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Design of a Compact, Low-inductance, 1200 V 6.5 MΩ SiC Half-bridge Power Module with Flexible Printed Circuit Board Gate-Loop Connection

Year: 2019 | Author: Grace Watt | Paper: P4.2
Half-bridge power module
Fig. 1. Half-bridge power module with Rogowski coil around bottom side wire bonds.
The properties of silicon carbide (SiC) devices make them desirable for use in power modules due to their high-power density capabilities, robust operation at higher temperatures, and low losses. SiC can be switched at higher frequencies than silicon (Si), enabling passive components to be smaller, thus reducing the size of fabricated modules. With these advantages comes the need for updated packaging techniques. The design of this power module includes new methods in order to obtain low inductance, compact size, and reliable performance using Wolfspeed’s 1200 V, 13 mΩ SiC MOSFET. Using the MOSFET body diodes increases the potential power density by eliminating the need for ex-ternal antiparallel diodes. Reduction of the gate loop inductance is achieved with a flexible PCB serving as the gate, desaturation point, and kelvin source interconnection. Lastly, a balanced/symmetrical direct bonded copper (DBC) layout increases the likelihood of current balance in the commutation loop. The final design is shown in Fig. 1 with the patterned DBC and the die on top, the base plate, and the housing. One of the more novel and challenging elements of the module fabrication process is attach-ing the flexible circuit on the bare die. This is due to the small area available for attachment, heat sensitivity of the die, and incompatibility of some attachment materials to the die. The flexible PCB is attached using a nanosilver sinter paste which can bond with the specialized nickel: gold top coat die. To test the functionality of the half-bridge SiC power module, a static and dynamic characteriza-tion will be performed. The static characterization will include output and transfer curves. A clamped inductive load tester allows for dynamic characterization under different load currents. The tester was designed with vias to easily attach to pins coming out of the flexible PCBs (Fig. 2). New techniques used in the design of this power module take advantage of SIC properties. Two power modules are designed to compare current sharing behavior: 1) the control sample with balanced die, and 2) the other with unbalanced die. The individual die currents are measured with Rogowski coils (Fig. 1) to determine the effects of the unbalanced die on power module performance.
Gate driver connections
Fig.2. Flexible PCB attach to gate driver PCB; size of PCB against a penny; pins for gate, kelvin, and desat shown from left to right from the top view.

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