Why Solid-State Transformers (SST) Rely Heavily on SiC/GaN Wide-Bandgap Devices？

The core operating principle of a Solid-State Transformer (SST) is to elevate the traditional power-frequency (50/60 Hz) conversion to medium/high frequency (tens to hundreds of kHz), thereby drastically shrinking the magnetic core volume. Traditional silicon-based IGBTs, constrained by material properties, typically achieve practical switching frequencies below 5 kHz in medium/high-voltage high-power scenarios, and generally not exceeding 20 kHz even in low-voltage applications.

They also suffer from large reverse-recovery losses and slow switching speeds, which fundamentally limit the high-frequency operation and miniaturisation of SSTs.

Modular Solid State Transformers for Adaptable Energy Integration

Core Advantages of SiC/GaN Wide-Bandgap Devices

High Voltage & High Frequency Compatibility

• SiC has a bandgap of 3.26 eV (silicon: 1.1 eV), a critical breakdown electric field strength approximately 10× that of silicon, and a thermal conductivity roughly 3× that of silicon.
• This enables SiC MOSFETs to operate reliably in medium-voltage (kilovolt-class) environments while supporting switching frequencies of 20 kHz to 100 kHz or higher.

Significantly Reduced Losses

• SiC MOSFETs exhibit virtually no minority-carrier storage effect; their switching process is primarily limited by parasitic capacitance charging/discharging, eliminating the tail-current problem inherent to IGBTs.
• In optimised SST designs, system efficiency reaches 96–98.5%, maintained across a wide load range — not just at full load.

Drastic Volume & Weight Reduction

• Following the inverse relationship between transformer volume and operating frequency (V ∝ 1/f), a 10× frequency increase reduces high-frequency transformer core volume by ~90%.
• At the system level, SST volume can shrink to one-third or less of a traditional power-frequency transformer, with weight reduced to approximately half.

High-Temperature Capability & Reliability

• SiC devices operate stably at junction temperatures ≥ 175 °C, offering far superior thermal stability compared to silicon devices — essential for harsh grid-side environments.

SiC vs. GaN: Division of Labour in SST

Two mainstream technology routes coexist in the industry:

SiC Route — High-Power Module Approach (Medium/High Voltage)

• Targets medium/high-voltage scenarios; commercial SiC MOSFETs now cover 1.7 kV to 15 kV voltage classes
• Widely used in SST input rectifier stages and high-frequency isolation DAB stages, fitting the cascaded architectures of 10 kV/35 kV medium-voltage grids
• 2026: commercial 10 kV SiC MOSFETs have entered the market, further reducing series-stack counts and simplifying system architecture

GaN Route — Distributed Small-Power Module Approach (Low Voltage / High Frequency)

• GaN bandgap ~3.4 eV; breakdown field >10× silicon, supporting MHz-class switching frequencies
• Commercial single-die voltage ratings mostly ≤ 900 V (typically 650 V)
• Companies such as Enphase adopt a distributed architecture: many small GaN HEMT modules combined — AC-side series connection for voltage sharing, DC-side parallel connection for current sharing; per-module voltage stress ~300 V, comfortably covered by 650 V GaN devices
• Advantages: faster response, inherent redundancy, cost efficiency at module level
• Challenges: higher system-level hardware complexity; multi-module architectures exhibit higher losses under light-load conditions

Cost & Industrialisation Context

Power electronic devices account for ≈40% of total SST cost — the single largest cost item.

With the SiC industry transitioning from 6-inch to 8-inch wafers, volume production costs are expected to fall significantly from 2026 onward, further accelerating SST commercialisation.

Application-Driven Demand

Key Scenarios

• AI Data Centres — single-site power reaching tens to hundreds of MW, cluster power up to GW scale
• Renewable Integration, Ultra-Fast Charging — emerging applications demanding extreme efficiency and power density

SST Performance Enabled by Wide-Bandgap Devices

• Direct conversion of 10–35 kV medium-voltage AC to 800 V high-voltage DC
• System efficiency up to 98.5%
• ~60% footprint reduction versus traditional UPS solutions

These performance targets are unattainable without SiC/GaN wide-bandgap devices.

Summary

SSTs adopt SiC/GaN wide-bandgap devices en masse because traditional silicon devices hit a ceiling in frequency, efficiency, and volume simultaneously. Wide-bandgap semiconductors, with their wider bandgap, higher breakdown field, and higher thermal conductivity, naturally match the SST's core demands for high frequency, high efficiency, and high power density.

• SiC dominates medium/high-voltage, high-power segments
• GaN focuses on low-voltage, high-frequency and distributed small-power module architectures

The two technologies co-operate to cover the full SST chain. Without wide-bandgap semiconductors, the commercialisation of Solid-State Transformers — from laboratory to grid, data centres, and renewable energy applications — would not be possible. They are the key enabling technology.

Edited From：Echo