Characterizing Nonlinear Field Dependent Conductivity Layers to Mitigate Electric Field Within (U)WBG Power Modules Under High Frequency, High Slew Rate Square Voltage Pulses

A vast majority of research on (U)WBG power modules has been going on to implement nonlinear resistive field grading material on metal-brazed substrates in reducing the electric field that is maximum at triple points (TPs). However, nearly all investigations have been conducted under either DC or 50/60
$\mathbf{H}_{\mathbf{Z}}$
sinusoidal AC voltages, even though the actual operation of envisioned (U)WBG power modules involves high-frequency square voltages with high slew rates. It has been validated by several studies that fast rise times of square voltages rapidly degrade the breakdown strength of insulation materials, leading to premature failure. Therefore, this paper introduces a nonlinear resistive field grading material or field-dependent conductivity (FDC) layer around the TP and metal edges to evaluate the electric field mitigation under a high frequency and high slew rate square voltage. The modeling and simulation of both coated and uncoated (U)WBG substrates were performed in COMSOL Multiphysics to assess the electric field reduction with the nonlinear FDC layer. The improvement of reduction in electric field under 100 kHz high slew rate square voltage is compared with that of 60 Hz. The results reveal a significant decrease in field stress at the TP, even under square voltages with fast rise times and high frequencies, when applying a nonlinear FDC coating, as opposed to the uncoated substrate. The influence of switching field
$(\mathbf{E}_{\mathbf{b}})$
and nonlinearity coefficient
$(\alpha)$
of nonlinear FDC layer is studied under 100 kHz square voltage, and it is concluded that
$\alpha$
and
$\mathbf{E}_{\mathbf{b}}$
should be more than 10 and less than 8 kV/mm, respectively to achieve effective performance of resistive field grading material.