Dr. Richard Bonner, Anqi (Angie) Fan, Daniel Pounds and Dr. Ehsan Yakhshi-Tafti; Advanced Cooling Technologies, Inc.
INTRODUCTION
In light-emitting diodes (LEDs), 70 percent to 80 percent of the applied electrical power is converted to waste heat. If this thermal energy is not properly dissipated, the resulting high operating temperatures lead to reduced brightness, shift in wavelength (color), and reduced life. Optical requirements impose the heat to be extracted through circuit boards designed to balance contradictory thermal management and electrical isolation functions [1-5]. In most general lighting applications, the heat must be ultimately be dissipated through natural convection heat sinks (where radiation may play a significant role) due to acoustic noise restrictions [6-14]. However, the thermal management requirements of emerging high intensity LED lighting products often exceed the practical limits of these passive cooling strategies. In the broader electronics cooling industry, heat pipes have been effectively applied as a means to extend the application window of passively cooled heat sinks through heat spreading enhancement. Although the general guidelines for integrating heat pipes in broader electronics cooling applications still apply to LEDs, their unique thermal requirements require adaptation in the implementation of heat pipes. This article explores the implementation and benefits of heat pipes in natural convection heat sinks and circuit boards for LED applications.
Although each LED may only dissipate a few watts of thermal energy, the small package footprint results in high heat fluxes. Despite heat spreading in the LED packages and circuit boards, these heat fluxes remain significant as the heat is dissipated across the various thermal interfaces between the LED package, circuit boards, and attached heat sinks [15-19]. By embedding heat pipes directly into circuit boards, the thermal interface between the circuit board and heat sinks can be minimized while heat spreading can be enhanced. At the heat sink level, heat pipes may be able to reduce the volume for natural convection heat sinks by 10 percent to 30 percent. Further, the luminaire design may dictate a remote heat sink due to other design considerations, such as aesthetics. In these cases heat pipes enable thermal connection of the remote heat sink to the LED circuit board package with minimal thermal gradient.
NATURAL CONVECTION HEAT SINKS
Heat pipes can be effectively embedded in a variety of heat sink form factors to improve conductive heat spreading. To clearly illustrate the benefits of using heat pipes, a radial heat sink with a heat pipe embedded in the core is chosen as a model case as illustrated in Figure 1. In general, heat pipes tend to have increased benefits as the length (z) of the heat pipe is increased. This is attributed to the low thermal resistance of the heat pipe’s vapor core, which is often negated when compared to other thermal resistances in the system. As a result, the heat pipe’s thermal resistance does not change significantly as the heat sink length is increased. To demonstrate this point, thermal finite element analysis was performed on a typical aluminum extrusion radial heat sink with and without embedded heat pipes (see dimensions in Figures 1 and 2). Heat was applied in a 1cm2 circular region on one side of the base of the heat sink. In a series of simulations, the length of the heat sink was increased while prescribing a heat transfer coefficient associated with natural convection (10 W/m2K). Radiation was not considered in the calculations. The simulation results are summarized in Figure 2, which shows the net reduction in thermal resistance (including the heat sink conduction and convection resistance) between the embedded heat pipe and baseline aluminum case. The improvement with heat pipes approaches 30 percent for heat sinks less than 30 cm long.
The results of Figure 2 are indicative of the performance improvement when the circuit board is directly connected to the base of the heat sink. Heat pipes show more dramatic improvement when the heat sink is remotely located from the circuit board. Figure 3 shows an infrared (IR) image of two heat pipe heat sinks operating in natural convection conditions with 30 Watts of applied heat. A FLIR A655sc camera with +/-2°C accuracy, 640 x 480 resolution, and a spectral range of 7.5μm to 14μm was used. All surfaces on the heat sink were coated with Medtherm optical black coating (specified to a 0.95 average absorptance over a 0.3 μm to 15μm spectral range). The photograph on the left shows the thermal gradient in a 30 cm long heat sink (the other dimensions are identical to those simulated in Figure 2). The increased thermal gradient near the base of the heat sink is noticeably higher than the opposite side of the heat sink. This is attributed to the evaporator thermal resistance of the heat pipe that must be overcome before heat is transferred into the highly conductive vapor core. After the heat is acquired in the first 10 cm of the heat sink’s length, the heat sink is nearly isothermal due to the low thermal resistance of the heat pipe vapor space. On the right, the same heat sink is remotely attached to the heat source. The heat pipe clearly demonstrates the transport of heat isothermally from the heat source to the heat sink and apply even distribution of heat to the heat sink. A slight increase in temperature is measured across the heat sink (< 0.5°C), due to the sensible heating of air rising through the heat sink.
METAL CORE PRINTED CIRCUIT BOARDS
Heat pipes can also be applied at the circuit board level to improve heat spreading performance [20]. There are several strategies for improvement. One strategy is to fully contain the heat pipes within the printed circuit board, as shown in Figure 4. If the ratio of the heat source to circuit board areas is sufficient, this can be an effective way to improve heat spreading at the board level while requiring minimal design changes to the lighting system. Figure 5 shows an IR image of a fully integrated LED/ circuit board/heat pipe assembly during operation.
Another closely related implementation strategy is to simply extend the heat pipe from the circuit board and directly attach the heat pipes to a heat sink. By directly connecting the circuit board to a heat sink using heat pipes, the thermal interface between the circuit board and heat sink is avoided. The improvement in thermal performance is very application specific, and depends on many of the factors described in the natural convection heat sink discussion.
CONCLUSIONS
LED lighting products have unique performance and thermal management challenges that must be addressed when implementing heat pipes. Heat pipes can silently and passively improve thermal performance, limit heat sink size, and increase reliability in LED applications. In natural convection radial heat sinks less than 30 cm in length, reductions in thermal resistance approaching 30% can be obtained by simply embedding heat pipes into the heat sink core. Heat pipes can also increase packaging flexibility by enabling remote heat sink location. Finally, overall thermal performance can be improved by directly attaching heat pipes to a circuit board. The performance increase is due to more effective heat spreading and reduction of thermal interfaces.
ACKNOWLEDGEMENT
This material is based upon work supported by the Department of Energy under Award Number DE-SC0004722. Special thanks to Jon Mott for providing valuable technician support for this work.
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Dr. Richard Bonner is currently the Manager of Custom Products at Advanced Cooling Technologies, Inc. (ACT), where he is responsible for developing new thermal management products. In 8 years at ACT, Dr. Bonner has led over a dozen government funded advanced thermal management programs on topics including heat pipes, dropwise condensation, thermoelectrics, junction level cooling, micro-channel cooling, and thermal storage. He has published over 20 papers and is currently the principal investigator on a Department of Energy funded program to investigate advanced heat pipe concepts for solid state lighting applications. Dr. Bonner has a B.S., M.S, and Ph.D. in chemical engineering from Lehigh University.
Anqi (Angie) Fan is a senior R&D engineer in the Electronics Products group at ACT. She received a M.S. in mechanical engineering from Univeristy of Toledo with research focused on computational fluid dynamics and heat transfer. Ms. Fan has 9 years of experience in thermal management and electronics cooling. At ACT, she has worked on various SBIR and commercial projects to develop advanced cooling solutions by performing thermal mechanical and electrical modeling, and experimentation. Recently, she was funded by the Army to develop coupled electrical and thermal model for wide bandgap electronics.
Daniel Pounds is a R&D Engineer in the Custom Products group at ACT. Mr. Pounds’ five-year research background includes heat and mass transfer modeling, two-phase fluid dynamics modeling and advanced level thermodynamics. Mr. Pounds currently holds a Masters and a Bachelors of Science in Mechanical Engineering from the University of Missouri – Columbia.
Dr. Ehsan Yakhshi-Tafti is a R&D Engineer at ACT. He received his Ph.D. in Mechanical Engineering from the University of Central Florida where he received the Award for Best Doctoral Dissertation. He has authored more than 15 scientific and technical papers in indexed journals and international peer-reviewed conferences, and holds 1 US patent. Most recently he has been working on developing novel thermal management systems (passive and active) for high heat flux electronics.
