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News Article

Designing innovative cooling solutions for high power modules

Thermal design specialist Klaus Olesen from Danfoss Silicon Power describes ways he used FloEFD, a 3D computational fluid dynamics (CFD) program, to dramatically improve efficiency and performance in power-module cooling systems.


I am a thermal specialist and researcher at Danfoss Silicon Power, a small division of Danfoss Drives that employs around 450 people working in Flensburg, northern Germany. Our group creates only customized products"”everything we produce is according to customer specification"”power modules and IGBT power modules, as well as MOSFET power modules for automotive and industrial motor drives. We also build power modules and power stacks for wind turbines and solar-power systems.

We use liquid cooling for many of these products because it has heat transfer coefficients that are several orders of magnitude higher than air cooling, enabling much higher power densities and more compact solutions. A product's lifetime operation and reliability is directly related to temperature management, so we use thermal simulation and analysis software based on computational fluid dynamics (CFD) to optimize our products.

The liquid cooling system for power modules needs to be highly efficient. Typically, the power module is placed on a cold plate (a heatsink with water flowing inside, See Figure 1); a thermal interface material (TIM) is used to fill the gaps where surfaces meet because the surfaces are wavy and uneven. When we used CFD to analyze the materials, we found that 20"“30% of the thermal resistance was in the TIM. So, to make our cooling system more efficient, we wanted to remove the TIM from our designs. We believed that if we could eliminate the TIM, we could reduce complexity and increase reliability. These considerations are especially important in applications that need long lifetimes such as wind turbines and electric/hybrid vehicles, where pump-out and dry-out effects of the TIM create problems.



Figure 1: Power module mounted on a cold plate.


When approaching a design like this, one of the most common methods is to have water in direct contact with the power module on the backside using pin fins (Figure 2). Pin-fin coolers are often used in the traction modules for electric car inverters; and although they are very efficient, pin fins have some drawbacks. For example, power modules with pin fins have temperature gradients inside them arising from operational heating. As water gets hotter, the power module's temperature rises from one end to the other. Also, pin fins are expensive.



Figure 2: Pin-fin cooling system.


To address these issues we came up with a completely different design, without TIMs or pin fins. We designed and built a liquid cooling system made of plastic to interface between the heatsink and power module. It has small, meandering channels on the top for higher cooling efficiency, distributing water with the same temperature on a large surface and providing homogenous cooling (Figure 3). We have no gradients in cooling and the plastic is less costly than metal. The product ShowerPower [1] is based conceptually on the water flow of rivers. Water flow is forced to rotate as it turns corners"”a swirl effect.



Figure 3: The ShowerPower liquid-cooling system.


To create a laminar flow, we designed small channels into the structure. But once we had laminar flow, we then had a buildup of the boundary layer which leads to poor thermal performance. To fix this issue, we applied the hydraulic swirl effect, which eliminated the boundary layer. We then had low pressure drop from the laminar flow and high performance from the swirl effect. An interlaced manifold structure on the backside of the new product also enables customizable, parallel cooling of multiple modules. Rather than creating 7,000 parts and taking three years to test the physical prototypes, we were able to use CFD to optimize the channel geometries (depth, height, length, etc.), for optimal thermal performance in a matter of weeks.

Let's take a look at a couple of examples of how we used simulation and analysis to optimize the cooling solutions for the power modules provided to our customers. The first example is a wind turbine converter. The customer specification was for differential pressure drop, P3 half-bridge modules at 1,700 V (1,000"“1,800 A), with six modules per cooler and one brake chopper (Figure 4).



Figure 4: P3 power module for a wind turbine.


We needed to find the right configuration for an optimal design. Wind turbine generators are huge assemblies of large power modules running in parallel, and they need homogenous cooling to be able to operate at the same temperatures. In this case, we used simulation to analyze the hydraulic balancing for multi-bathtub assemblies (Figure 5). Our simulation system incorporates FloEFD, a full-featured 3D CFD analysis solution built into major MCAD systems such as Creo, CATIA V5, Siemens NX, and Solid Edge. FloEFD enables engineers to move CFD simulation into the design process early to examine trends and dismiss less desirable design options. It can help reduce the entire simulation time by up to 75% compared to traditional CFD solutions.



Figure 5: Looking at the optimal configuration for hydraulic balancing of multi-bathtub assemblies.

Figure 6: Simulation results for the first hole design configuration.






Figure 7: Simulation results for the second hole design configuration.


Figure 8: Simulation results for the third hole design configuration.


This example has seven power modules that receive cooling water in parallel. The small holes needed to be adjusted individually to get the same flow rate. We used simulation to analyze the results when varying seven inlet holes. For the first design, we wanted to optimize the cooling system so that the modules have the same temperature. But the result was inhomogeneous; the first module was much hotter than the one next to it (Figure 6).

These results led us to try another design. In this case, the last one was too hot, and the middle one was too cool (Figure 7). The third design was a little bit better (Figure 8). Using CFD, we could run 700 iterations to reach an optimum solution quickly. If we were doing physical prototypes, the cost of the aluminum parts would have been several thousand euros, and it would have taken several weeks to have just one supplied.

The second example is a transfer-moulded, half-bridge module (650 V/800 A, 6 x 6 cm). In this case the module could not be allowed to have more than 100 millibar differential pressure drop. This is a huge challenge; it meant that the cooling of each power module needed to be as good as possible, and the application's three modules needed to be cooled equally well. The flow rate was lower per area with water temperatures higher. This was a difficult challenge; at times, working with dual heating and struggling to get the right boundary conditions for our simulations that would match the experimental conditions. The boundary conditions for volume flow rates typically were 5"“7 L/min per inverter at 70"“90 °C. The glycol-to-water ratio was 50/50. Environmental pressure was assigned to the outlet. For Joule heating, self-heating on bond wires and conductor tracks were also considered.

We also wanted to incorporate thermal radiation in our simulations. Because we only supply the one module, one component of a large system, radiation is difficult to take into account when we do not know the rest of the system. We could not have solved it without being able to use CFD combined with our customer's CAD files.

When developing this new cooling technology, we had to address one of the known challenges of designing large power modules with ceramic substrates that are soldered to a copper base plate. When it is soldered at 250°C, the solder solidifies to 220°C at cool down, and then the ceramic substrate does not shrink because of the low coefficient of thermal expansion of the substrate compared to the copper. This process results in small indentations or depressions on the backside"”a few hundred microns deep, which create small gaps through which water can take a shortcut, passing between the baseplate and the plastic cooling module. Because of this phenomena, our colleagues for years had been saying this type of design will never work. So to get around this seemingly impossible problem, we decided to simply include the bypass as part of the system (Figure 9).



Figure 9: Simulation of using the bypass in the cooling system design.





Figure 10: Thermal performance improves while differential pressure drop is reduced.


With this change in approach and some deep analysis, we found that we could actually use the bypass to reduce the differential pressure drop and thermal resistance (Figure 10) for overall improved performance.

Conclusion

One of the benefits of using FloEFD as part of our computational fluid dynamics thermal simulation and analysis is that we can get a deeper understanding of the physics of the systems we are working with. We often receive a STEP CAD file from the customer for each new project, and we cannot edit a STEP file, but this is not an issue for FloEFD, which can work with that file format. CFD programs often have a problem with round and small features, crashing while attempting to mesh them. So typically, I would have to spend two long days of work to remove all the round features before I could run the meshing process. But with FloEFD, I can skip that step; the software "˜eats everything raw'"”in other words, as we receive it, and measures it accurately and quickly. My team can use the simulations to separate different physical effects, focus on one and then investigate what happens with that particular physical effect. This allows us to experiment and come up with novel designs that we could only do with a CFD tool that works well in our CAD system, enabling us to collaborate closely with our customers for their custom products.


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