On Gate Drivers for MOS-Controlled Power Devices and dv-dt Filters for Train Traction Converters
Abstract: In this thesis, gate-drive solutions and a proposed dv/dt-filter have been investigated with focus on train traction line- and motor converters with dv/dt constraints. Low losses on the power devices enable reductions in weight, volume, and cost of the converter components such as cooling parts and enclosures. The power devices considered in this thesis are the silicon carbide (SiC) junction field-effect transistor (JFET) and the metal oxide semiconductor field effect transistor (MOSFET) as well as the well-known silicon insulated-gate bipolar transistor (IGBT) and the silicon p-i-n diode. The study on the JFET has shown that the switching losses can be significantly reduced compared to the IGBT, but complex gate-driver solutions are required for high utilization in terms of loss reduction. For the commonly used silicon IGBT and the p-i-n diode, the dv/dt across the diode at the turn-ON process of the IGBT was found to be highest with low phase current and low temperature. On the other hand, the most critical operation points loss-wise in a train is when the power device junction temperature and current are highest. Therefore, a proposal meeting this tradeoff has been addressed in this thesis tailored at minimal cost for the new half-bridge package. By using this proposal, the dv/dt at low currents are kept within the required limits although the losses are reduced at high currents. For the SiC MOSFET, an investigation has been performed using the operation point in order to minimize the switching losses given a certain dv/dt requirement. In addition, separate control of the di/dt and dv/dt were implemented. Contrary to the bipolar IGBT and p-i-n diode, the dv/dt for the MOSFET and the JBS diode increases with commutated current and temperature. Therefore, the benefit of doing such optimization is less beneficial for these devices than for the IGBT and p-i-n diode.In addition, phase short-circuit investigations have been performed using the SiC MOSFET with a gate-driver proposal meeting the application requirements. The results show that short circuits with fast current rise can be terminated. However, due to the lower thermal mass compared to the IGBT, the detection and termination times must be shorter using the MOSFET. Finally, a new dv/dt filter concept has been proposed. This concept uses the stray inductance present in the inter-connections between the power devices and the convert output terminals as a filter component. By so doing, the devices can be fully utilized even though the output dv/dt levels are in the range of the levels for which an IGBT equipped converter exhibit with ultra-low additional filter losses. As a consequence, the switching frequency can be four times higher for the same switching losses compared to a solution for which the required dv/dt is achieved by adjusting the MOSFET from the gate-driver.
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