This paper proposes a new method that diagnoses multiple aging effects in power electronic modules minimally 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 ⦟Z_th(jw) at certain bandwidths. Thus, the phase offset between device loss excitation at specific frequencies and the corresponding junction temperature response differs for different aging effects, which is used by the proposed method. Therefore, the method excites periodic conduction losses at moderate bandwidths via small-signal manipulation of the gate-source voltage and measures the phase delay between gate-source and temperature-dependent drain-source voltage. This phase delay results primarily from the dynamic response of the thermal impedance, because the phase-delayed junction temperature impacts the on-state resistance. Thus, changes of the phase delay allow identifying the above mentioned degradation modes. In contrast to previous publications, the proposed method makes sensor calibration, device loss calculation as well as high-bandwidth measurements obsolete.
For the practical realization of the proposed method, an excitation and extraction circuit has been designed that allows phase delay measurements with high resolution over multiple frequency decades. Experiments that were conducted with SiC MOSFETs using differently manipulated thermal interfaces demonstrate that the implemented method can effectively diagnose changes of the thermal interface that result from different degradation modes.