Thermal Design with CFD of the power electronics at low frequencies

Thermal Reliability

IGBT Transistors are exposed to strong thermo-mechanical load variations, which lead to aging, material fatigue abrasion and finally outage. The switch losses and the resulting temperature rises in the IGBT semiconductors can be considered as constant for frequencies of 50 Hz and more. For lower frequencies, the ON and OFF switching is so slow, that it results in a time-dependent temperature behavior of the chips. The life expectancy of an IGBT type is defined by the number of temperature cycles; this rapidly falls by increased amplitude of the chip temperature variation.
IGBT chips used for the control of the traction motors of metros might experience during their period of use up to 1 to 10 million load changes with a temperature variation between 15K and 40K. When neither the chips nor the connections can be upgraded, then the temperature variations must be reduced with more efficient cooling.

 Surface temperatures of the IGBTs and diodes for a 0.1 Hz frequency
Surface temperatures of the IGBTs and diodes for a 0.1 Hz frequency
(video frequency is real frequency), fan on the left

 

Surface temperatures of the IGBTs and diodes for a 1 Hz frequency
Surface temperatures of the IGBTs and diodes for a 1 Hz frequency (video frequency is real frequency), fan on the left

Transient thermal and flow CFD Calculations

In this example, the IGBTs and diodes are integrated into a Semikron power module, which is mounted on a heat sink. The produced losses will be removed by forced convection through the cooling fans.

These calculations have been performed with the commercial software FloEFD by switching on the transient option. The time step is one-hundredth of the period. The time dependence of the losses is given as input. It is known for such applications that the heat radiation is negligible as the temperatures are too low, therefore, it does not need to be simulated. The natural convection is calculated in the casing by switching on the gravitation option of the solver. Thanks to the friendly user interface, the embedment in the CAD tool and a multi-processor solver, the results could be reached quickly.

The calculation domain has been spilt into ½ million cells for ½ a module. For the calculated worst case with a frequency of 0.1 Hz, the IGBT temperatures vary between 45 and 60°C. This corresponds to only 1 million cycles or one-year of normal operation of an on-shore wind power station.

 Air temperatures in an IGBT module casing for a  0.1 Hz frequency
Air temperatures in an IGBT module casing for a 0.1 Hz frequency, (video frequency is real frequency)

Air temperatures in an IGBT module casing for a 1 Hz frequency
Air temperatures in an IGBT module casing for a 1 Hz frequency, (video frequency is real frequency)

Low frequencies for on-shore wind power stations generators

 On-shore wind power stations are mostly equipped with double-fed asynchronous generators with lip-ring rotors. The rotor windings are excited through an inverter with a variable low-frequency current, typically between 0.1 and 10 Hz.
It enables the production of current with the network frequency directly at the generator, independently from the load changes caused by wind variations. The transistors of the inverter, IGBT or MOSFET, show strong temperature variations; a more efficient thermal path must be designed with the help of 3D-CFD.