Unlike conventional 50/60 Hz transformers, many solid-state transformer (SST) designs use power converters and a medium-frequency isolation transformer to regulate voltage and power flow.

This changes the requirements for magnetic core materials. Higher operating frequencies can reduce transformer size, but they also introduce different core-loss, saturation, and thermal constraints.

SST technology is also being explored for AI data center power distribution, where medium-voltage AC conversion and 800 V DC architectures are attracting interest.

1. What Is a Solid-State Transformer?

A solid-state transformer uses semiconductor-based converters and magnetic components to transform voltage and control electrical power flow.

Many isolated SST designs contain three functional stages:

AC/DC conversion: Converts the incoming AC supply into a controlled DC link.
Isolated DC/DC conversion: Transfers power through a medium- or high-frequency transformer, providing galvanic isolation and voltage conversion.
DC/AC conversion: Produces an AC output when required.

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SST working principle showing AC/DC conversion, isolated DC/DC conversion through a medium-frequency transformer, and optional DC/AC output.

The arrangement varies by application. Some SSTs supply DC output directly, while others use modular converter cells or different isolation topologies.

Despite the name, a solid-state transformer does not necessarily eliminate magnetic components. In isolated designs, the transformer remains an essential part of the power conversion system.

A 2026 review in the IEEE Open Journal of Power Electronics examines modular SST converter topologies and identifies medium-frequency transformer design as a challenge in isolated DC/DC conversion.

2. Solid-State Transformer vs. Conventional Transformer

Conventional power transformers operate directly at the AC supply frequency, generally 50 or 60 Hz. Their magnetic cores are designed for efficient voltage transformation and long-term operation.

An SST introduces semiconductor switching and electronic control. Where galvanic isolation is required, an internal transformer can operate at a frequency substantially higher than the grid frequency.

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Conventional 50/60 Hz transformer compared with an SST using power electronic converters and a medium-frequency isolation transformer.

The main differences are summarized below.

Feature	Conventional Transformer	Solid-State Transformer
Main function	Electromagnetic voltage transformation	Controlled power conversion and voltage transformation
Operating frequency	Typically 50/60 Hz	Grid-frequency input with higher-frequency converter stages
Magnetic components	Line-frequency transformer core	Often includes a medium- or high-frequency isolation transformer
Voltage regulation	Transformer taps or external regulation equipment	Electronic regulation within converter limits
DC output	Requires additional rectification	Can be integrated into the system
Power flow control	Limited without additional equipment	Can support active and bidirectional power flow, depending on design
Main loss sources	Core and winding losses	Semiconductor, magnetic, winding, and auxiliary losses
System complexity	Comparatively low	Higher, including controls, protection, and thermal management

Does an SST Have Higher Efficiency?

An SST is not inherently more efficient than a conventional transformer.

Traditional power transformers can achieve very high efficiency, particularly at large power ratings. An SST introduces semiconductor conduction and switching losses, even when its internal magnetic components are smaller.

Its potential advantage may appear at the distribution-system level, where several conversion stages can be replaced or integrated.

3. Why SSTs Use Medium- and High-Frequency Transformers

Increasing the operating frequency of an isolation transformer can reduce the magnetic core area required for a given voltage, number of turns, and magnetic flux density.

For sinusoidal excitation, the transformer voltage equation is:

Vrms=4.44fNBA

Where:

Vrms is the RMS induced voltage.
f is the operating frequency.
N is the number of turns.
B is the peak magnetic flux density.
A is the effective magnetic core cross-sectional area.

The reduction in core size comes with additional winding, insulation, and magnetic-loss constraints.

Higher Frequency Changes Core Loss

Core loss generally increases as frequency rises, although the relationship depends on the magnetic material, flux-density swing, and excitation waveform.

A material with acceptable losses at 50/60 Hz may perform differently at the medium frequencies used in an SST isolation stage.

Winding and Insulation Constraints

Skin and proximity effects increase AC winding resistance, making conductor geometry and winding arrangement important for controlling copper losses.

Medium-voltage SSTs also require insulation systems capable of withstanding the electrical stresses between windings, the core, and grounded components.

These constraints can limit transformer miniaturization even when a higher switching frequency allows a smaller magnetic core.

A 2026 review of SST qualification challenges discusses medium-frequency transformer insulation, medium-voltage semiconductors, and qualification requirements as barriers to wider deployment.

4. Core Materials for Solid-State Transformers

Ferrite, amorphous alloys, nanocrystalline alloys, and thin electrical steel are among the materials considered for SST isolation transformers.

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Magnetic core material options for SST isolation transformers, including ferrite, amorphous alloy, nanocrystalline alloy, and electrical steel.

Comparing Core Material Options

Material	SST-Relevant Advantage	Main Limitation to Evaluate	Potential Application
Ferrite	High electrical resistivity helps limit eddy-current losses at high frequencies	Lower saturation flux density than many iron-based alloys; temperature-dependent behavior	High-frequency isolation transformers
Amorphous alloy	Thin-ribbon construction and favorable core-loss potential under suitable conditions	Frequency-dependent losses, mechanical stress, and processing effects	Selected medium-frequency transformers
Nanocrystalline alloy	High permeability and favorable loss characteristics over selected frequency ranges	Flux-density swing, heat treatment, mechanical stress, and core-loss behavior	Selected medium- and high-frequency magnetic components
Thin electrical steel, including GOES	Relatively high magnetic flux capability and established core construction methods	Increasing losses at higher frequencies, particularly with thicker laminations	Line-frequency and certain lower-medium-frequency designs

Can Electrical Steel Be Used in an SST?

Grain-oriented electrical steel (GOES) is widely used in conventional power transformer cores because of its magnetic characteristics and established manufacturing processes.

At higher frequencies, eddy-current losses become more significant, particularly with thicker laminations.

Thin electrical steel laminations have also been investigated for medium-frequency transformers, where allowable core loss remains a key design constraint.

5. Core Loss and Magnetic Design Challenges in SSTs

SST isolation transformers may operate under excitation waveforms that differ from the sinusoidal conditions commonly used for conventional magnetic material testing.

The converter topology and switching strategy determine the voltage applied to the isolation transformer. Depending on the design, the resulting magnetic flux waveform may be approximately triangular, trapezoidal, or otherwise non-sinusoidal.

Core Loss Under Non-Sinusoidal Excitation

Magnetic core loss includes hysteresis, eddy-current, and other dynamic loss contributions.

Under converter excitation, waveform shape and the rate of magnetic flux change influence the resulting losses. A material that performs well under sinusoidal testing may not retain the same relative advantage under a different waveform.

Engineers may estimate these losses using generalized Steinmetz-based methods, with parameters fitted to the selected material and excitation conditions. Finished-core measurements can then be used to check the estimates.

Saturation and Flux-Density Swing

The usable magnetic flux-density swing affects transformer dimensions and magnetic losses.

A larger flux-density swing can allow fewer turns or a smaller core for a given voltage-time product, but generally increases core loss. The available saturation margin depends on the resulting peak flux density.

Temperature and Finished-Core Performance

The magnetic properties of a finished core may differ from those of the original ribbon or sheet material.

For amorphous and nanocrystalline cores, heat treatment, mechanical stress, and assembly conditions can influence magnetic performance.

For example, ribbon loss measurements may support initial material screening, but they do not independently establish the losses of a wound core operating under the SST converter's actual waveform.

Research on high-frequency magnetic cores for SSTs also examines the effects of core geometry and winding integration on magnetic design.

6. SST Applications: Eaton and AI Data Center Power Systems

SSTs are being developed for AI data center power distribution, including medium-voltage AC to 800 V DC conversion architectures.

Eaton's Medium-Voltage Solid-State Transformer

In June 2026, Eaton announced its MV SST 2.0 solution for AI data center power systems.

The announced design converts 10 kV medium-voltage AC input to 800 V DC output, integrating power-electronic conversion into an architecture intended to simplify power distribution.

Siemens and Reinhausen's 800 VDC Development

Siemens and Reinhausen announced plans in August 2026 to develop a modular SST for AI data centers. The proposed system is designed for grid connection voltages up to 36 kV and 800 VDC output.

Neither announcement establishes which specific magnetic core material grades are used inside the isolation components.

7. What Magnetic Material Buyers Should Verify

For an SST isolation transformer, material selection begins with the converter's operating conditions.

Verification Item	What to Check
Operating frequency	Intended frequency range of the isolation transformer
Excitation waveform	Actual converter excitation and available test waveform
Flux-density swing	Expected operating range and saturation margin
Core-loss data	Loss values at specified frequency, flux density, waveform, and temperature
Material dimensions	Ribbon or lamination thickness and applicable tolerances
Magnetic properties	Saturation flux density, permeability, and relevant test conditions
Finished-core performance	Available core test results and differences from material-level data
Processing requirements	Applicable heat treatment, winding, handling, and assembly conditions
Material traceability	Grade, batch identification, and available inspection documents

Comparing Material and Finished-Core Data

Consider two candidate materials: one with a reported loss at 10 kHz and another tested at 20 kHz.

If the flux-density swing and excitation waveforms also differ, the published values cannot establish which material would produce lower losses in the same SST transformer design.

Buyers can use ribbon datasheets for initial screening and request finished-core test data where available. If the supplied data does not cover the intended SST operating conditions, the transformer manufacturer will need additional verification.

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