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Kinetic Cooling for Thermal Management of High Power Electronics
Keywords: KineticCooling, Fan Heat Sink, Technology
The fan-cooled heat sink is the most widely used active-cooling solution in the electronics industry. As shown in Figure 1(a), the cooling architecture uses two principal components: a finned heat sink to spread heat from a chip (or other heat-generating components) across a large surface area and an air mover (e.g., fan or blower) to blow cooling air past the heat-sink fins. A third component – copper heat pipes – is typically used to aid heat transfer to (or within) the finned heat sink in applications where heat needs to be transferred or spread across relatively large distances. The resulting architecture typically uses an air mover that occupies a significant portion of the overall cooler volume, quite often over 50% of the cooling volume. Although generally acceptable when there is sufficient space available for the cooling solution, fan-sinks that use large air movers are not suitable for space-constrained applications. Kinetic Cooling addresses the limitation of traditional fan-sinks by making the air mover thermally conductive and integrating it into the heat transfer path. This is shown in Figure 1(b), where a Kinetic Cooling heatsink is shown. In the Kinetic Cooling architecture, an additional heat-transfer path is introduced: a portion of the heat is transferred into the rotating heat-sink impeller, the Kinetic Cooling Engine, across a small air-gap region. The rotating metallic blades of the heat-sink impeller generate cooling airflow which cools both the metallic rotating blades as well as the stationary metal fins around the perimeter. Figure 1: Comparison on conventional fan-sink with kinetic cooling By including the air mover into the heat-transfer path, the ratio of heat-transfer surface area to overall cooler volume can be increased as much as 2X compared to the traditional fan-sinks. The thermal conductance (inverse of thermal resistance) of the conceptual cooler in Figure 1(b) can be approximated as where RT is the total thermal resistance of the cooler, RS is the thermal resistance of the stationary portion of the cooler (heat pipes + metal stationary fins), RR is the thermal resistance of the rotating portion of the cooler (baseplate spreader and Kinetic Cooling Engine), and the last term accounts for preheating of the air by the rotating component before the air is used to cool the stationary fins. The preheating term is smaller in magnitude than RS, meaning that RT < RS (i.e., the thermal resistance of the Kinetic Cooler is lower than the thermal resistance of the traditional fan-sink. By being able to achieve a lower thermal resistance at a given cooler volume and airflow, Kinetic Cooling provides favorable trade offs between three critical performance variables: thermal resistance, acoustics, and volume. The overall enhancement in performance of a given kinetic cooling system over a similarly sized fan-sink is dependent on two factors, the enhancement of the thermal characteristics of a spinning heat sink over a stationary one coupled with the resistance to heat flow from the baseplate to the impeller. Figure 2 shows the effect of a spinning heat sink over a stationary heat sink for the same airflow generated over the heat sink. For the stationary heat sink a 10.6CFM was forced through the impeller and for the spinning heat sink 10.6CFM was induced by the rotation of the impeller. The adjacent table shows the performance characteristics of the two situations. At the same total CFM flowing through the heat sink, the spinning heat sink dissipates nearly 40% more heat than a stationary fin. This indicates that just populating the central portion of the heat sink in Figure 1(a) with stationary fins and driving airflow through it is not necessarily as effective as using a kinetic cooling solution. The second issue to consider in a kinetic cooling solution is the resistance of heat transfer to the spinning heat sink from the baseplate. This will be dealt with in the full presentation. Additionally, examples of system level design will be shown where the kinetic cooling solution offers significant performance/acoustics/size advantages over conventional forced air solutions.
Raghav Mahalingam, Vice President Business Development and Product Strategy
CoolChip Technologies, Inc.
Somerville, MA
USA


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