Heat pipes get the heat out of multikilowatt electronic devices

Heat pipes are basically vacuum-sealed tubes. Yet they keep high-power electronics cooler than traditional heat sinks.

Power semiconductors are encountering the same thermal control situations experienced by the microprocessor industry in the early 1990s. As microprocessor speed increased, so did the amount of heat generated by the microprocessor chip. Depending on the particular application, the heat released ranged from 10 to 150 W. For cost savings, this heat is typically rejected to ambient air. Conventional cooling approaches, such as extruded heat sinks, placed under constrained conditions were insufficient to meet the ever-growing cooling needs of the micro-processor. The computer industry found a solution using heat pipes. Today, heat pipes are used extensively for cooling microprocessors in laptops, desktop computers, high-performance servers, and workstations.

In its simplest sense, a heat pipe is a heat mover or spreader. It takes heat from a source, such as power semiconductors, and moves or spreads it to a region where it’s more readily dissipated. The heat pipe moves this heat with a minimal drop in temperature. A typical heat pipe is a sealed and evacuated tube that contains a porous wick structure and a very small amount of working fluid. The porous wick is typically a sintered-powder metal that lines the internal circumference of the tube. The central core of the tube is left open to permit vapor flow.

Each heat pipe has three sections: an evaporator, an adiabatic, and a condenser. As heat enters the evaporator section, it is absorbed by vaporizing the working fluid. The generated vapor travels down the center of the tube through the adiabatic section to the condenser section where the vapor condenses, giving up the latent heat acquired during vaporization. The condensed fluid is returned to the evaporator section by gravity or by capillary action in the porous wick structure. Heat-pipe operation is completely passive and continuous. This makes heat pipes quite reliable as there are no moving parts to fail.

In comparison to microprocessors, the heat released from large power semiconductors, such as IGBTs, diodes, or thyristors, can range into multiple kilowatts — a factor of 10 to 30× more heat. As electronic packaging trends towards smaller and lighter assemblies, ridding multiple kilowatts of heat from power semiconductors to ambient air becomes more of a challenge.

For example, two groups of six IGBTs each generate 6 kW of heat. One group is attached to a state-of-the-art aluminum extruded heat sink while the other group uses a heat-pipe heat-sink assembly. Both heat sinks operate under the same performance conditions and were modeled using Mentor Graphics FloTherm computational fluid-dynamics (CFD) software. The large extruded heat sink was 42-in. long × 24-in. wide × 3-in. high. Correspondingly, the heat-pipe assembly was 42-in. long × 11-in. wide × 9-in. high. Airflow rate and temperature at the cooling inlet was 600 cfm at 40°C. The goal was to create a small, air-cooled, and lightweight heat-sink package that kept the IGBTs within their operating temperature limits.
The alternative is to use a liquid-pump loop system that ultimately transfers the heat to air. Most companies try to avoid this scenario due to reliability, maintenance, and cost issues.

Heat-sink descriptions
The aluminum heat-sink profile is a state-of-the-art large-profile extruded heat sink with the highest aspect ratio available. Consequently, the implied advantage is improved thermal control. The heat-sink profile measures 24-in. wide × 42-in. long × 3 in. high. The fin pitch is 2.5 fins/in. Each fin was 0.08-in. thick with a base thickness of 0.67 in. The weight and volume of this heat sink is 151 lb and 3,024 in.3, respectively.
The heat-pipe heat sink uses standard 0.75-in.-diameter heat pipes embedded in an aluminum plate under the power semiconductors. The pipes extend from the plate to a remote fin stack. The 11 heat pipes absorb heat from the electronics and transport it to the plate fins. The fins are cooled by forced convection. The aluminum mounting plate is 17-in. long × 12-in. wide × 0.98-in. thick. The fin stack is 19.3-in. long × 10.8-in. wide × 9-in. deep. The plate fins are 0.02-in. thick with a fin pitch of 10 fins/in. This heat sink weighs 70 lb and occupies an overall volume of 2,200 in.3

Heat-sink CFD analysis
Six 5 × 5-in. IGBTs, handling 1 kW each, were applied to each heat sink abutting the others in a two by three array. A 0.003-in. layer of interface material (with a k =1 W/m‑K) was assumed between the IGBT array and the heat sink. In each case, a 40°C ambient temperature and a volumetric flow of 600 cfm was used. The air was fully ducted through each heat sink.