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PhD Thesis In-situ condition monitoring of IGBT power modules by real-time junction temperature measurement Dr. Marco Denk, University of Bayreuth In the powertrain of hybrid and electric vehicles IGBT power modules take a central role in view of functionality, efficiency, reliability and costs. Especially due to harsh operating conditions and very different mission profiles the power modules in electric cars are exposed to high and often uncertain loads. When thinking about higher power densities, new control algorithms or the implementation of power electronics in im- mediate vicinity to the electric machine, the widely unknown exposure of the IGBT se- miconductors in the real system is a growing problem and hinders engineers to further decrease the inverter size and to implement new functionalities. To bring light into the darkness, this work develops an innovative sensor system that measures the loading of an IGBT during its real operation in the target application. As a key parameter the sensor system in figure 1 measures the IGBT junction tem- perature during the unaffected switching operation of the IGBT and in a way that is suitable for series production. For this purpose, the parasitic temperature sensitive internal gate resistor of the IGBT is measured with a slightly modified gate driver cir- cuit, that superimposes the negative gate voltage with a high frequency identification signal. As it can be seen in figure 1, the gate driver consists of a modified output stage, an integrated sensor circuit that generates a very noise immune sensor output voltage and an automatic calibration function to pair the gate driver with a certain power mo- dule. A prototype of the gate driver was implemented into the voltage source inverter of hybrid transmission and successfully tested during inverter operation with motor currents up to 1000 A. As an example, figure 2 shows the measured junction temperature cycles during the start-up of the electric vehicle with a low-frequency motor current. Today, the gate dri- ver concept is the first working sensor system that enables real-time junction tempe- rature measurement of conventional, non-modified power modules with high accuracy and technical feasibility. This significantly helps engineers to examine and to qualify new functionalities and inverter designs and to improve the reliability and efficiency of power electronics in the future. Additionally, the developed gate driver consists of an online Rainflow algorithm that analyzes the measured junction temperature in search of closed temperature cycles. The implementation is made possible by a reduction of the computation effort by 85 % compared to a conventional Rainflow algorithm. Each temperature cycle is parameteri- zed with three characteristic parameters and stored on an 8 kB EEPROM as a frequen- cy distribution. In case of longer road trials, the gate driver records all relevant loading information in a very compact form and makes them available for further analyses. This especially helps to resolve uncertainties in the lifetime calculation of power mo- dules, for example to investigate the impact of different hybrid strategies and control algorithms onto the power module lifetime. As a last functionality this work presents a new method to determine the health-state of a power module and its cooling system in the field. It is found that the amplitude of the low-frequency temperature cycles, that appear during the start-up of the vehicle, give specific information about the health-state of the IGBT chip solder and deeper material layers. This enables the monitoring of the ageing process of power modules in the real target application and to qualify the empirical lifetime models used in to- day’s lifetime calculations. In future this information may be used for diagnostics and early fault detection. At a glance, this work developed and investigated: • a new method to measure the IGBT junction temperature during the regular inverter operation. • a simplified online Rainflow algorithm with significantly reduced calculation and storage effort (-85 %). • a new method to identify the health-state of the IGBT power module and its cooling system in the field. In practice, these innovations provide valuable insights in the behavior of power modules in their target application and thus help engineers to further improve the efficiency, reliability, power density and costs of voltage source inverters in the future. I would be very happy and it would be an honor for me to present and discuss some of these topics at the CTI Symposium in Berlin. Figure 1 Developed IGBT gate driver with integrated real-time junction temperature measurement. Figure 2 Measured IGBT junction temperature cycles during low-frequency inverter operation.
