Measurement of IGBT High-Frequency Input Impedance in Short Circuit

Insulated-gate bipolar transistors (IGBTs) operated in short circuit become instable in certain driving and load conditions. The induced oscillations can compromise robustness and reliability of the entire power converter. The stability of the device inserted in a real system can be analyzed using the theory of linear oscillators that requires the knowledge of input or output impedance of the device in real operating conditions. In this paper, we present an experimental procedure for measuring in pulsed mode the small-signal impedance of a power device biased in any test conditions. The small-signal input impedance of a 650-V 20-A IGBT operated in short circuit has been measured as a function of the frequency. This input impedance has been used to extract the stability map of the IGBT in short circuit, which allows us to easily predict the test conditions where the IGBT becomes instable. The validity of this stability map has been confirmed by a large-signal time-domain characterization of the IGBT operated in short circuit. The proposed technique is very useful to design driving circuit able to avoid instable operations.