Thermal Management and Heat Rejection for LED Cooling By Dr. Paul A. Magill, VP of Marketing and Business Development • Nextreme Thermal Solutions, Inc.
LEDs, with their order of magnitude improvement in energy efficiency, are being considered for use in a wide range of applications. They are used as low-energy indicators but also for replacements for traditional light sources in general lighting and automotive lighting. The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in communications technology.
From a lighting standpoint, LEDs present many advantages over traditional light sources including lower energy consumption, longer lifetime, improved robustness, smaller size and faster switching. One of the key advantages of LED-based lighting is its high efficiency, as measured by its light output per unit power input. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. A conventional 60 W to 100 W incandescent light bulb will produce around 15 lumens per watt, and standard fluorescent lights will produce up to 100 lm/W. LEDs have been produced with outputs as high as ~140 lm/W.
In contrast to their significant advantages, LEDs are relatively expensive and require more precise current and heat management than traditional light sources. The most challenging custom LED lighting design is that which contains an airtight enclosure or housing. Some applications restrict the use of enclosure ventilation or fans for forced air convection. When heat generated by the LED lights cannot escape, the sealed enclosure will begin to develop an oven effect. LEDs require a different type of thermal management system than has traditionally been used for incandescent or compact fluorescent lighting.
In addition to the issues of airtight enclosures, smaller packaging and higher heat densities, as the temperature increases, the LEDs may begin to suffer from thermal stress. The junction temperature can directly affect the performance and longevity of the LED. As the junction temperature rises, a significant loss of output (luminosity) can be expected. The forward-voltage of an LED is also dependent on the junction temperature and as the temperature rises, the forward voltage decreases, which can cause excessive current drain on other LEDs in the array. This in turn would lead to thermal runaway conditions and ultimately to the failure of the device. High temperatures can also affect the wavelength of an LED fabricated using gallium arsenide, gallium nitride or silicon carbide.
Today’s cooling systems take advantage of convection, conduction and/or radiation to move heat efficiently away from the heat generator. One of the problems with LEDs introduced as replacements for commercial lighting today is that there is no infrastructure for heat removal out of the back side of the light source; instead, conventional lighting relies on convection from the front side. High heat fluxes require cooling systems for a wide variety of devices and LEDs are no exception. Depending on the power levels of the LED, an active thermal management solution may be required.
A thermoelectric cooler (TEC) is one such solution that uses the Peltier effect to create a heat flux between the junction of two different types of materials. In essence, a thermoelectric cooler is an active solid-state heat pump that transfers heat from one side of the device to the other side against the temperature gradient (from cold to hot), with consumption of electrical energy. Thin film TECs allow the heat removal system to be scaled to the device size.
To put the scope of the problem in perspective, consider the current cooling solution for computer chips. A 3 GHz Pentium IV computer chip generates 70 W to 100 W of heat over a 35 mm by 35 mm footprint, requiring dissipation of about 8 by 104 W/m². The specifications for cooling an LED package require dissipation of 6 by 106 W/m², or nearly 100 times the flux found in today’s computer heat sinks.
Cooling of this magnitude is a challenge for all but thin films TECs; however, the power consumption necessary to cool the entire area would not be practical. So a philosophy of cooling only what needs to be cooled has been developed. In the case of CPUs, the hot spot can be cooled by placing the TEC as close to the heat source as possible. In other words, cool the device not the package.
Cooling only the device means that the TEC must be sized as close as possible to the device size. Thin film TECs allow the user to shrink the package and cooling system by increasing the cooling power of the TEC. While this is an advantage, the problem of where to dump the heat still remains. For LEDs, this implies that the thermal management system must be looked at in its entirety.
The heat equation is an important partial differential equation describing the distribution of heat (or variation in temperature) in a given region over time. For a function u (x,y,z,t), which is a measurement of the temperature T, of three spatial variables (x,y,z), and the time variable t, the heat equation is
u = u(t, x, y, z) is temperature as a function of time and space; or equivalently
where k is a constant.
The other important equation governing is,
Q = U + W
In thermodynamics, the change in heat is equivalent to the heat flowing into the system. Combining these two equations and in the absence of any work being performed Q, is proportional to the change in temperature.
Q = kΔT
Hence for passive systems, cooling by the conduction of heat is a linear function of temperature and a constant related to the material properties of the solid. This constant k may be a function of many variables including T, P and V.
Active Cooling and Package Size If it is our desire to continue to shrink the overall size of our devices, we must shrink the size of the thermal management system. Given that passive heat removal is only a linear function over distance of the temperature difference, we must put work into the system to obtain a greater rate of cooling and hence a smaller device.
An example of cooling LEDS with thermoelectrics is shown in Figure 1. Integrating the TEC in the package lowers the LED junction temperature by some 20°C.
Active thermal management devices such as, thermoelectric coolers (TECs) have been employed to provide this additional heat pumping. An example of the type of heat pumping offered by a TEC for cooling below ambient is shown in Figure 2.
One of the drawbacks to using TECs is that, in doing work to move the heat in an almost discontinuous manner, more heat is added to the system. This heat will be added downstream of the device to be cooled but will be added to the overall system heat that needs to be rejected at the next level.
If TECs are applied for cooling in the same manner as passive thermal elements (in other words - everywhere), we will end up with a larger problem than the one we solved. A more cost-effective and efficient approach—and one that is only possible with TECs—would be to cool only what is necessary. In other words, scale the thermal management system to the size of the heat problem.
Active Thermal Management Must be Employed Close to the Heat Source
Since heat in the passive case flows linearly, any material between the TEC and the heat source will have a temperature drop across it. This will act to increase the temperature difference that TEC must pull. As already mentioned the TEC is a heat pump doing work (moving heat) in a manner whose efficiency depends upon the temperature difference it is required to generate. Minimizing this temperature difference will improve the efficiency of the TEC and reduce the additional heat added at the system level.
Today, TECs are manufactured using two methods. Bulk TECs are manufactured using pressed sintered powders; thin film TECs are manufactured using semiconductor manufacturing processes. Each has advantages, but the most obvious difference is that the thin film TECs are 10 to 20 times smaller in each linear dimension and can be sized to almost any scale
One of the drawbacks to using TECs is that, in doing work to move the heat, in an almost discontinuous manner, waste heat will be added to the system. This heat will be added downstream of the device to be cooled and will passively flow out of the system. The flow of heat through a material system per unit time Q/Δt = kΔT/Δt may be viewed in an analogous manner to that of current flowing through an electrical circuit V=IR. Here the heat flow is analogous to the current.
The implication is that the thermal resistance from the TEC to ambient will limit the amount of heat that can be pumped out of the package enclosure. If the thermal resistance is too high, then too much of the heat will go into a temperature drop across the package and will raise the temperature of the hot side of the TEC (TA in Figure 2) causing the entire device to float up in temperature.
A good heat rejection path is important for thermal management regardless of whether you are using an active or passive thermal management system. The heat rejection, as it states, is the path you intend to move the heat along until you finally dump it into the ambient surroundings or into a thermal reservoir. The more resistive this path is to the flow of heat, the greater the temperature rise that will occur as heat is lost into this material. Conversely, the larger the heat load you have to move, the lower the thermal resistance of your heat rejection needs to be. Failure to have appropriately designed your thermal rejection path relative to the heat load will result in unacceptably high temperatures back at your device.
Summary
TECs have long been used in the telecom industry for solving thermal management issues. In the case of LEDs, while these optoelectronic devices have been increasing in functionality and their packaging has decreased in size, the scalability of the TECs has been slow to follow. The use of thin film TECs in heat rejection solutions offers the kind of scalability required in an industry driven by size and cost.
Dr. Paul Magill is the VP of Marketing and Business Development for Nextreme, Inc. Paul has more than 20 years of experience in the electronics and optoelectronics industry with expertise in sensors and laser diode applications as well as electronics and MEMS packaging and manufacturing. He is an experienced entrepreneur who has a track record in founding and building technology companies including Unitive Electronics Inc., Optical Process Automation and, more recently, Avo Photonics. Paul holds a BS and Ph.D. in Physics from the University of North Carolina. Paul can be reached at pmagill@nextreme.com.
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Thermal Management and Heat Rejection for LED Cooling
One of the drawbacks to using TECs is that, in doing work to move the heat in an almost discontinuous manner, more heat is added to the system. This heat will be added downstream of the device to be cooled but will be added to the overall system heat that needs to be rejected at the next level. 