Solid-state transformers (SSTs) are employing compact high-power medium-voltage (MV) medium-frequency transformers (MFTs) and enable the power transfer between galvanically isolated DC systems. Considering a typical SST isolation stage, i.e., a 166 kW unregulated series-resonant DC–DC converter acting as a DC transformer (DCX) with equal input and output MV DC voltages of 7 kV (1:1-DCX), we derive component- and system-level performance limits of air-cooled realizations with either an air-core transformer (ACT) or with a magnetic-core transformer (MCT). We describe the design of two fully rated MFT prototypes in detail and provide a comprehensive experimental characterization to validate the derived performance limits, including the dielectric losses and the analysis of stray magnetic fields. The realized ACT and MCT prototypes achieve measured efficiencies of 99.5% and 99.7% at gravimetric power densities of 16.5 and 6.7 kW/kg, respectively. Considering 10 kV SiC MOSFETs, calculated system-level efficiencies of the ACT-based and MCT-based 1:1-DCX result in 99.0% and 99.2% at the nominal operating point, with similar part-load efficiency characteristics. This article concludes with an application-oriented qualitative evaluation of the two concepts.

The galvanic isolation of solid-state transformers (SSTs) is typically realized with a medium-frequency (MF) magnetic-core transformer (MCT). Previous demonstrations indicate that achieving highly power-dense and lightweight MCTs imposes several challenges on the design because of stringent requirements related to insulation and cooling. This work investigates the achievable efficiency of an optimized 166 kW/7 kV air-core transformer (ACT), which is a core part of a dc transformer (DCX), i.e., an unregulated dc–dc SST with a voltage scaling defined by the transformer turns ratio. The ACT features relatively low complexity of the construction, comparably high coupling values, and high efficiency. Modeling, optimization, and construction of the realized ACT are explained and guidelines regarding insulation, cooling, and shielding of the magnetic stray flux are discussed in detail. Furthermore, the prototype is experimentally validated to demonstrate its full functionality. In the investigated DCX, which is based on a series resonant converter (SRC) topology, the realized ACT is found to achieve a full-load efficiency of 99.5% and an unprecedented gravimetric power density of 16.5 kW/kg. With the use of 10 kV SiC MOSFETs, the complete DCX is estimated to reach an efficiency of 99% at 166 kW output power.

This paper analyzes the potential of air-core transformers (ACTs) for realizing medium-voltage high-power isolated DC-DC converters. ACTs, i.e., transformers without magnetic cores, are particularly interesting for their simple construction and reduced weight. However, the reduced magnetizing inductance, the reduced magnetic coupling, and the stray fields are challenging aspects for the design of ACTs. A comprehensive model of the ACT (e.g., magnetic field patterns, skin and proximity losses, shield’s eddy currents, harmonics, insulation constraints, and thermal limit) is proposed and verified with measurements obtained with a prototype. Afterwards, a complete multi-objective optimization of a series resonant converter (SRC) operating as a DC transformer (DCX) between two 7 kV buses with a rated power of 166 kW is conducted. Two different geometries are considered for the ACT: concentric cylindrical coils and planar spiral coils. As a result, the optimal ACT, (operated at 162 kHz) features extreme power densities of 7.5 kW/dm3 and 31 kW/kg, which confirms the superiority of ACTs regarding the gravimetric power density. The calculated efficiencies are 99.5 % and 98.7 % for the ACT and the complete DC-DC converter, respectively. Finally, the different trade-offs are highlighted and analyzed, e.g., mass, volume, efficiency, switching frequency, part-load behavior, and insulation distance.

Quasi-2-level (Q2L) operation of multi-level bridge-legs, especially of flying-capacitor converters (FCC), is an interesting option for realizing single-cell power conversion in applications whose system voltages exceed the ratings of available power semiconductors. To ensure equal voltage sharing among a Q2L-FCC’s switches, the voltages of a Q2L-FCC’s minimized flying capacitors (FCs) must always be balanced. Thus, we propose a concept for load-independent FC voltage balancing: For non-zero load current, we use a model predictive control (MPC) approach to identify the commutation sequence of the individual switches within a Q2L transition that minimizes the FC or cell voltage errors. In case of zero load current, we employ a novel MPC-based approach using cell multiple switching (CMS), i.e., the insertion of additional zero-current commutations within a Q2L transition, to exchange charge between the FCs via the charging currents of the switches’ parasitic capacitances. Experiments with a 5-level FCC half-bridge demonstrator confirm the validity of the derived models and verify the performance of the proposed load-independent balancing concept.