Selecting the right power conversion topology for a DC fast charger above 60kW is one of the earliest and most consequential architectural decisions on a project. Get it wrong and you will be re-spinning hardware mid-certification. This article compares the Two-Level Full-Bridge LLC resonant converter and the Three-Phase Vienna Rectifier across the criteria that matter most in a commercial DCFC programme.
The Candidates
Full-Bridge LLC operates the primary H-bridge at resonance, achieving zero-voltage switching (ZVS) for all primary switches across the full load range. The resonant tank (series inductor, series capacitor, shunt inductor) defines the gain curve. Output regulation is achieved by varying switching frequency around resonance.
Vienna Rectifier is a three-phase, three-level boost PFC topology used on the AC front-end. It is not a DC–DC stage — it rectifies and boosts the three-phase mains to a high-voltage DC bus while maintaining near-unity power factor and low harmonic distortion.
These two topologies are not direct alternatives to each other. In a typical high-power DCFC, both may appear in the same power chain: the Vienna Rectifier as the AC–DC front-end, and a Full-Bridge LLC (or DAB) as the isolated DC–DC stage. The selection question is usually about which to prioritise when trade-offs arise, and which topology to use where the two overlap in function (e.g., single-phase high-power AC–DC conversion).
Efficiency Comparison
At 150kW and above, the LLC converter can achieve peak efficiencies above 97% when the resonant tank is tuned to the nominal operating point. However, the LLC efficiency curve is narrow — efficiency degrades significantly away from resonance, which matters for DCFC units that must regulate output voltage across a wide range (200V–1000V for CCS).
The Vienna Rectifier achieves >97% power factor across a wide load range but its efficiency as a pure rectifier stage is bounded by boost converter losses. At full load, a well-designed Vienna Rectifier front-end adds approximately 1.5–2% loss on top of the DC–DC stage.
Component Stress
LLC primary switches see full DC bus voltage (typically 800V for a 400V three-phase input system), requiring 1200V-rated devices. The ZVS turn-on eliminates switching loss but body diode reverse recovery must be managed.
Vienna Rectifier switches see only half the DC bus voltage per device — a significant advantage at power levels where 650V SiC MOSFETs offer better figure-of-merit than 1200V devices. This is the Vienna Rectifier's strongest argument at 150kW+.
Thermal Implications
Heat distribution differs significantly between the two topologies. The LLC concentrates losses in the transformer core and resonant inductor (core loss is frequency-dependent), while the Vienna Rectifier distributes losses more evenly across six switch positions. For a liquid-cooled chassis, the LLC's hot spot on the magnetic components complicates cold plate design.
Certification Boundary Conditions
IEC 61000-3-12 harmonic limits strongly favour the Vienna Rectifier for three-phase AC connections — its inherent three-level switching reduces current harmonic content without additional filtering. An LLC front-end on three-phase power requires a larger EMI filter to meet the same standard.
For UL 2594 and IEC 61851, neither topology has an inherent advantage — compliance is determined by the system-level design, not the power stage topology.
Conclusion
For a high-power DCFC above 100kW with a three-phase supply, the Vienna Rectifier is the right choice for the AC–DC front-end when harmonic compliance and switch stress are the dominant concerns. The Full-Bridge LLC is the right choice for the isolated DC–DC stage where a wide output voltage range and high peak efficiency are required. Using both in series gives you the best of each topology's strengths.