SiC converter for electrical vehicle - DC-link ripple

Abstract

The increased focus towards electrification of the transportation sector increases the need for smaller, lighter, cheaper and more efficient inverters for use in drivetrains. In most of these inverters, the DC-link capacitor makes up a large portion of the total inverter volume and weight. It is therefore interesting to analyze the DC-link current and voltage ripple which is one of the major dimensioning factors of the DC-link capacitor. Throughout this thesis, simulations of the DC-link ripple are performed to evaluate how well the actual ripple can be predicted for a PMSM-drivetrain. In addition to this, an investigation of the impact of switching from Si-IGBTs to SiC-MOSFETs is performed. To accomplish this and to evaluate the simulation model, the simulation model was compared to both theoretical calculations as well as with actual measurements on a drivetrain. By comparing simulation results with actual measurements on a drivetrain, it was showed that the fundamental components of the DC-link current ripple can be simulated with 5 % accuracy for the evaluated operating points. This proves that system-level transistor models produce a good result in relation to their complexity. However, the simulated DC-link voltage was not as accurate. This can be derived to the DC-link capacitor model which did not accurately enough reproduce the transfer function from the injected inverter current to the voltage ripple on the DC-link. It was also shown that the impact on ripple was negligible when using SiC-MOSFETs as opposed to the original Si-IGBT if all other parameters were kept constant. SiC-MOSFET was shown to have advantages over Si-IGBT in that the switching frequency could be increased, as well as increased efficiency. The efficiency could especially be increased when using reverse conduction which showed no impact on overall ripple levels. By having a simulation model that can accurately reproduce the DC-link ripple, better dimensioning of the DC-link capacitor and other converters attached to the DC-link can be performed. Also, parameter optimization for ripple minimization can help to reduce cost, size, weight and materials used in inverters for EV/HEV applications. Keywords: SiC,