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Geometrical Optimization of Medium-Frequency Air-Core Transformers for DCX Applications | IEEE Journals & Magazine | IEEE Xplore

Geometrical Optimization of Medium-Frequency Air-Core Transformers for DCX Applications


Abstract:

This article analyzes the potential of air-core transformers (ACTs) for realizing medium-voltage high-power isolated dc–dc converters. ACTs, i.e., transformers without ma...Show More

Abstract:

This article 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. Afterward, a complete multiobjective 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 tradeoffs are highlighted and analyzed, e.g., mass, volume, efficiency, switching frequency, part-load behavior, and insulation distance.
Page(s): 4319 - 4335
Date of Publication: 04 January 2022

ISSN Information:


I. Introduction

Isolated dc–dc converters are essential building blocks of modern high-power energy conversion systems, such as fast electric vehicle chargers, renewable energies, datacenter power supplies, drive systems, or more electric aircraft [1]–[4]. The usage of medium-frequency transformers and wide bandgap semiconductors allows the construction of more compact and efficient dc–dc converters [5]–[7]. Nevertheless, due to their limited gravimetric power density (i.e., 2–12 kW/kg), the magnetic-core transformers (MCTs) are typically the heaviest components of power electronic systems. This is particularly problematic for airborne or battery-powered applications [2], [8]–[11].

References

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