Cooling Calculations of Power Electronics with CFD

 

Power electronics is the key component to ensure efficient and sustainable electric production and consumption. It is used in order to adjust renewable energy sources like wind and solar 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 plant, gas turbine power plants... 

Power modules with diodes, IGBT or mosfet

Temperature related problems like thermo-mechanical stresses are identified are the highest threat on 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.
The conventional semiconductors like diodes, IGBT and MOSFET are mostly used as switches 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

Heat sinks and thermal contact materials TIM

In order to dissipate the produced heat losses, the power modules are generally installed on common heatsinks. Passive heatsinks are designed to function without fan: the free convection flow will evacuate the heat. Aktive 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 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 to the heat sink occurs with conductive Thermal Interface Materials. For practical cases, it is unclear how the TIM is filled; the resulting thermal resistant is vaguely estimated, which results in a supplementary imprecision of the thermal calculation. The best method is to measure the thermal resistance, complete measurement systems like T3ster can be therefore utilized.

Casing or enclosure for invertors or rectifiers  

The power modules are placed in electrical cabinets with other devices like resistors, condensers and inductive components.
The smallest calculation model includes only the package, if more details are necessary in order to estimate the chip temperatures, then the simulation should be done at board, rack or even room-level.

Thermal and flow CFD calculations

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 analyze 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.
CFD Tools such as FloTherm or FloEFD work with a Cartesian grid and specialize in ventilation and cooling of the electronics. They are much more suitable than tools with complex grid generation.

Streamlines in the enclosure of a frequency converter
Streamlines in the enclosure of a frequency converter