Designer's guide to system voltage, topology, and submodule power

Designing a solid-state transformer (SST) with SiC power modules starts with defining the system architecture. Key parameters such as medium-voltage input, DC-link and output voltages, isolation requirements, submodule power rating, medium-frequency transformer design strategy and converter topology determine the electrical stress, converter configuration, and SiC module choice for each stage.

In practice, SiC module selection depends less on a single voltage rating and more on where the module sits inside the SST architecture: grid-frequency active front end (AFE), isolated DC-DC stage, bypass path, OR-ing or eFuse protections.

To understand how these stage-level requirements translate into real designs, it is essential to first look at how a solid-state transformer is structured and operates.

What is a solid-state transformer (SST) and how does it work?

SST is a modern, semiconductor-based high-power transformer. Instead of a single heavy iron-core winding operating at mains frequency (50/60 Hz) like a traditional low-frequency transformer (LFT), an SST uses power electronic converter stages and a medium-frequency isolation transformer to transfer energy in a much lighter, more compact form.

Effectively, an SST is like replacing an old landline phone with a modern smartphone.

For example, recent Infineon collaborations show the direction of SST development: DG Matrix’s SiC-enabled SST platform is reported to be at least 30% smaller and lighter than conventional transformers.

Solid State Transformer Visual Datacenter

By breaking the power conversion into multiple steps, an SST can provide flexible outputs (AC and/or DC) while vastly reducing volume and weight. The steps include:

  • Active front end (AC to DC)
  • Isolated DC-DC converter (for voltage transformation and isolation via a medium-frequency transformer)
  • Final DC-AC or DC-DC output stage

Additionally, SSTs integrate gate drivers, high-speed digital controllers (MCUs), voltage/current sensors, and solid-state isolators for communication and control. These are all coordinated to ensure safe, stable power conversion.

Why is SiC essential for solid-state transformers (SSTs)?

Traditional silicon IGBTs and MOSFETs struggle to meet the simultaneous high-voltage and high-frequency demands of an SST. However, SiC devices offer:

  • High-voltage capability: SiC devices support kV-level operation needed for medium-voltage grids
  • High-frequency switching: Enables smaller transformers and higher power density
  • Lower losses: Improves efficiency across multiple conversion stages
  • Thermal robustness: Supports high junction temperatures and long lifetimes
  • Reliability: High-voltage SiC devices are widely used in renewable-energy applications such as battery energy storage and photovoltaic

In addition to CoolSiC™ power modules, Infineon’s 2000 V CoolSiC™ discrete MOSFETs can be an option for selected SST converter stages requiring greater design flexibility.

What should engineers define first before selecting SiC modules for SST?

Designing an SST with SiC begins by nailing down a few critical system parameters – each with direct implications for your architecture and component choices:

Grid input and output specs

What are your input voltage and output requirement?

E.g., a medium-voltage 10 kV AC input feeding a ±400 V DC or 800 V DC output. High input voltage means you need multilevel front-end converters (using series-connected mid-voltage SiC modules), whereas a low-voltage input could allow simpler topologies.

Isolation and stage count

Does your design require galvanic isolation, and where?

If yes, include an isolated DC-DC stage (using a medium or high-frequency transformer). Decide if you are building a single-stage SST or a two/three-stage SST (common for MV to LV conversion). This choice sets the DC-link voltages on each side of the transformer and determines the voltage classes of SiC devices at each stage (e.g., 1.2 kV vs. 3.3 kV modules).

Submodule power and modularity

Will you use modular building blocks? If so, what is the power per building block?

For large power ratings, SSTs usually split the load across multiple identical converter building blocks. Picking a building block power (for instance, 100 kW per building block in a 1 MW system gives 10 building blocks, plus redundancy) influences how many SiC modules go in series (for voltage) or parallel (for current). It also influences the medium-frequency transformer design strategy. There is a trade-off: high power modules (fewer power modules) vs. low power modules (better granularity and redundancy)

Topology selection

Which converter topologies suit each stage?

For example, a medium voltage active front end might use a cascaded half-bridge, while the isolation stage could use a dual active bridge converter. A careful topology pick ensures each stage can leverage SiC strengths – e.g., multilevel converters exploit SiC’s efficiency at high switching speeds by spreading voltage stress, whereas simpler two-level designs often require higher-voltage devices to support the same system voltage. Topology ties everything together, so ensure it aligns with your voltage levels and module choices.

What are the applications of solid-state transformers?

Frequently asked questions about SST

1. What challenges arise when integrating SiC modules into SST designs?

Designing with SiC in SSTs introduces challenges such as managing high dv/dt switching, controlling electromagnetic interference (EMI), ensuring proper insulation at medium voltage levels, and optimizing thermal performance. These factors require careful layout, gate driver design, and system-level insulation planning to ensure reliable operation.

Switching frequency directly affects transformer size, efficiency, and losses in an SST. Higher switching frequencies enabled by SiC allow smaller and lighter magnetic components, but they also increase switching losses and EMI, requiring a careful trade-off between efficiency, power density, and system complexity.

SiC devices can operate at higher junction temperatures, but effective thermal management is still critical in SSTs due to high power density. Engineers must consider heat sink design, cooling methods (air or liquid), and thermal cycling to ensure long-term reliability and optimal performance.

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