Design Points and Main Problem Countermeasures for SST Solid-State High-Frequency Transformers

Solid State Transformer (SST) high-frequency transformer (10 kHz–200 kHz) design essentially compresses three functions—'medium voltage isolation, high-frequency power, and module redundancy'—into a magnetic component with less than 1/10 the volume of traditional designs. Below, the core essentials that 'must be done right the first time' and the most common failure-prone issues are summarized across five dimensions: 'Materials – Electromagnetic – Insulation – Thermal – Process,' along with the latest literature and prototype data for practical implementation.

I. Key Design Points

Magnetic Core Material and Frequency Window

• Below 20 kHz: Nanocrystalline ribbon (Bs ≈ 1.2 T, pcore ≈ 0.25 W/kg @ 20 kHz/0.2 T) offers the highest power density.
  
• 20–100 kHz: Power ferrite 3C95/PC200 (Bs ≈ 0.4 T, pcore ≈ 0.35 W/kg @ 100 kHz/0.1 T) is cost-effective with mature supply chains.
  
• 100 kHz: PC200H or 4F1 is required, but Bs is only 0.32 T, necessitating 'thin core legs + interleaved winding' to reduce ΔB; otherwise, the magnetic flux saturation margin is insufficient.
  

Winding Configuration and High-Frequency Loss Control

• Medium-voltage side (over 3 kV): Use 'segmented copper foil (thickness less than or equal to skin depth) + polyimide film' or 'PCB-Litz multilayer boards' to limit layers to 2 or fewer. This reduces proximity loss by 60 percent.
  
• Low-voltage, high-current side: Use 0.08 mm copper foil or 0.05 mm × 100-strand Litz wire, ensuring foil thickness/strand diameter ≤ 2δ @ 100 kHz.
  
• Interleaved winding (½P-S-½P) is the 'standard configuration' for SSTs, halving the equivalent layer count *m* and reducing AC resistance by 50 %–70 %.
  

Insulation and BIL Coordination

•  Lightning Impulse Level (BIL) does not decrease with frequency: A 10 kV system still requires 95 kV BIL, corresponding to winding spacing ≥ 16 mm and oil gap or potting resin thickness ≥ 3 mm.
  
•  Adopt 'graded insulation' + electric field shielding rings to reduce the maximum electric field strength from 3.5 kV/mm to 2.2 kV/mm, with partial discharge < 5 pC.
  
•  Nanocrystalline cores must be sleeved with 0.2 mm polyimide tubing and potted with silicone gel to prevent sharp edges from puncturing insulation.

Leakage Inductance Integration and Soft Switching

•  DAB topologies require 5–15 % leakage inductance as resonant/commutation inductance. Achieve this by 'adjusting winding spacing + adding air gaps to the core' in one step, avoiding the 15 % volume penalty of  external inductors.
  
•  Air gaps are only placed at ½ the height of the center leg to form 'semi-distributed air gaps,' pulling stray flux away from windings and reducing eddy current loss by 20 %.
  

Thermal-Structure Integration

•  Thermal flux density in 1 MW-class SST high-frequency transformers can reach 15 W/cm³, requiring 'core-winding-heat sink' 3D thermal network design.
  
•  Nanocrystalline cores can operate up to 120 °C, but potting silicone has an upper limit of 180 °C. Use hybrid cooling with 'aluminum cold plates + thermal grease + localized resin potting' to keep hot spot temperature     rise < 35 K.
  
•   Modular shell structure, single module 15–30 kW, N+1 redundancy, hot-swap maintenance time < 5 min.
  

II. Typical Engineering Failure Modes and Solutions

III. Design Parameter Quick Reference (Copper conductor, 100 °C operating temperature)

IV. One-Sentence Summary

The 'sweet spot' for SST high-frequency transformers is: 20–50 kHz using nanocrystalline + interleaved copper foil, achieving 95 kV BIL via graded insulation + potting silicone, integrating leakage inductance directly into the magnetic component, with module power of 20–30 kW and N+1 redundancy.

Get these four points right the first time, and by 2025, mainstream prototypes can achieve ≥ 98.5 % efficiency, power density ≥ 15 kW/L, and expected reliability of 10 years MTBF.