silicon film technology that vents air from the LEDs to the heatsink, which in turn reduces the size of the heatsink to independent passives. One-third of the cooling solution. Synthetic jet technology, or SynJet, "has no rotating or friction components," said Tom Dalton, senior vice president of marketing at Nuventix. Instead, it uses LED-driven power to make the film vibrate 50 times per second—he estimates that the power consumption is about 0.35-1W—these adiabatic pulses create a better convection heat dissipation method, which is better than a fan. The resulting laminar gas flow is better. Reed said. With this technology, SynJet can last for five years, allowing the silicon film to expand beyond the limit by 10%, Nuventix's Dalton said.

Although some manufacturers still have concerns about moving parts on active cooling, most agree that active cooling is more efficient than passive cooling. “The more cooling and quiet, the better the situation,” Gershowitz said.

Hybrid heat pipe

Heat pipe technology is a mature heat dissipation mechanism adopted by computers. It combines the characteristics of both active and passive cooling solutions, and has also attracted the interest of OEMs and lighting manufacturers. Essentially, the heat pipe is made up of a sealed metal tube that is internally filled with a heat-conducting liquid that carries heat away from the LED and is sent to a remote cooling system. Although the effectiveness of the system is diminished due to the increased distance between the heat source and the heat sink, the heat pipe still makes the overall components unique. “This separation is an excellent design opportunity,” Gershowitz said. For example, a heat pipe in a compact hoisting luminaire transfers heat to a creative ceiling. “Designers can come up with some very attractive solutions,” he said.

Figure: Heat pipe technology

FrigoDynamics has applied some new techniques to this technology. The German company created an effective hybrid cooling technology that maximizes heat dissipation by passing heat pipes through strategically shaped, geometrically shaped passive radiator metal blades. “The heat pipe is the element that drains heat from one side of the radiator to the other,” Reed said. “This is very efficient at rejecting heat and has a very uniform and constant temperature across the radiator.”

bright future

The technology that has not been proven in practice has also attracted interest from some curious vendors. Sandia Corp. has long been committed to the development of the Sandia Radiator, a single-piece metal fan that relies on air-floating bearings to rotate, eliminating the potential for physical bearing failure.

If you don't talk about heat-dissipating media materials, the discussion about thermal management strategies is not comprehensive, it is filled with tiny gaps between the LED metal backplane and its heat sink. The choice of which thermal pad material is filled in the air gap is critical to LED heat transfer. Phase change materials are a plausible, effective material that can be selected for both plastic and metal heat sink substrates, Gershowitz said.

Cambridge Nanatherm also focuses on these complex challenges at the micro level. The British company recently introduced a nano-ceramic dielectric layer for LED PCBs with a thermal conductivity of 7W/mK. MBPCBs typically use an epoxy fill with a thermal conductivity between 1-3 W/mK. Nanotherm claims that their MBPCBs are 20% more efficient at exporting heat from LED chips than aluminum-based PCBs on the market.

In addition to advances in thermal management strategies, as the efficiency of LEDs rises – CREE's McClear is expected to reach 65% by 2020 – LED cooling will no longer be a problem in luminaire design. Acuity's Mark Hand predicts that low-power LEDs will strategically shift to their own heat dissipation, and even eliminate the need for specialized thermal management strategies. “At the very top of the ruler, people will try and get more lumens from a smaller space, and they will start to focus on thermal polymer.” This can be molded into a diversified LED package, he said.

Purui's Gershowitz believes that even if LED efficiency progresses, thermal management needs to be properly retained. “Every step forward...not to reduce power or reduce cooling, OEMs will use the same cooling solution, but use a larger light source.” The good news is that the speed of thermal management will allow OEMs and lighting. Manufacturers have expanded solid-state lighting to more markets without fear of burning.