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Posted on 29 June 2019

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Liquid Cooling Meets Advanced Requirements of Power Electronics

So far nobody has succeeded in designing an electronic component with a 100% efficiency, so power that is lost during operation is transformed into heat. In order to avoid exceeding the acceptable temperature limit and subsequent destruction of the component, this heat needs to be removed as effectively as possible.

By Rainer Selisko, Thermal Management Sales & Technical Support, Mersen Germany and Austria 

 

Advanced power electronics are gradually reaching the limits of air cooling solutions. The trend to attain higher and higher power in more compact sizes requires efficient solutions, which can be achieved by liquid cooling. Possible misgivings based on the presence of water in a touchy electric environment are no longer an issue today; up-to-date technologies like vacuum brazing have dispelled these concerns. Today’s market offers users of power semiconductors a variety of technologies to choose from. However, the pros and cons for the individual application demand careful evaluation. Not only thermal performance, but also electrochemical, mechanic and hydraulic challenges have to be taken into consideration when designing the technologically ideal cooling plate.

Entry Level Technology for Liquid Cooling

Cooling plates with embedded pipe meanders are the so-called entry level technology for liquid cooling. S-shape copper tubes or, in exceptional cases, stainless steel tubes are mounted into an aluminum base plate. The most common version is a serpentine copper tube glued into a one-sided pre-machined base plate. The pipe bends may be positioned inside or outside the aluminum plate. These pipes are squeezed and milled on the surface to reach an even contact plane. The alternative is to insert the copper tubes into channels drilled centered and longitudinal into the plate, which requires brazing the connections after assembly. The thermal contact is achieved by expanding the tubes, for instance, by pulling a mandrel through them.

Cooling plates equipped with copper tubes can be manufactured at a competitive price and are used for low- to medium-high thermal requirements. Copper is an excellent heat conductor which also tolerates contaminated cooling liquids. The thermal resistance between the tube and the aluminum plate makes the cooling effect on the complete contact surface of the cooling element slightly inhomogeneous. When positioning the electronic components onto the cooling plate, the limited bending radius of the tubes has to be observed and this does not always support a space-saving layout. Although tubes made of stainless steel resist corrosion and aggressive substances pretty well, they do have a low thermal conductivity and are more difficult to work with. The meander can only be mounted into the aluminum plates from one side, otherwise the end turns would have to be welded for centered tubes. A special cooling plate design is manufactured by utilizing stainless steel tubes embedded in cast aluminum. Cast aluminum, however, has a slightly lower heat conductivity compared to extruded aluminum, and requires its own casting mold, depending on size.

Liquid Cooling Plates Utilizing Aluminum Profiles

If aluminum can be used in direct contact with the cooling liquid, a pure aluminum cooling plate sports considerable advantages. The heat transfer between the tube and the plate is eliminated, enabling the entire assembly to become lighter in weight. The basis is a profile with longitudinal drilled holes. This profile is then equipped with cross drillings to redirect the coolant flow, this requires plugging the unused openings. An alternative is to machine the direction change channels at the plate end and to close them with a welded-on cover. To reach better flow characteristics in the straight channels, turbulators are inserted in some cases. The biggest advantage is a simple, costeffective design. Obviously, on the other hand, some restrictions in the positioning of water channels, hot spots and mounting holes are inevitable. Design-based sharp corners in the channels may also cause a high pressure drop in the cooling circuit.

Milled Aluminum Cooling Plates

To optimize the cooling circuit according to the components’ positions and thermal requirements, designs equipped with milled water channels are an attractive solution. The cooling liquid can be directed exactly under the hot spots of the power semiconductors and around their mounting holes. A high flexibility is reached for the positioning of various components on one cooling plate, which at the same time serves as a support for the electronic elements. An exact cross-section for the desired coolant flow can be dimensioned, thus achieving an optimized liquid velocity and a minimum pressure drop. Correct designing of the channels creates optimized turbulences in the cooling liquid which in turn achieve the best possible heat transfer. Machined base plates must of course be closed with a cover; the simplest solution being to bolt it with a gasket. However, this entails a certain risk of leakage which increases with larger plates. A solid and tight joint can be achieved by welding the lid to the base; by classic gas-shielded, friction or laser welding.

Vacuum Brazing is First Choice

A completely homogeneous connection between the milled base plate and the cover plate can only be achieved through vacuum brazing. This technology combines all the advantages of the above-mentioned thermal and hydraulic properties with other merits such as an optimized heat transfer to both sides of the coolplate, extreme mechanical strength, absolute tightness and extremely long life. An aluminum plate with machined cooling channels is used as a base. The cover consists of two parts: the brazing sheet and the actual cover plate. The thin aluminum brazing sheet is plated on both sides with an aluminum alloy with a low melting point, positioned between the base plate and the cover. The entire stack is then fixed with a clamping device and heated in a vacuum furnace. The temperaturetime curve during the process demands a tight control in order to reach a completely homogeneous heat diffusion and a temperature between 580 and 600°C in the cooling plate. During the process the liquefied plating of the brazing sheet creates a completely homogeneous bond with the adjacent aluminum parts. Many years of development and experience in this field of technology enable Mersen to master this sensible brazing process to absolute perfection.

Cooling plates manufactured in a vacuum brazing process show an excellent heat transfer on both sides of the cooling circuit, making them the perfect choice for double-sided placement. Because of the consistent connection of each wall between the channels with the cover, complete sealing is achieved, not only to the exterior, but also between the individual channels. The resulting excellent high-pressure resistance gives certitude that the coolplate will not buckle when subjected to hydraulic shocks in the cooling circuit which could lead to destruction of the electronic components. Vacuum brazing technology can also be used to manufacture multi-layer coolplates in a single brazing run. Even very complex cooling circuits, for instance countercurrent circuits for a more homogeneous temperature distribution below one or more components, can be designed.

Vacuum-Brazed Cooling Plates for Standard Applications

Based on experience with a multitude of customer-oriented technical solutions, Mersen is in the process of developing a range of vacuumbrazed cooling plates for standard applications. The new standard product line will combine all the advantages of high-tech coolers for small and medium quantities at attractive prices. The first version that will be shortly available off the shelf is multifunctional and can be used for two IGBTs measuring 140 mm x 190 mm or three IGBTs 130 mm x 140 mm. All mounting holes are predrilled and G3/8 thread water connections are provided. The cooling circuit has been designed for countercurrent, achieving an excellent temperature distribution on the entire contact surface. The Rth for an IGBT 140 mm x 190 mm (heat transfer area 102 mm x 190 mm) reaches 14 to 4.5 K/kW, depending on the cooling liquid composition and flow rate.

Thermal, hydraulic and mechanic optimum: Design of a water cooling plate manufactured with Mersen vacuum brazing technology

Here is an application example for two IGBTs 140 mm x 190 mm, both emitting a power loss of 1000 W: using water with an inlet temperature of 40°C and a flow rate of 10 l/min, the plate surface only reaches a maximum temperature of 46°C on the hottest spot. The temperature difference underneath the semiconductors is less than 2°C, the pressure loss in the cooling circuit stays below 400 mBar.

The new generation of standardized water cooling plates for IGBTs offers an affordable high-tech cooling solution, even in small quantities

 

 

 

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