Diagnosing Degradation in Power Modules Using Phase Delay Changes of Electrical Response

This paper proposes a new and simple method that diagnoses multiple aging effects in power electronic modules min-imally invasively and without the necessity of expensive sensors. Different degradation modes, e.g., fatigue of solder and thermal interface layer, influence the phase of the thermal impedance frequency response function
$\angle \boldsymbol{Z}_{\text{th}}(j \omega)$
at specific bandwidths. Thus, tracking changes of the phase between periodic device-loss excitation at specific frequencies and the resulting junction temperature response allows identifying these degradation modes. This is exploited by the proposed method for degradation diagnosis. The method excites periodic conduction losses at selected frequencies via small-signal manipulation of the gate-source voltage and measures the phase delay between gate-source and drain-source voltage. The measurable phase delay results partially from the dynamic response of the thermal impedance, because the phase-delayed junction temperature impacts the on-state resistance and therefore the drain-source voltage. Conse-quently, changes of the measurable phase delay allow identifying changes of
$\angle Z_{\text{th}}(j\omega)$
and thus diagnosing the above mentioned degradation modes. The paper features a detailed analysis of the proposed method as well as an effective excitation and extraction circuit that allows phase delay measurements with high resolution over multiple frequency decades. Experiments with silicon carbide MOSFETs demonstrate that the implemented method can effectively diagnose changes of the thermal path between the device and the coolant that result from different degradation modes.