Ringing frequency extraction for health monitoring of power transistors

The detection, diagnostic, and prognostic of power transistor degradation play a key role in increasing the reliability and safety of power electronic converters (PECs), especially in aeronautic and astronautic fields. In a PEC, high-frequency oscillation may occur during the switching transients of power transistors, which is known as ringing. The switching ringing reflects the intrinsic parameters of power transistors. Evaluating the change over time of these parameters characterized from ringing becomes a key indicator to assess the aging status of the PEC with respect to transistor degradation. The leg midpoint voltage (νM) is often used to capture the ringing information. However, dc offset voltage and numerous harmonics in νM make it difficult to acquire useful information. In this paper, the voltage across the stray inductance is used to extract the ringing waveform during transistor switching, and the fast Fourier transform is used to capture the ringing frequency. The measured ringing frequency is associated with transistor's junction capacitance or internal parasitic inductance, and the detected shifts in frequency can indicate transistor degradation.