M More efficient and compact electric vehicle (EV) chargers promote sustained transportation electrification towards a low-carbon economy. Advanced three-phase (3-Φ) bidirectional AC/DC converter systems with buck-boost functionality, e.g., with an extended output voltage range of 200 V to 1000 V, could be employed as standard building blocks in various galvanically isolated EV chargers, but could promisingly also serve as future RCD-based non-isolated EV chargers characterized by significantly increased efficiency and power density compared to state-of-the-art isolated EV charger solutions, and are accordingly reviewed and analyzed in this thesis comprehensively considering both the current DC-link (current source) and the voltage DC-link (voltage source) topologies.

A 3-Φ current DC-link buck-boost (bB) PFC AC/DC converter system, where a 3-Φ buck-type current source rectifier (CSR)-stage and a subsequent 3-L boost-type DC/DC-stage are combined through one advantageous shared DC-link inductor, is first selected from the category of current source converter systems. The buck functionality is achieved by operating the CSR-stage solely in 3/3-PWM without switching the DC/DC-stage such that the DC/DCstage only creates the inevitable conduction losses but zero switching losses. In the boost-mode, the DC/DC-stage steps up the rectified voltage of the 3-Φ mains to a required large output value and, beneficially, shapes the DC-link current to follow the upper envelope of the absolute value of the 3-Φ currents to eliminate the zero switching state (shoot-through state) of the CSR-stage, i.e., the CSR-stage operates with 2/3-PWM. These loss-optimum operations, i.e., reduced number of switching instants due to clamping of one phase of the CSR-stage or the DC/DC-stage, and minimum possible DC-link current for any operating point, are guaranteed by a proposed synergetic control strategy. These favorable features are experimentally verified, including conducted EMI measurements, using a 10 kW hardware demonstrator with a power density of 6.4 kW/dm³ (107.5 W/in³) and a peak efficiency of 98.8%. The measured ultra-flat efficiency surface proves the expected highly efficient operation over wide output voltage and power ranges.

Furthermore, the synergetic control concept of the 3-Φ current DC-link bB PFC AC/DC converter system is extended to regulate two independent DC outputs for heavy-duty EVs. The loss-optimal operations are still retained, i.e., the reduced number of switching instants due to clamping of one phase of the CSR-stage (switching only two out of the three phases, i.e., 2/3-PWM) or individual clamping of the DC/DC-stage’s two half-bridges, and minimum possible DC-link current for any operating point. Experimental confirmation of the proposed control scheme using the built 10 kW demonstrator system is provided, and a significant measured efficiency improvement, e.g., from 97.9 % to 98.4 % (0.5 %) at 10 kW, is demonstrated, which is largely independent of output voltage asymmetries and load asymmetries.

Targeting non-isolated EV chargers based on the analyzed 3-Φ current DC-link bB PFC AC/DC converter system, a virtual grounding control (VGC) is proposed to operate the DC/DC-stage to compensate the low-frequency (LF) common-mode (CM) voltage resulting from the CSR-stage modulation and thus controls the LF CM voltage between the DC output and protective earth (PE) to zero. This enables further a direct connection of the DC output midpoint to PE, where an additionally proposed ground current control (GCC) prevents nuisance tripping of mandatory RCDs by regulating the measured LF CM ground current. The proposed concepts are verified on the realized 10 kW hardware demonstrator considering Terra-Terra (TT) and Terra-Neutral (TN) grounding systems. The proposed GCC successfully limits the LF CM leakage current to < 6 mA RMS, i.e., significantly below typical RCD trip levels, and, using the human-body impedance model according to UL 2202, achieves a test voltage of 110 mV that is clearly below the most stringent limit (250 mV) of the standard, which provides a viable solution for future non-isolated EV chargers.

Besides the aforementioned current DC-link topologies, such bidirectional AC/DC buck-boost converter systems can also be realized in the form of widely-analyzed voltage DC-link topologies. A three-level (3-L) realization of the 3-Φ voltage DC-link rectifier stage facilitates small EMI filters and hence compact converter realization so that a T-type (Vienna) voltage source rectifier (VSR)-stage is selected as front-end. To achieve buck-boost functionality, the boost-type VSR-stage must be combined with a buck-type DC/DC stage, which again advantageously is realized as a 3-L structure to reduce the magnetics volume and to enable controllability of the voltage DC-link midpoint potential. Thus, a 3-Φ voltage DC-link boost-buck (Bb) PFC AC/DC converter system is selected and studied from the category of voltage source topologies. For high output voltages, the VSR-stage continuously modulates all three phases to regulate the output voltage (3/3-PWM) while the DC/DC-stage remains clamped to avoid switching losses. For low output voltages, the DC/DC-stage advantageously controls the DC-link voltage to enable 1/3-PWM (only one of the three bridge-legs operates with PWM at any given time) of the VSR-stage with reduced switching losses. Furthermore, a novel 2/3-PWM scheme for the output voltage transition region, where output voltages are in between the buck-mode and the boost-mode operation limits, guarantees operation with minimal losses, i.e., the minimum number of the VSR-stage bridge-legs operating with PWM, and with the minimum possible DC-link voltage, for any output voltage. These advantageous operations are ensured by a proposed synergetic control concept which achieves a seamless transition between the loss-optimum operating modes. A comprehensive experimental verification, including pre-compliance EMI measurements, using a 10 kW hardware demonstrator with a power density of 5.4 kW/dm³ (91 W/in³) and a peak efficiency of 98.8% confirms the theoretical analyses.

Finally, based on two realized hardware demonstrators, a comprehensive comparison of current and voltage DC-link buck-boost AC/DC converter systems is provided regarding hardware realization, synergetic control implementation, efficiency characteristics, and conducted EMI noise emissions. The thesis concludes with a summary of the main results and a discussion of future research areas.