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Posted on 01 August 2019

Water Coolers for New Converter Systems

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These converter systems require new customized cooling solutions

Increasing power density is one of the main tasks developers of new power converter systems face. When modern more powerful IGBT modules are used in wind, large industrial drive, hybrid vehicle and other applications, engineers also need to adjust the water cooling system. Type and number of modules, thermal, fluid dynamic and mechanical specifications and the technologies used to manufacture a cold plate usually require customized solutions.

By Dr. Konrad Laufs, AMS Technologies

 

Requirements

The scope of a water cooler usually includes the cooling of a set of power semiconductors, their mechanical support, and optionally flow distribution, cooling of other components or electrical conduction for thyristor cells. Mechanical constraints in terms of overall dimensions and positioning of inlet and outlet connections may have a great impact on the cold plate design and the achievable performance as shown below. One of the preferred orientations is vertically mounted cold plates with inlet/outlet facing front and easy removal of air before startup. Fittings should be considered as part of the water cooler as they may account for up to 15% of the pressure drop. Different mounting patterns of M5 or M6 thread holes of the modules and possibly other holes are natural obstructions for the water channels. This may seem a trivial task for a water cooler designer, however space between screws is limited and it can lead to significant changes in cooling performance when screw patterns are changed from one generation of an IGBT module to the next.

Thermal requirements vary from 11 K/kW down to 5 K/kW per IGBT module and a heat dissipation of 1 to 2 kW/module. Of course the referenced parameters must be defined carefully. In a cold plate with 4 or 6 modules in series on a water channel the fluid temperature rise may account for up to 30% or more of the thermal resistance when the inlet temperature is referenced. Often test benches at customers use the original semiconductor components and supply DC power to the IGBT chips which leads to a somewhat conservative result as hot spots are generated solely at IGBT chips without the heat spreading effect of the diodes. Cold plate manufacturers typically use uniform heat source dummies that allow for a better comparison between water coolers. The difference between hot spot and uniform heating can be 1 to 3 K/kW/module.

Fluid flow rates vary from 6 to 30 l/min for allowable pressure drops of 300 mbar to 1.5 bar. For the fluid circuit typically a water:glycol compound is specified, a maximum system pressure and a maximum particle size in the fluid. The latter is important in case micro-channel structures are considered.

The surface flatness for the IGBT mounting area is typically in the range of 20µm/module. If the system pressure is high the change in flatness must be specified.

Makes of aluminum cold plates

The industry has shown preference for two types of water coolers, some of which are shown in figure 1. The first is based on latest extrusion profiles. They are an advancement of commonly used designs that use gun-drilled plates or extrusions with circular holes as main structure. Instead of using circular holes new comb-shaped holes are extruded that offer 2 to 4x more surface area for heat transfer. Fluid distribution is achieved through end pieces that are either bolted on and sealed by O-rings or that are inserted and sealed with a friction stir weld (FSW). The cost advantage as compared to other solutions remains while gaining a significant increase in performance. The drawback is that the most demanding thermal requirements cannot be satisfied as well as there are constraints in the design for certain applications. O-rings are typically good for 10 years.

Water coolers for modules 130x140, Primepack3, SkiiP3 and 140x190

The second type of water coolers is a FSW plate, see figure 2. Water channels are machined into an aluminum base plate into which a cover plate is inserted and sealed by FSW. The method has shown to be beneficial for all the different types of requirements as it allows for several flow patterns such as snake-shape channels, countercurrent channels, round or rectangular pin fins or other geometries, adjustment of cross section or flow path layout. FSW is a reliable technology already used in tens of thousands of water coolers and also used for space- and aircrafts. The relatively low thermal load during the welding process yields a wrap free highly accurate cold plate. Correctly designed FSW have shown to withstand 1 million cycles of 0 to 4 bars without any sign of fatigue.

Light-weight 12 mm water cooler with friction stir weld

Thermal resistance and flow configurations

Water cooler design is to a bigger part based on experience and empirical findings. Apart from thermal and fluid dynamic parameters also questions of feasibility and cost effectiveness need to be incorporated. However, numerical investigations are becoming more accurate and more common.

Figure 3 depicts the measured performance curve of an extrusion cold plate suitable for SkiiP3 4-fold modules. The heat source for testing covers the whole area of 4 modules and is uniform. The two thermal resistance curves depict the difference between maximum cooler temperature and referenced temperatures inlet versus outlet. At 10 l/min the Rth value is 6 K/kW with respect to inlet and 4.5 K/kW with respect to the outlet, which leads to the conclusion that the temperature rise in the fluid accounts for 25% of the thermal resistance when the inlet is referenced. The maximum recommended flow is related to the maximum fluid velocity of 2.5 m/s which is considered safe in order to prevent erosion.

Performance curves of VK-390-215-18 suitable for SKiiP3 4-fold

Representative of today’s cold plate specifications a set for 4 modules 89x250 mm² and its various configurations and performance curves shall be considered. Figure 4 depicts a sketch of 4 extrusion cold plates clamped between a distribution and a collector channel. Configuration A shall be the reference for comparisons. It stands for a diagonal flow pattern where all 4 plates are supposed to be cooled evenly. At a nominal flow of 20 l/min one cold plate has a flow of 5 l/min resulting in a fluid temperature rise of 7 K underneath the IGBT module for 2 kW of heat dissipation. Configuration B stands for the preferred inlet/outlet from one side. As compared to configuration A a thermal resistance rise of 24% is noted.

Performance curves for 4x Primepack3 modules Rth-module

Configuration C uses parallel flow underneath modules 1 and 2 in series with a parallel flow underneath modules 3 and 4. Pressure drop is increased as anticipated, however the configuration’s thermal resistance did not benefit from a higher fluid velocity as one may expect.

Configurations D and E use the 4 coolers in series flow. The flow cross section in config D has been reduced resulting in higher fluid velocities, a pressure drop increase from 300 to 700 mbar and a significant reduction in thermal resistance by 37%.

A solution that evens out the fluid temperature rise is configuration E where coolers 1 and 2 have the original flow cross section and coolers 3 and 4 the reduced flow cross section, i.e. higher fluid velocities. Compared to configuration A pressure drop is increased from 300 to 550 mbar and thermal resistance is reduced from 9.8 K/kW to 8 K/kW.

This one set of cold plates has shown that just by varying flow routing and adjusting the flow cross section a wide range of results may be achieved.

The task is to put the best of the variety of design options to work for a customer’s application.

 

 

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