Cooling Calculations of Power Electronics with CFD

Power electronics are the key component ensuring efficient and sustainable electric production and consumption. It is used to adjust renewable energy sources like wind power stations and photovoltaics to the grid frequency as well as to supply the right frequency for the speed control of a drive train. It is used for LED headlamps, electric cars, wind energy power plants, gas turbine power plants...

Power modules with diodes, IGBT or MOSFET

Temperature-related problems like thermo-mechanical stresses are the highest 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 to achieve the highest power density with the least amount of space.
The conventional semiconductors, diodes, IGBT, and MOSFET are mostly combined in compact power modules. There are modules with or without conducting base plates.

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. 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 aluminium and have a very low thermal resistance. If heat sinks are not enough, the heat can be moved away by a water-cooling system or even heat pipes. All these cooling methods can be well calculated with 3D CFD.

The thermal contact with the heat sink occurs with conductive Thermal Interface Materials. For practical cases, it is unclear how the TIM is filled; the resulting thermal resistance is vaguely estimated, which results in a supplementary imprecision of the thermal calculation. The best method 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 more details are necessary to estimate the chip temperatures, then the simulation should be done at the board, the rack or even the 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 during the concept phase. The 3D Computational Fluid Dynamics is the ideal tool to precisely analyse the temperature and flow field; hot spots can be localized and eliminated. With relatively little effort, 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 recommended 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.