New Third-Harmonic Injection Modulation Reducing the DC-Link Energy Buffer Requirement of Phase-Modular Three-Phase Isolated PFC AC/DC Converter Systems

Realizing an isolated three-phase AC/DC converter system with three front-end single-phase PFC rectifier modules referenced to a common star point allows low DC-link voltage levels of 400V in contrast to standard three-phase boost-type PFC rectifiers operating with a DC-link of 800V. Hence, the usage of superior 600V GaN switches instead of 1.2kV SiC MOSFETs and accordingly a high system efficiency is enabled. The main drawback of this phase-modular approach, however, is that each of the modules is subject to a twice-mains frequency power pulsation, which is inherent in single-phase AC/DC operation, and requires bulky electrolytic DC-link capacitors to buffer the pulsating input power. In this paper, a new modulation method is presented which takes advantage of the three-phase and/or star connection of the three single-phase PFC rectifier modules and injects a common-mode (zero-sequence) component, e.g., a 3
^rd
harmonic voltage component into the AC-side voltage formation of the rectifier switching stages, which shifts part of the power pulsation to higher frequencies and accordingly reduces the low-frequency peak-to-peak voltage ripple and RMS current of the DC-link capacitors. A detailed analysis reveals that this method allows to cut the DC-link capacitor volume by up to 30% without the need of increasing the DC-link voltage reference setpoint or increasing the voltage rating of the capacitors and power semiconductors.