Why Solid-State Transformers Need High-Frequency Magnetic Cores

Modern power electronics are getting attention these days. As the energy changes toward renewable integration, electric vehicle charging, and smart grid infrastructure, traditional transformers are being pushed aside in favor of a more intelligent alternative, which is the solid-state transformer (SST).

In other words, solid-state transformer is the high frequency transformer built around advanced high frequency magnetic cores.

Understanding why these cores matter requires looking at what solid-state transformers actually can do. And why doing it at high frequency changes everything about the physics involved.

What Makes a Solid-State Transformer Different

A conventional transformer operates at grid frequency such as 50 or 60 Hz depending on where you are in the world. It works reliably, but it's essentially a passive device. It can step voltage up or down, but it can't regulate power flow, respond to grid disturbances, or communicate with control systems. Its physical size is also directly tied to frequency: lower operating frequency means larger cores and heavier copper windings.

A solid-state transformer replaces the passive magnetics of a traditional transformer with a power electronics conversion stage. AC input is first rectified, then processed by a high-frequency inverter, pushed through a compact high frequency transformer, and delivered to the output stage. The entire process happens at switching frequencies ranging from tens of kilohertz to several megahertz — hundreds of times faster than the grid itself.

This high-frequency switching is what enables SSTs to be compact, controllable, and bidirectional. But it also places extraordinary demands on the magnetic components at the center of the design.

The Physics behind High-Frequency Core Requirements

To understand why ordinary transformer cores fail at high frequency, consider what happens inside a magnetic material during each switching cycle. Every time the current direction changes, the magnetic domains inside the core must realign. At 60 Hz, this happens 120 times per second. At 100 kHz, it happens 200,000 times per second.

Each realignment involves energy loss — a phenomenon known as core loss, which includes two primary mechanisms such as hysteresis loss and eddy current loss.

Hysteresis loss occurs because magnetic domains resist switching. The area enclosed by a material's B-H curve represents the energy dissipated as heat per cycle. At low frequency, this loss is manageable. At high frequency, it multiplies proportionally with frequency and can cause catastrophic overheating in the wrong material.

Eddy current loss is caused by induced circulating currents within the core material itself. In conductive metals, these currents can generate enormous amounts of heat at high frequencies. Standard silicon steel laminations — perfectly adequate at 60 Hz — become virtually useless at 100 kHz because their electrical conductivity allows eddy currents to run wild.

This is why high frequency core materials are engineered from the ground up with low electrical conductivity and carefully controlled magnetic properties.

Core Materials That Make High-Frequency Operation Possible

The requirements for a high frequency core used in solid-state transformers are demanding and non-negotiable:

• Low core loss at the operating frequency
• High saturation flux density to minimize core volume
• Low coercivity for easy domain switching
• High electrical resistivity to suppress eddy currents
• Thermal stability across a wide temperature range

◆ Ferrite Cores

Ferrite is a ceramic-like compound of iron oxide mixed with nickel, zinc, or manganese. It is most common material used in high frequency transformer designs for SSTs operating in the 20 kHz to 500 kHz range. Its intrinsically low electrical conductivity nearly eliminates eddy current losses, and modern MnZn and NiZn formulations offer excellent permeability and saturation characteristics.

The main limitation of ferrite is its relatively modest saturation flux density, typically around 0.3 to 0.5 Tesla. For very high power applications, this means larger core volumes are needed to avoid saturation, partially offsetting the size advantages of high-frequency operation.

◆ Amorphous and Nanocrystalline Alloys

For applications demanding higher power density, amorphous metal alloys and nanocrystalline materials offer a compelling alternative. These materials are produced by rapid solidification of molten alloy, which creates a disordered atomic structure that dramatically reduces hysteresis loss compared to crystalline metals.

Nanocrystalline cores in particular have become attractive for high-power solid-state transformer designs. They offer saturation flux densities above 1.2 Tesla, which is more than double typical ferrite along with extremely low core losses at mid-range frequencies between 10 kHz and 200 kHz. Their permeability is also significantly higher, which reduces the number of turns needed in the winding and simplifies thermal management.

Amorphous cores occupy a complementary position in the SST material landscape. Iron-based amorphous alloys deliver saturation flux densities in the range of 1.5 Tesla and exhibit core losses significantly lower than silicon steel, making them a practical choice for SST designs operating at switching frequencies between 1 kHz and 50 kHz. Where nanocrystalline materials excel at mid-to-high frequencies, amorphous cores are often preferred in medium-frequency converter stages where higher flux density is the priority and cost efficiency matters at scale. 

◆ Silicon Carbide and Wide Bandgap Synergy

It's worth noting that the push toward higher switching frequencies in SSTs is closely tied to advances in semiconductor technology. Wide bandgap devices like silicon carbide (SiC) MOSFETs and gallium nitride (GaN) transistors can switch at megahertz frequencies with relatively low losses. As these devices become more cost-effective, the magnetic cores used with them must follow — pushing material development toward ultra-low-loss formulations capable of performing reliably above 1 MHz.

Core Geometry and Winding Design at High Frequency

The choice of high frequency magnetic cores material is only part of the challenge. Geometry matters enormously at high frequency, for reasons rooted in electromagnetic skin effect.

At high frequencies, current naturally concentrates near the surface of a conductor rather than flowing uniformly through its cross-section. This increases effective resistance and winding loss. To counteract this, designers use litz wire bundles of individually insulated fine strands, which distribute current across many small conductors and keep resistive losses manageable.

Core geometry also affects leakage inductance and winding capacitance; both become problematic at high frequency. Planar transformer designs have become popular in SST applications. They offer excellent repeatability, low leakage inductance, and superior thermal management compared to conventional wound designs.

Why Getting the Core Right Determines SST Efficiency

The efficiency of a solid-state transformer is primarily determined at two points: the semiconductor switching stage and the magnetic core. Unlike semiconductors, which can be upgraded as device technology advances, the magnetic core is fixed once a design is committed. A poor core choice locks in losses that cannot be designed around later.

In a medium-voltage SST operating at 10 kW or more, even a 0.5% improvement in core efficiency translates into meaningful reductions in heat generation, cooling system size, and operating cost over the product's lifetime. At utility scale — where SSTs are being deployed in traction power systems, data center infrastructure, and renewable energy converters — these margins become commercially significant.

This is why engineers working on SST development invest heavily in core characterization. Loss testing at actual operating frequencies and temperatures, not just catalog specifications, is essential.

Core manufacturers who understand the specific demands of high frequency transformer applications, including flux density, waveform distortion from PWM switching, and thermal cycling. These are differentiated partners in SST development, not just commodity suppliers.

Looking Ahead

Solid-state transformers are playing important role in EV fast-charging infrastructure, railroad traction systems, and grid-edge power conversion. Each of these applications demands magnetic cores that can sustain low-loss operation over millions of switching cycles, across wide temperature ranges.

The continued refinement of high frequency core materials and the precision engineering of the transformers built around them — will be a defining factor in how quickly solid-state power conversion scales from niche application to mainstream infrastructure.

For engineers entering this space, the message is clear: master the magnetics, and the rest of the design falls into place.