Heat-Pipe/Heat-Sink Technology Improves 6-kW Cooling

The heat generated from large power semiconductors such as IGBTs, diodes, or thyristors can be as high as several kilowatts. While conventional extruded heat sinks have been one of the most widely used solutions for cooling high-power semiconductors, removing the heat created with current and future devices may require an alternative solution.

Coupling tried-and-true heat-sink technology with a heat pipe is one alternative to traditional heat-dissipation solutions. Another technique is to use a liquid pumped-loop system that ultimately rejects the heat to air. However, this solution typically suffers from reliability, maintenance, and cost issues.

A heat pipe, in its simplest sense, is a heat mover or spreader. It acquires heat from a source, such as a power semiconductor, and moves or spreads it to a region where it can be 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 with a porous wick structure and a very small amount of working fluid on the inside (Fig. 1). The porous wick structure, such as sintered powder metal, lines the internal diameter of the tube. The center core of the tube is left open to permit vapor flow.

The heat pipe has three sections: the evaporator, adiabatic, and condenser. As heat enters the evaporator section, it is absorbed by the vaporization of the working fluid. The generated vapor travels down the center of the tube through the adiabatic section to the condenser, where the vapor condenses and gives up its latent heat of vaporization. The condensed fluid is returned to the evaporator section by gravity or by capillary pumping in the porous wick structure. Heat-pipe operation is completely passive and continuous. There are no moving parts to fail, so a heat pipe is very reliable.

Looking at a grouping of six IGBTs that generate 6 kW, we will compare the performance from a state-of-the-art aluminum extruded heat sink against that of a heat-pipe/heat-sink assembly in the same conditions. Both cooling techniques were modeled using Flotherm CFD software. Each IGBT assembly has a 5- × 5-inch footprint and generates 1 kW. The IGBTs were arranged in a 2 ×3 array (Fig. 2). A 0.003-inch-thick layer of interface material (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.

HEAT-SINK COOLING

The aluminum heat-sink profile that was evaluated represents a large-profile extruded heat sink with a high aspect ratio. Consequently, the implied advantage is improved thermal control. Measuring 24 (W) × 42 (L) × 3 (H) inches, the heat sink featured a fin pitch of 2.5 fins per inch.

Each fin was 0.08-inches thick and the base thickness was 0.67 inches. The heat sink weighed 151 lbs. and had a volume of 3,024 in.³.

Using Flotherm, an idealized “wind tunnel” was constructed around the extrusion. The upstream and downstream surfaces were open for passage of air through the domain. A uniform 600-cfm flow of air at 1 atm pressure and 40°C was specified on the upstream surface.

The remaining sides were made to coincide with the corresponding surfaces of the extrusion. These faces were given a symmetry boundary condition.

The computational software domain was broken down into 478 × 79 × 11 cells. Five cells were used to resolve the flow profile between each pair of fins. The extrusion base thickness was broken down into five cells. Using these parameters, the maximum temperature attained under the IGBTs was 130°C, well beyond the temperature limitations of the IGBTs. Therefore, the extruded heat sink cannot meet the required thermal performance.

HEAT PIPE/HEAT SINK COOLING

Next, the IGBT cooling system employed a heat-pipe/heat-sink assembly (Fig. 3). Standard 0.75-in. (diameter) heat pipes are embedded in an aluminum plate under the power semiconductors and extend from the plate to a remote fin stack. Heat from the electronics is absorbed by eleven heat pipes and transported to the plate fins, which are cooled by forced convection.

The aluminum mounting plate measures 17 (L) × 12 (W) × 0.98 (thick) inches. The fin stack is 19.3 (L) × 10.8 × (W) × 9 (D) inches. Plate fins are 0.02 inches thick and the fin pitch is 10 fins per inch. This heat sink weighs 70 lbs. and occupies an overall volume of 2,200 in.³.

In this application, the IGBTs have a maximum junction temperature of 125°C and a package thermal resistance of 0.04°C/W [4]. Using this information, the case temperature, TCASE, for the IGBTs is 125° - 40°C = 85°C.

If the incoming ambient air is 40°C, the remaining temperature drop to dissipate the heat is T_CASE - T_{AMBIENT AIR} (85° - 40°C = 45°C). This information was used as input for the following computational fluid dynamics (CFD) analysis. In all situations, the selected mode options included steady turbulent flow and conduction, negligible radiation heat transfer, and negligible buoyancy effects. Air properties varied with temperature.

The heat-pipe assembly consisted of two sub-models: the fin pack and the heat-input IGBT mounting plate. In addition, each heat pipe was constructed from two components:

1. A high-conductivity (k = 50,000 W/m-K) cuboid represented the high effective pipe conductance along the pipe's length.

2. A thin enclosure was specified to coincide with the boundaries of the high-k cuboid. The effective thickness and thermal conductivity of this enclosure were chosen to represent the thermal impedance associated with radial transfer of heat into or out of the pipe across the pipe/fin or plate interface, pipe wall, and liquid-saturated wick structure.

IGBT MOUNTING-PLATE SUB-MODEL

The fin sub-model was solved first; each heat pipe was assumed to be carrying an equal share of the total heat load. This sub-model yielded the operating temperature of each heat pipe. These temperatures then served as boundary conditions to the mounting-plate sub-model.

Again, an idealized wind tunnel was constructed around the fin pack. The upstream and downstream surfaces were open for the passage of air through the domain. A uniform 600-cfm flow of air at 1-atm pressure and 40°C was specified on the upstream surface. The remaining sides were made to coincide with the corresponding surfaces of the fin pack. These faces were given a symmetry boundary condition. The fin-pack domain was broken down into 46 × 43 cells in the plane of each fin. Five cells were included between each pair of fins.

The domain boundaries coincided with the boundaries of the chill plate. A symmetry condition was applied to all boundary surfaces, except for the one through which the heat pipes would pass. On this surface, temperature boundary conditions were applied to each heat pipe based on the results of the fin sub-model. The domain was separated into 46 × 43 × 9 cells.

Overall, the results of the CFD analysis of the heat-pipe assembly (representing the IGBT mounting plate surface temperatures) show very uniform temperatures over the entire mounting plate as shown in Fig. 4. This is expected because of the nature and performance of heat pipes. Any temperature variation is associated with the location of the heat pipe within the fin stack, i.e. the air is being heated as it passes through the fin stack.

The maximum temperature under the IGBTs is 80°C, well under the temperature limits of the IGBTs. The CFD analysis indicates that the heat pipe assembly achieves the desired cooling results.

The heat-pipe assembly was tested and the results were compared to the predicted CFD results. It was not necessary to test the extruded heat sink because Thermacore, under numerous occasions, showed that their modeling approach for extrusions yields results within a 5% accuracy.

RESULTS

In the heat-pipe assembly test, six heater blocks simulated the high-power IGBTs. Thermocouples were mounted under each heater block to measure the temperature of the plate. Other thermocouples were mounted in the airflow stream for calorimetric purposes. A steady-state heat load of 6 kW and 40°C ambient conditions were maintained. Airflow through the fin stack was achieved with two 300-cfm, 48-Vdc brushless fans. The acoustical noise rating associated to these fans was 57 dBA, below the Bellcore specification for acoustical noise suppression rating of 65 dBA^[5].

REFERENCES

At these test conditions, the average measured block temperature was 76°C, which yields a thermal resistance of 0.006°C/W for the entire heat sink. In comparison, the CFD results for the heat-pipe assembly agreed to within 5% with the measured results. Consequently, the test results verified the validity of the modeling approach.

1. The heat-pipe/heat-sink assembly was able to effectively dissipate 6 kW of heat and keep the IGBTs below their rated temperature limits. In comparison, the heat-sink extrusion alone could not meet the cooling requirements.

2. The entire IGBT mounting plate for the heat-pipe assembly is very uniform in temperature (approximately 10°C variation). Any temperature variation is associated with the location of the heat pipe within the fin stack, i.e. the air is heated as it passes through the fin stack. Uniform IGBT cooling is an important parameter when considering the effect of temperature on the quality of the output waveform.

3. The heat-pipe unit weighed 70 lbs., 81 lbs. lighter than the heat-sink extrusion.

4. The heat-pipe assembly has an approximate volume of 2,200 in.³, 27% less than the heat-sink extrusion's 3,024 in.³.

5. Heat-pipe technology allows designers to reject multiple kilowatts of heat from power semiconductors directly to ambient air, an important conclusion considering an alternative is a liquid-pumped loop system that has inherent long-tem reliability (leaks), maintenance (pump failure, fluid cleanliness, filtering), and corresponding cost issues.

6. Fan or blower noise is becoming an issue with cooling power semiconductors. The heat-pipe assembly used a dual-fan-pack solution that achieved an acoustic level of 57 dBA, below the 65-dBA ratings listed in the Bellcore specification for acoustical noise suppression.

1. Dunn, P.D., Reay, D.A., Heat Pipes, Fourth Edition, Pergamon Press, Copyright 1994 Elsevier Science Ltd.

2. Silverstein, Calvin, C., Design and Technology of Heat Pipes for Cooling and Heat Exchange, Hemisphere Publishing Corporation, Copyright 1992.

3. Faghri, Amir, Heat Pipe Science and Technology, Copyright 1995 Taylor & Francis.

4. Powerex Product Selector Guide, Publication No. 12A-200, HP-10K-12/99.

5. Bellcore Specification, Generic Requirements for Electronic Equipment Cabinets, GR-487- CORE, Issue 1, June 1996, Section 3.29.