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RESEARCH

Optimization of Electric-Field Grading Plates in a PCB-Integrated Bus Bar for a High-Density 10 kV SiC MOSFET Power Module

Module cross-section
Fig. 1. SiC 10 kV power module cross-section
  High-voltage SiC MOSFETs have the potential to drastically improve the size and efficiency of power systems due to their higher operating voltages and faster switching speeds. To realize this potential, a 10-kV high-density (4 W/mm2) package with low parasitic inductance (4.4 nH) was developed. The high-density design is enabled by minimal (6 mm) spacing between the terminals of the module. This is nearly six times closer than the terminals of the CREE XHV-9 10 kV SiC module, which has a terminal spacing of 37 mm. The reduction in terminal spacing is made possible by fully enclosing the terminals, which circumvents the creepage and clearance distance requirements (Fig. 1).
   To reduce the electric field strength in the air surrounding the interface, copper traces inside the PCB are used as field- grading plates that shift the peak electric field from the air to the FR4 dielectric, which has a higher breakdown field strength than air. The geometry and location of the field- grading plates is critical to their effectiveness. To design the geometry, a numerical optimization technique is used in conjunction with finite element analysis (FEA). The system is first decomposed into critical 2D design regions, which are then parameterized, and the locations of field crowding are identified. A weighted cost function is formulated using the breakdown strength of the materials and optimized using an interior-point algorithm with finite difference derivatives.
   One of these optimized cross-sections is shown in Fig.2. The optimization results in copper conductor shapes that contain the high electric field strength in the FR4 dielectric, where the field can be supported without partial discharge. Meanwhile, the electric field in air is kept below the breakdown strength of air, resulting in safe, reliable, partial-discharge-free operation. The optimized laminate bus bar and optimized module housing were built and experimentally demonstrated a partial discharge inception voltage of 11.6 kV rms (16.4 kV peak) under 60Hz sinusoidal excitation.
optimized cross-section
Fig. 2. Optimized cross-section supports 10 kV bus voltage without exceeding the breakdown strength of air.

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