Why Amorphous Metal Cores Are the Starting Point for Medium-Frequency Loss Reduction

Amorphous metal alloys offer inherently lower eddy current losses than grain-oriented silicon steel because their ultra-thin ribbon structure and near-zero long-range atomic order suppress the classical eddy current mechanisms that dominate at elevated frequencies. This intrinsic advantage makes them the preferred core material for medium-frequency transformers operating in the 1 kHz–20 kHz range—a regime increasingly central to solid-state transformers, dual active bridge (DAB) DC-DC converters, and high-power-density power electronics.

The dataset underpinning this analysis encompasses over 60 patent records and peer-reviewed literature entries. Key assignees appearing most frequently include Metglas Inc., AlliedSignal Inc. (later Honeywell), Hitachi Metals Ltd., Westinghouse Electric Corp., and academic institutions from China, Japan, and Europe. The dominant technical approaches fall into four categories: magnetic field annealing, bulk lamination and adhesive bonding, winding geometry optimisation, and core structural design—including gap management, shell-type geometries, and laser surface treatment.

A critical but often overlooked property is that, unlike non-oriented electrical steel, amorphous magnetic material (AMM) is effectively immune to the skin effect in the 1–20 kHz carrier frequency range under inverter excitation. Research from Toyota Technological Institute confirmed that electrical steel’s iron loss decreases with increasing carrier frequency under Si-IGBT excitation due to the skin effect, whereas AMM rings remain virtually unaffected. This means that iron loss prediction models for AMM in medium-frequency converters must account for inverter harmonic content, but can treat the material as skin-effect-resistant across this range.

Magnetic Field Annealing: The Primary Lever for Minimising Core Losses

Magnetic field annealing is the single most important process step for realising low eddy current losses in amorphous metal cores: heating to 355–375°C for 30–60 minutes in an applied magnetic field minimises exciting power and enables operation at 1.40–1.45 T with exciting power below 1 VA/kg at 60 Hz, as specified in Metglas annealing method patents. This process sets the easy-axis orientation in a direction that favours lower high-frequency core loss, a concept first patented by Westinghouse Electric Corp. in 1986.

The mechanism is precise: by applying a controlled DC or low-frequency magnetic field at a non-coincident angle to the primary AC exciting field during core fabrication, the easy-axis orientation is configured to minimise losses under operating conditions. AlliedSignal Inc. and Metglas Inc. established that annealing to minimise exciting power—rather than total core loss—yields strips with exciting power below 0.5 VA/kg at 60 Hz and an operating induction of 1.40–1.45 Tesla, allowing cores to operate at higher flux densities while physically reducing the magnetic component volume. Multiple patent families from both assignees, filed across jurisdictions including the EP, Canada, Australia, and WO, establish this as the foundational process for commercial amorphous transformer production.

“Annealing amorphous alloy cores at 355–375°C for 30–60 minutes in an applied magnetic field enables exciting power below 1 VA/kg at 60 Hz and operating induction of 1.40–1.45 T—the process parameters that make medium-frequency amorphous transformers commercially viable.”

For large-scale transformer cores, Metglas refined this process to address production reproducibility. Their India-filed annealing method patent specifies precise thermal parameters: heating in a magnetic field to 355–375°C with a soak time of 30–60 minutes, minimising power loss in a reproducible manner for production environments. Hitachi’s approach, described in a Canada patent, specifies annealing at a core centre temperature of 300–340°C with a holding time of at least 0.5 hours and a magnetic field intensity of no less than 800 A/m, yielding magnetic properties superior to conventional amorphous alloys at a lower annealing temperature.

Boron content variation in the ribbon feedstock introduces a further complication. Research from China Iron & Steel Research Institute showed that boron fluctuation in amorphous alloy ribbons shifts the optimal annealing temperature window and causes loss deterioration. Their work also demonstrated that distributed air gaps in wound cores can significantly increase effective core loss, with simulations and experiments validating that minimising these gaps and applying superior annealing techniques produces appreciable core loss reductions.

The most recent advance in material-level loss reduction comes from Hitachi Metals’ laser surface treatment of Fe-based amorphous ribbons, described in a 2023 EP patent. Linear laser irradiation marks are applied orthogonal to the casting direction with a surface unevenness difference of 0.25–2.0 µm, achieving iron loss values below 0.150 W/kg at 1.45 T—a critical threshold for compliance with international transformer efficiency standards as monitored by bodies such as IEC and ISO.

Structural Design Strategies: Lamination, Shell Geometry, and Gap Control

Beyond material treatment, the physical architecture of the core assembly is the second most critical determinant of eddy current losses at medium frequencies. Bulk adhesive lamination of amorphous strip into polyhedral forms suppresses inter-layer eddy current paths and achieves core losses of at most 12 W/kg at 5 kHz and 0.3 T, enabling direct use in switch-mode power conditioning circuits from 1 kHz to over 200 kHz, as demonstrated in Metglas Inc.’s US bulk laminated amorphous metal inductive device patent (2005).

The loss formula defined in Metglas’s low core loss amorphous metal magnetic components patent—L = 0.005f(B_max)^1.5 + 0.000012f^1.5(B_max)^1.6—quantifies performance across the 50 Hz to 20,000 Hz range for polyhedral bulk amorphous components, demonstrating substantially superior performance compared to silicon steel over the medium-frequency range. This formula is directly applicable to design engineers selecting core geometries for switch-mode and resonant converter applications, where standards bodies such as IEEE and IEC increasingly define minimum efficiency thresholds.

Shell-type core geometries are particularly favoured for medium-frequency transformer design. A study from Shenyang University of Technology (2022) presents a shell-type medium-frequency transformer (MFT) design at 20 kVA/10 kHz using amorphous alloy. The area product design method was used to maximise efficiency and power density while minimising loss and volume, with FEM simulations confirming that magnetic flux density, loss, and temperature rise all remained within acceptable bounds for the fabricated prototype. This validates amorphous alloy for the 10 kHz operating range when correct geometric design methods are applied.

Leakage flux from air gaps is an independent source of eddy currents in surrounding conductive structures. Delta Electronics’ US patent (2009) addresses this by introducing a conductive case with an eddy-current-diminishing opening positioned opposite the air gap, coupled with an interposed insulator, to intercept and redirect stray flux paths that would otherwise induce losses in adjacent metallic enclosures. LSIS Co., Ltd.’s EP patent (2019, active) introduces cut sections at the upper and lower ends of both primary and secondary coils arranged concentrically around the core: by removing conductor material precisely at the regions most exposed to leakage flux, the circulating eddy current paths within the coil conductors are interrupted.

For laminated open-type cores with multi-part structures, research from the University of Zagreb (2021) demonstrates that multi-part slender cores reduce eddy current losses, but that accurate loss computation requires the A→,T→−A→ FEM formulation. By eliminating redundant degrees of freedom, convergence rate improves by at least a factor of two compared to the A→,V−A→ formulation—an important practical consideration for engineers running FEM-based optimisation of complex core assemblies.

Winding Optimisation and Frequency-Dependent Conductor Effects

At medium frequencies, winding losses due to the skin effect and proximity effect become significant contributors to total transformer loss, often comparable to or exceeding core eddy current losses. Winding interleaving is the primary structural optimisation to decrease eddy current losses in conductors at elevated frequencies, as proposed in analysis of power electronic transformer designs from 25 kHz to 400 kHz — a technique equally applicable to amorphous core designs at lower medium-frequency ranges of 1–20 kHz.

Conductor topology at high frequencies was analysed in depth by researchers at Missouri University of Science and Technology (2019), who investigated circular, square, and foil conductors at 20 kHz and 20 MHz using FEM. The study found that the skin effect increases AC winding resistance while decreasing leakage inductance as frequency rises, and that foil conductors and carefully arranged winding structures can mitigate these effects. For medium-frequency transformers operating in the 5–20 kHz range, this implies that Litz wire or foil windings with appropriate interleaving are essential complements to low-loss amorphous cores.

The interaction between amorphous core iron loss and the type of power electronics driving waveform is non-trivial. Research from Toyota Technological Institute (2018) found that AMM rings remain virtually unaffected by the skin effect in the 1–20 kHz carrier frequency range under both Si-IGBT and GaN-FET inverter excitation. This distinctive property means that amorphous cores can be used confidently across the medium-frequency range without loss penalties from skin-effect-induced flux redistribution—a significant advantage over conventional steel cores in PWM-driven applications.

For integrated magnetic structures in resonant converters, analysis from the University of Chinese Academy of Sciences (2021) demonstrates how integrating two resonant inductances and a transformer into a single magnetic core with decoupled but coupled inductance paths reduces the number of turns required per inductance, thereby lowering winding conduction losses. The Pareto optimisation procedure used enables co-optimisation of efficiency and power density—an approach directly applicable to amorphous core medium-frequency converter designs. According to IEC and efficiency standards tracked by IEA, integrated magnetic designs represent a key pathway to meeting next-generation energy efficiency targets in power conversion equipment.

Performance Benchmarks: Efficiency and Loss Data Across Core Materials

A direct comparison of core materials for medium-frequency transformer applications benchmarks the practical performance ceiling achievable with amorphous and nanocrystalline cores. Toroidal nanocrystalline transformers tested in a 1.1 kW dual active bridge converter at 20 kHz achieved 98.5–99.2% efficiency and 12 W/cm³ power density, as reported by Naresuan University (Thailand, 2021). Nanocrystalline materials are derived from amorphous precursors and share many loss-reduction mechanisms, making this a directly relevant benchmark for amorphous core designs at similar frequencies.

Research from Gdynia Maritime University (2022) confirmed that energy efficiency of a full-bridge DC-DC converter strongly depends on the core material’s frequency-dependent loss characteristics, with experimental validation using ferrite, iron powder, and nanocrystalline ring cores. This reinforces that correct material selection for the operating frequency range is the primary design lever—and that amorphous alloy, positioned between ferrite (lower saturation flux density) and nanocrystalline (higher cost), occupies a practical optimum for many medium-frequency applications.

For aerospace applications at elevated frequencies, amorphous alloy cores were found to significantly reduce idling losses in an 18-pulse transformer-rectifier unit, as reported by Ufa State Aviation Technical University (2017). Numerical calculations confirmed reductions in mass, volume, and no-load losses relative to conventional steel cores, with thermal field simulations validating that nominal operating temperatures remained within acceptable limits. This demonstrates the integrated benefit of amorphous materials for weight- and loss-sensitive medium-frequency power conversion—a finding consistent with efficiency frameworks promoted by the U.S. Department of Energy and international bodies tracking transformer energy efficiency.

“Toroidal nanocrystalline transformers in a 1.1 kW dual active bridge converter at 20 kHz achieved 98.5–99.2% efficiency and 12 W/cm³ power density — a performance benchmark directly relevant to amorphous core designs at similar medium frequencies.”

The innovation trend across this dataset is a clear progression: from foundational material patents on annealing and alloy composition in the 1980s–1990s, toward structural and process optimisations (bulk lamination, laser treatment) in the 2000s–2010s, and most recently toward system-level integration with power electronics (resonant converters, DAB topologies, GaN-based excitation) and rigorous FEM-based loss analysis tools in the 2020s. Engineers designing medium-frequency transformers today have access to a mature, multi-layer toolkit — but realising the full potential of amorphous cores requires applying all four strategies in combination: optimised annealing, bulk lamination, shell-type geometry with gap minimisation, and Litz or foil winding interleaving.