Cooling Calculations of Power Electronics with CFD

Power electronics are the key component ensuring efficient and sustainable electric production and consumption. They are used to adjust renewable energy sources like wind power stations and photovoltaics to the grid frequency or to set the inverter frequency for the speed control of a drive train.

Power modules with diodes, IGBT or MOSFET

Temperature-related problems like thermo-mechanical stresses are the greatest threat to reliability. Exact compliance with the limit temperatures of the chips is mandatory; otherwise, the life expectancy will be dramatically shortened. Efficient cooling is indispensable in order to achieve the highest power density with the least amount of space. Semiconductors such as, diodes, IGBT, and MOSFET are mostly combined in compact power modules.

Surface temperatures in an IGBT casing
Surface temperatures in an IGBT casing

Thermal management with heat sinks and thermal contact materials TIM

To dissipate the produced heat losses, the power modules are generally installed on common heatsinks. A conducting base plate can be placed between the modules and the heat sink in order to spread the heat transversally. Passive heatsinks are designed to function without a fan: the free convection flow will evacuate the heat. Active heat sinks have an electric-driven fan, and the cooling takes place by forced convection. Heat sinks are usually made of aluminum and have a very low thermal resistance. If heat sinks are not enough, the heat can be removed by a water-cooling system or even heat pipes. All these cooling methods can be well calculated with 3D CFD.

When assembling the power module to the heat sink, small air bubbles remain, which greatly increase the thermal resistance. A thermal interface material in the form of a gap pad or thermal paste is therefore used. For practical cases, it is unclear how the TIM is filled and compressed; the resulting thermal resistance is roughly estimated, resulting in an additional imprecision of the thermal calculation. The best practice is to measure the thermal resistance, which should be used as input for the calculation.

Thermal and flow CFD calculations

The smallest calculation model includes only the package; if further details are necessary to estimate the chip temperatures, then the simulation should be done at board, rack, or even room-level.

Streamlines through the heat sink of an IGBT power module
Streamlines through the heat sink of an IGBT power module

The cooling of the power electronics is so important that it should be sketched as early as possible during the concept phase. The 3D Computational Fluid Dynamics is the ideal tool to precisely analyze the temperature and flow field; hot spots can be localized and eliminated. It is possible to obtain fast and effective computational results. These will make viewable optimization potentials which can then be achieved with further thermal simulations.

Thermal models of PCBs

Printed circuit boards consist of several layers of copper and conductive connections (conductor tracks); the connections between the layers are made with holes (vias); the whole volume is filled with an electrically insulating material such as FR4. From a CFD and CAD point of view, a PCB is a very complex geometry. The PCB can either be modeled as a complex model, or it can be reduced to an anisotropic square block.
If the geometry of a PCB is available, it can be exported as a CAD model in STEP format. The CAD model can be manually corrected and simplified until accepted by the CFD tool. This operation can take several days even for an experienced CAD professional. Otherwise, commercial CFD software offers tools to simplify the PCB model, such as the FloEDA bridge for FloEFD; this is suitable for regular use only.
It is recommendable to thermally simulate the power electronics as early as the concept phase. As a rule, the complete geometry of the PCB has not yet been created; only the number and thickness of the copper layers are known. It is not possible to use general values for the thermal conductivity of the PCB here, especially since the thermal vias improve the heat conduction through the PCB. In a company, substitute models can be used for a category of PCBs; these were determined using earlier models. A manufacturer of LED headlights may have standard PCB thermal conductivity values according to their function: the turn signal, high beam, low beam, and stop PCBs.