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RESEARCH

Medium Voltage PCB-based DC Bus Design Considering Current Handling Capability

Test setup
Fig. 1. Short circuit test setup for PCB bus
  To fully utilize the higher switching speed offered by medium voltage wide bandgap (WBG) devices, parasitic inductance must be minimized to limit voltage overshoot at switching transitions. A planar power bus structure can reduce the loop inductance by using larger conductor planes with antiparallel current conduction paths and conductors with minimal spacing. The traditional laminated power bus uses thicker conductors providing a high current capability, but fabrication limitations leave a relatively poor insulation quality due to internal defects such as small air voids. Although the solid dielectric can withstand a higher electric field (E-field) intensity, localized breakdown, or partial discharge (PD), occurs within voids at a much lower intensity leading to premature insulation failure. The partial discharge inception voltage (PDIV) can be increased by increasing the conductor spacing, but this is counterproductive to the goal of parasitic inductance reduction.
  A PCB-based bus was implemented utilizing geometric techniques for a well-controlled E-field. The manufacturing process provides less defects allowing the bus thickness to be reduced by approximately 75% compared to the traditional laminated while increasing the PDIV from 5.6 kV to 10.5 kV. Multiple parallel layers were used for +dc and -dc to allow a current handling capability of at least 85 A.
  During short circuit (SC) fault events, a high peak current can create a large force on the conductors, especially in high current density areas. This force can effectively peel the copper from the dielectric; thus, voids are introduced and the PDIV is decreased. A SC testbed has been constructed to allow SC testing up to 4500 Apk; see Fig. 1. A new bus is used to determine the initial PDIV and the phase resolved PD pattern (PRPD) is recorded as shown in Fig. 2. Repetitive SC events are generated with 20 seconds between pulses to allow capacitor recharging and ensure no degradation is caused from copper self-heating. Every 20 SC events, the PDIV is checked and PRPD recorded to aid in identifying insulation degradation. This work will be used to further improve the layout of PCB-based buses to ensure the long-term reliability.
Partial Discharge
Fig. 2. Phase resolved partial discharge pattern for bus before short circuit testing; PDIV=10.5 kV, QIEC=9.4 pC.


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