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EMI Evaluation of a 1.7 kV SiC MOSFET Module with Organic DBC Substrate

Prototype layout
Fig.1. (a) ODBC-based half-bridge layout, (b) Half-bridge schematic, and (c) ODBC module prototype.
  This work evaluates the electromagnetic interference (EMI) of a silicon carbide (SiC) MOSFET power module with emerging organic direct bonded copper (ODBC) substrates. The thin, ductile organic insulator in ODBC substrates enables thick copper (Cu) to be used. The thick Cu enables higher current capacity, improved heat spreading, and can eliminate the need for a baseplate. However, the thin dielectric of the ODBC increases the electrical coupling within the power module. In particular, the ODBC-based module has increased capacitive coupling between the module terminals and the mounting structure (i.e. ground network) compared to a conventional ceramic-DBC-based module. Capacitive coupling between the power module terminals and the ground network is known to be a significant contributor to the propagation of common mode (CM) currents in high-power converter systems. Therefore, as part of this work, a detailed analysis of the CM implications of the module design presented herein was conducted.
  A 1.7 kV SiC MOSFET half-bridge module was fabricated using a 50 mm × 50 mm ODBC substrate as shown in Fig. 1(c) . The layout of the module is shown in Fig. 1(a). The Cu pads are colorcoded to correspond to the nodes in the schematic shown in Fig. 1(b). The half-bridge was designed to minimize CM noise by reducing the S1D2 area to be less than 30 % of the total substrate area. The ODBC module was simulated in a custom inverter testbed, where the input and output impedance of the system are equal (Zin = Zout). This allows for a simplified common-mode equivalent model (CEM) of the testbed.
  The ratio of the S1D2 capacitance (Cag) to the total module capacitance to ground (Cbp ) can be manipulated to minimize or cancel the leakage current through the baseplate. The testbed and model were previously verified with a Cree BM2 SiC MOSFET power module. As shown in Fig 2, the baseplate leakage current of the ODBC module is reduced by nearly 20 dB across a wide frequency range, and by 30 dB at 5-10 MHz. This evaluation is valuable because it demonstrates that proper application of a suitable CM model can be employed to reduce emissions in a deterministic fashion, even without the introduction of CM filters.
Leakage current
Fig. 2: Predicted leakage current through module baseplate in (a) time-domain, (b) frequency-domain for practical configuration of EMI testbed with commercial module (red) and ODBC module (blue)

   This work was supported by the Office of Naval Research (ONR) with the grant number N00014-16-1-2956

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