IGBT Temperatures at Low Frequencies

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 failure. The switching losses and the resulting temperature rises in the IGBT semiconductors can be considered constant for frequencies of 50 Hz and more. For lower frequencies, the ON and OFF switching cycles are 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 decreases with increased amplitude of the chip temperature variation.
IGBT chips used for the control of the traction motors of metros might experience during their service life 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 improved 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 generated losses are dissipated by forced convection via cooling fans.

These calculations were performed with the commercial software Simcenter FloEFD by enabling the transient option. The time step is one-hundredth of the period. The time-dependent losses were defined as input. Thermal radiation is known to be negligible in such applications due to the low temperatures; therefore, it was not simulated. Natural convection inside the casing was calculated by enabling the gravity option in the solver. Thanks to the user-friendly interface, CAD tool integration, and a multi-processor solver, results were obtained quickly.

The calculation domain was divided into 0.5 million cells for half a module. In the calculated worst-case scenario at a frequency of 0.1 Hz, IGBT temperatures varied between 45 and 60°C. This corresponds to only 1 million cycles, or one year of normal operation for an onshore 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)

Onshore Wind Power Station Generators

 Onshore wind power stations are mostly equipped with doubly-fed asynchronous generators with slip-ring rotors. The rotor windings are excited via an inverter with a variable low-frequency current, typically between 0.1 and 10 Hz.
This enables the production of current at grid frequency directly from the generator, independent of load changes caused by wind variations. The inverter transistors (IGBTs or MOSFETs) experience strong temperature variations; a more efficient thermal path must be designed using 3D CFD.