Thermal Impedance Spectroscopy for Non-Destructive Evaluation of Power Cycling

Due to energy transition, e.g. the "Energiewende" in Germany, the amount of power electronics in field of renewable and distributed generation increases. This means, that new power generation sources in the grid require a considerable increase of applied power electronic components, e.g. converters for doubly fed induction machines of wind turbines or for photo-voltaic applications. In the face of changing load depending on environmental exposures, e.g. wind strength or sunshine, temperature swings lead to thermo mechanical stress within the layer interfaces of the installed power modules. The thermo mechanical stress evokes accelerated ageing of those electronic devices. On the one hand, reliability in the electric power distribution grid is required. On the other hand, it is intended to avoid an over dimensioning of the components in the grid (taking into consideration the costs). Therefore, research regarding characteristics of critical power electronic components gets a high priority, to ensure long operation time and to prevent over dimensioning. Testing and evaluating the reliability of power semiconductors in terms of active strain is done by active power cycling. With active power cycling conduction losses during turned-on load current are used to heat up the Device Under Test (DUT). In off-state the DUT is cooling down. The temperature swing is a function of the applied load current. Since the device is heated by its own thermal losses, the power cycling is called "active". This process will be done periodically until the component reaches the end-of-life. This paper deals with a non-destructive method to investigate the thermal path of power semiconductor devices in order to detect changes or failure in the thermal interface during the power cycling. The aim of this research is to evaluate reliability of power modules depending on application strain and potentially condition monitoring in the application. The method is illustrated using measurement results of an active power cycling test of a standard power semiconductor.