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By treating the inductive coupling probes and the electrical system to be monitored as three cascaded two-port ABCD networks, the in-circuit impedance of the electrical system can be extracted with ease. The proposed method simplifies the setup calibration and allows real-time impedance monitoring of critical electrical system to be implemented without interrupting its normal operating conditions. In this paper, the theory behind the proposed ABCD networks approach is described. The proposed approach has overcome the poor measurement accuracy of the conventional inductive coupling calibration method above 30 MHz and has achieved good accuracy up to 100 MHz with a maximum measurement deviation of 3.6%. Finally, using a dc-powered motor as a practical example, the ability to detect the change of in-circuit impedance of the motor under different operating conditions is demonstrated.

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Understanding the causes of a power device's defect plays a crucial role in failure and reliability analyses, and hence, the need of health diagnosis. Heath diagnosis involves the measurement of pre-defined electrical parameter of a power device under its usual operating condition and the analysis of its deviation from its norm. Instead of the conventional direct measurements of voltage and current waveforms in time-domain, this paper proposes a non-intrusive measurement method that measures the on-state impedance of a power device in frequency-domain. The proposed method eliminates direct electrical contact with the device-under-test (DUT) and it has the ability to detect the deviation of the on-state impedance from its norm for health diagnosis purpose.