Advances in LED Thermal Management

LEDs are often described in marketing materials as "cool" lighting, and in fact LEDs are cool to the touch because they generally don't produce heat in the form of infrared (IR) radiation. On the other hand, LEDs generate heat in the diode semiconductor structure (in addition to photons) and this heat must exit the system through conduction and convection. Consequently, luminaire designers must be conscious of potential heat dissipation challenges and how those challenges may affect LED performance, longevity, and even lamp safety.

Elevated junction temperatures have been shown to cause an LED to produce less light (lumen output) and less forward voltage. Over time, higher junction temperatures may also significantly accelerate chip degeneration, perhaps by as much as 75 percent with an increase from about 100°C to 135°C during regular use.

Engineers and material scientists have been and are developing new LED-related thermal management solutions including improved drivers, diaphragm-driven forced convection methods, better heat sinks, and even the introduction of graphite foam as a cooling medium. This article will first describe three junction temperature considerations--basic thermal resistance, power dissipation, and junction temperature measurement--then briefly look at advances in each of the aforementioned approaches to improved LED thermal management.

Junction temperature considerations

When considering LED thermal management, there are generally three factors that tend to act on junction temperature. These are the ambient air temperature, the thermal path between the LED junction and the surrounding environment (the thermal path, of course, should be optimized to encourage natural heat convection) and the LED's efficiency.

Ambient temperatures will vary by application, so that luminaire designers will want to pay attention to how designs will be used in real world environments. For example, a few years ago the Rensselaer Polytechnic Institute's Lighting Research Center and the Alliance for Solid-State Illumination Systems and Technologies experimented with LEDs in various open air, semi-ventilated, and enclosed environments. Board temperatures for a 12-watt LED reached 60°C in the enclosed environment, a 26-watt LED's board temperature rose to 119°C.

As for efficiency, LED efficiency will vary based on several factors with some devices converting as much as 80 percent — or perhaps even more — of the input electrical power to heat. Efficiency becomes more of an issue as LEDs increase in power. For example, when LEDs were primarily used as indicator lights, current levels were only a few milliamps whereas hundreds of milliamps or even amps are becoming commonplace in present day applications.

Measuring thermal resistance, power dissipation, and junction temperature

In an excellent application note associated with its XLamp XR family of LEDs, Cree offers suggestions regarding how to measure LED thermal resistance, power dissipation, and junction temperature. The thermal resistance between two points, which is often measured in degrees C per watt, may be thought of as "the ratio of the difference in temperature to the power dissipated." This ratio should be calculated for the LED junction to the thermal contact or solder point typically found at the bottom of an LED package and for the thermal contact to the ambient. The sum of these measurements represents the thermal resistance for the LED as a whole.

Power dissipated, again according to Cree, "is the product of the forward voltage and the forward current of the LED." To assure satisfactory lifetime of the device, good efficiency, and proper LED color, junction temperature must be maintained within a specified band. Junction temperature (T_j) may be calculated by adding the product of the LED's overall thermal resistance (R) and the power dissipated (P_d) to the ambient temperature (T_a).

T_j = (R x P_d) + T_a

Further information on calculating LED junction temperature can be found in a previous article Calculating LED Junction Temperature in Lighting Applications in DigiKey’s Lighting TechZone^SM.

Improved LED drivers and temperature sensors

Since electrical characteristics, such as the forward voltage of the LEDs, will drift with temperature this has to be taken into account when designing driver circuitry. Thus LED drivers are at the front line of LED thermal management. Manufacturers such as National Semiconductor or Texas Instruments, for example, are developing and producing LED drivers and companion temperature sensors that work together to adjust the current flow to an LED based on that LED's junction temperature profile. The temperature sensors are designed with enough margin to be able to both detect an over-temperature problem and at the same time not trigger a false alarm under normal operating temperatures.

LED drivers also often employ a thermal shutdown failsafe mechanism, so if the drivers exceed a specified temperature, typically 125°C to 150°C, they will turn off along with the LED. The driver-as-thermal-manager approach may also be combined with occupancy monitoring solutions that will reduce an LED's lumen output when it is turned on in an unoccupied room or as a room's natural light illumination changes over the course of a day.

Improving the heat sink

Without good heat sinking, the junction temperature of the LED rises, and this causes the LED characteristics to change. Heat sinks seek to transfer heat from the LED to the air, which as a more fluid medium, may naturally move the heat away from the LED, helping to keep the junction temperature lower. When considering any heat sink luminaire designers should consider the heat sink's surface area, aerodynamics, thermal transfer, and mounting (including flatness).

Generally, thermal transfer takes place on the heat sink's various fin surfaces, so that a heat sink with a greater surface area either as the result of more fins or a larger overall physical size should move more heat away from the LED. On balance, however, heat sinks should also be aerodynamic so that heated air may move quickly and freely, as a result a heat sink with many small but densely packed fins may actually discourage air flow and undermine the expected advantage of having greater surface area.

As a further consideration, there should be a balance between surface area and fin thickness, since a thinker fin tends to offer superior thermal transfer when compared to relatively thinner fins. So again, if a heat sink maker increases the thermal transfer rate with thick fins, there is a cost in terms of total surface area. Lastly, the contact point between the heat sink and the LED should be as flat as possible, if there is space between the LED and the heat sink mounting, the thermal path will not be as direct.

To address the various requirements for surface area, aerodynamics, thermal transfer, and even mounting flatness, researchers at The Singapore Institute of Manufacturing Technology have proposed a new method of heat sink manufacturing called liquid forging. This technique allows for pore-less heat sink designs in very complex geometric shapes specifically selected to boost airflow without surrendering too much surface area.

According to the researchers, "liquid forging is an innovative hybrid casting and forming process [wherein] molten metal/alloy is poured into a die cavity and squeezed under pressure during solidification to form metal components in a single process."

Heat sinks manufactured using the method and a combination of aluminum alloys with a copper base have been shown to have superior thermal performance (at approximately four times better thermal conductivity) than more typical, commercial extruded, machined, and die-cast heat sinks.

The technique also allows for more intricate fin and pin design to potentially improve heat convection thanks to a better balance between thermal mass and surface area. While the non-porous surface created with liquid forging eliminates air pockets or other air flow obstacles. There is, of course, still work to be done before liquid forged heat sinks are readily available, but the technology is promising.

Separately, companies such as Nuventix are combining improvements in heat sink design with forced convection to dramatically increase the movement of air through the heat sink. The Nuventix solution is called SynJet. SynJet uses an oscillating diaphragm to create pulses of high velocity and rather turbulent air flow. This airflow pulls air in its wake, which is referred to as entraining, and thereby increases the volume of air moving over the heat sink. According to Nuventix, this airflow also improves heat transfer.

Figure 1: Nuventix SynJet significantly improves airflow.

The SynJet is in production and readily available for luminaire designers to employ.

Graphite foam for cooling

Another advance in LED thermal management comes from the U.S. Department of Energy's Oak Ridge National Laboratory, which has developed graphite foam that wicks heat away from an LED lamp, reducing operating temperatures by 10 degrees or more.

This particular LED thermal management advance is aimed at large outdoor lighting systems, such as those used by municipalities for roadside lamps or by businesses in parking lots and structures.

The foam's graphite crystal structure contains a combination of internal air pockets and networked ligaments that wick heat away from the LED in a fashion similar to a more conventional heat sink. Graphite foam is light and porous with 25 percent density, making it easy to machine into heat sinks, but with the superior thermal conductivity afforded by the pure-carbon material (compared to conventional metal heat sinks).

This technology is already being employed in some designs and may spark research into similar material science solutions for LED thermal management.

Summary

High temperatures can both shorten the life and impact the brightness of LEDs. Studies have shown that decreasing the operating temperature by 10 degrees can double the lifetime of LEDs. As a result, virtually all luminaires require some type of heat-sink or other thermal management technology. This article has examined junction temperature, discussed how to measure thermal resistance and power dissipation, and presented several solutions for reducing LED operating temperatures.