Power Electronics

Thin Cavity Cooling of Electronic Circuitry

Thin Cavity Fluidic Heat Exchanger, or TCFHE, is a simply constructed, small volume, high performance effective method for cooling critical electronic devices at the chip, board, module, and system levels. Its high thermal performance originates from a very high velocity air flow, a very thin boundary layer, and high air utilization. It can efficiently and reliably cool critical regions in electronics at very low cost, in a small space, with high design flexibility, and with no concern for air leakage causing damage. The TCFHE resolves the conflict in high-speed electronics to completely seal a compact enclosure for shielding and packaging purposes while also removing high heat loads. The device is technically positioned between fan and heat sink methods and water cooling methods of cooling. When fan and heat sink methods or packaging constraints cannot obtain the needed thermal performance, the TCFHE is the next best choice over water cooling.

Fig. 1. shows the cross-section for a basic TCFHE for a simple planar structure. It consists of a thermal load, heat transfer plate, a cavity spacer, a cover plate, a gas inlet, and a gas outlet. The cavity spacer can be a feature of the heat transfer plate or the cover plate instead of a separate item. Heat produced by the thermal load is conducted to a heat transfer plate, which can be planar, tubular, or another shape. Compressed air or other gas passes by the plate through a thin gap (<1-10 mils) at high speed and carries the heat away. At the expense of higher inlet pressure, a thinner gap provides better cooling, which is not casually obvious. Although heat sink fins or channels could be used in the gap, they are less efficient, less reliable, and more costly than a thin-gap spaced surface. The TCFHE structure can be embedded within a printed circuit board or IC package as well.

Test data for a prototype TCFHE was taken using cavity gap thicknesses of 3, 5, 10, and 20 mils (using paper). The prototype is shown in Fig. 2, Fig. 3, and Fig. 4 comprise three main pieces. A set of graphs were produced to measure cooling performance using a load power of 3.6 watts and an ordinary shop compressor for an air source. The cooling area was 1 inch long by 1.25 inches wide and air inlet temperature was about 29°C. The prototype was thermally insulated for worse case test purposes using bubble wrap. Electronic pressure gauges (plenum located), flow meter, and temperature sensors (not shown) were used for air measurements.

The lowest thermal resistance of 2.8 °C/watt was obtained for a cavity gap thickness of 3 mils, an air flow rate of 0.24 CFM, and an air inlet pressure of 23 PSI. This corresponds to an air velocity within the cavity of about 9,100 feet per minute (104 MPH), which is about 10 times faster than the fastest air velocity of fan and heat sink combinations. The decreasing difference between the outlet air temperature and the load temperature indicates that the incoming air heats up quickly from the load heat source. A longer cavity length or higher flow rate would not improve performance significantly. Thinner air gaps will result in even lower thermal resistance at the expense of higher required air pressure. Other prototype experiments have been close to Mach 1 with thermal resistance less than 1 °C/watt.

TCFHE technology is applicable from a micro to macro scale in power generation and distribution, defense, automotive, electronic, chemical, biomedical, and other industries. TCFHE structures can be used for air conditioning, refrigeration, combustion heaters, engines, motors, transformers, lighting, supercomputers, telecom, server farms, integrated circuits, and other devices. Building infrastructure capital and energy costs can be reduced when such products are used, due to remote location of the gas flow source and heat exhaust. The gas used could be air, nitrogen, oxygen, Freon, or similar gases at hot, room, or cold temperatures and can be direct or alternating pressure. It could be hot gas produced by internal or external fuel combustion or cool Freon gas from refrigeration systems. The gas flow could originate from a gas compressor, compressed gas tank, ducted fan or blower, combustion chamber, or other pneumatic source. The technology also works well with liquids.

Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, and Fig. 12 plot the TCFHE characteristics vs. air flow rate. Technology details can be viewed at the U.S. Patent and Trademark website, application 20100078155.

Contact Dr. Steve Morra at Third Millennium Engineering to discuss any technical details, applications, or business arrangements for using TCFHE technology in a product (972-491-1132, [email protected]). An evaluation kit based on the prototype is also available, as are consulting and development services.

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