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  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).