Don Noren/Noren Products

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Heat pipes combined with a heat sink permit a power module to remain sealed while fins exposed to ambient air remove waste heat.

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It is very difficult to cool high power "power modules" and TO-3 transistors in sealed enclosures that protect them from dirt and other contaminants. One effective cooling method is heat pipes combined with a heat sink that allows the module to remain sealed while fins are exposed to ambient air to remove waste heat. Use of heat pipes offers an improvement over other classical methods of heat removal.

One classic approach to cool high power loads is to use an oversized enclosure with a bulky heat sink. Another way is an internal sink chimney relying on the outside airflow though the chimney. A third method is a massive extruded heat sink placed on the back of the cabinet wall. Other approaches include an air conditioner attached to the enclosure or water cooling. Each of these oversized solutions are difficult to apply and cause hidden expenses.

Heat pipes are a good solution to this difficult cooling problem. Heat pipes have an inherent thermal conductivity over a thousand times that of copper due to the boiling and condensing fluid on the inside. This thermal conductivity of heat pipes, allows the designer to provide a cooling solution with a smaller, lighter-weight package.

We considered several physical arrangements within an enclosure to determine the optimal packaging for performance and function. Initial surveys showed that the top of a cabinet is usually available to accept the heat sink. In addition, the heat pipes could be made more efficient and less expensive in this position.

The easily assembled arrangement is shown in Figure 1 . Here, a flange holds a set of finned heat pipes that enter the enclosure and terminate into a flat input plate. The flange seals into the enclosure and is leak tight. The fins are a high density, efficient design surrounded by a fan housing with a built-in fan. 

Figure 2 shows the thermal performance with various power inputs. The temperature rise at the input pad was 28.5°C above the ambient 20°C air with a 1500W input. This is s a very low heat sink thermal resistance of only 0.19°C/W.

In Figure 3 we see that the thermal resistance of the unit decreases with higher ambient temperatures. This occurs because the boiling and condensing heat transfer coefficients of the working fluid in the heat pipe are a function of the pressure inside the heat pipe. As the heat pipe pressure (and temperature) increases, the coefficient decreases to make the heat pipe much more efficient. This increased efficiency balances well with electronics that require better cooling at higher ambients.

The placement of the input pad has less than 1.4°C difference in the temperature of the components, as shown in Figure 4 . The bottom of the input pad is the coldest due to some flooding in the heat pipe and the center is the hottest due to minimum fluid wetting. The top of the heat pipe receives the cold return fluids from the condensing fins and remains cooler than the middle.

We designed the unit for use with an integrated fan and studied its performance at higher airflows. Figure 5 shows that at 700 CFM of air, the DT drops to 11°C, resulting in a thermal resistance of only 0.011°C, which is 57% better than with a 180 CFM fan.

To use Figure 5 efficiently, the pressure drop in the fin stack was measured and reported. Figure 6 shows that the pressure drop at 700 CFM is 0.6" of water. This is obviously for a low-cost blower.

Finned heat pipes are very efficient and have an inherently low cost per watt dissipated, i.e., less than 20¢/W. Their use simplifies the overall cabinet and reduces costs. The portion inside the enclosure required for the input of the heat pipe is only 18² in. This arrangement allows use of a much smaller enclosure, making the final system very compact and less expensive. Also, the input pad with flange and fins provides a predetermined performance.

Most other systems require heat sinks mounted inside enclosures where they only have increased air temperatures to cool them. In our systems, the heat pipe fins are directly outside where the cooler ambient air cools them directly, making the system much more efficient and at no increase in cost. This approach removes heat inside the enclosure so that the high-powered dissipating units do not increase the cabinet's ambient and affect other more temperature-sensitive components inside the enclosure.

SPECIFICATIONS