This paper proposes a measurement technique applied to an unbiased multi-chip power module that is able to distinguish between junction temperature variation and bondwire lift-off. Compared with traditional approaches, the proposed technique requires only one test to monitor and discriminate these two conditions. The proposed technique relies upon the injection of high frequency/low power signals into the Gate-Emitter terminals of an IGBT and then analysing the corresponding frequency response of the Gate-Emitter circuit. It is verified that the imaginary part of the Gate-Emitter impedance (Z_(GE, Imag)) changes in opposite directions. A high number of bondwire lift-offs leads to an increase of Z_(GE, Imag), whereas junction temperature rise leads to a drop in Z_(GE, Imag). Furthermore, it is shown that the real part of Gate-Emitter impedance Z_(GE, Real) increases with both increased number of bondwire lift-offs and rise in junction temperature. In this paper, a small-signal model for the Gate-Emitter circuit of a 3.3 kV/100 A IGBT power module is developed and its frequency response is examined and analysed with SaberRD. Furthermore, practical tests are also carried out with a network analyser.
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