MHz-Range FRA for High-Frequency Transformers: Diagnosing Solid-State Power Electronics
Solid-state transformers (SSTs) and medium-frequency transformers (MFTs) operate at 10–500 kHz, far above the 50/60 Hz of conventional grid transformers. Their cores use nanocrystalline or ferrite materials, and windings are often foil or Litz wire. Applying a Transformer Frequency Response Analyzer to these high-frequency assets requires modifications: extended frequency range (up to 50 MHz), understanding ferrite core behavior, and specialized interpretation. This article presents FRA techniques for modern power electronic transformers.
High-Frequency Transformer Construction Differences
MFTs and SSTs differ from line-frequency transformers:
Core materials: Nanocrystalline (e.g., Vitroperm) or ferrite (e.g., N87) have higher resistivity and lower hysteresis losses, leading to different low-frequency FRA signatures.
Winding types: Litz wire, copper foil, or PCB-embedded windings. These have distributed capacitance and lower self-resonant frequencies than conventional round wire.
Leakage inductance optimization: SSTs often require precise leakage for resonant converters. FRA measures leakage inductance as the low-frequency roll-off.
No oil insulation: Air or encapsulation (epoxy, silicone) has lower permittivity, shifting resonances higher in frequency.
Expected FRA Signatures for Healthy High-Frequency Transformers
A typical 10–500 kHz SST transformer produces:
Low-frequency band (10 Hz – 1 kHz): Very low amplitude (-60 to -80 dB) because ferrite permeability drops below 10 kHz. Nanocrystalline retains permeability down to 100 Hz, giving -40 to -60 dB.
Operating band (1–500 kHz): Flat response (within ±2 dB) with 1–3 broad resonances due to leakage inductance and winding capacitance.
High-frequency band (500 kHz – 50 MHz): Multiple sharp resonant peaks from winding parasitic resonances. Foil windings produce fewer peaks than Litz.
Detecting Core Cracking in Ferrite Transformers
Ferrite cores are brittle and prone to cracking under mechanical or thermal stress. FRA detects core cracks as:
Low-frequency band amplitude increase (2–6 dB below 100 kHz) due to reduced core losses (crack lowers effective permeability but also reduces loss).
Shifting of the first resonant peak upward by 5–15% because cracked core lowers inductance.
Asymmetrical changes if only one core leg is affected (e.g., in EE or UU cores).
Case Example: Ferrite Core Crack in 100 kW SST Transformer
A 100 kW solid-state transformer (20 kHz operation) experienced increased temperature rise and audible noise. FRA from 100 Hz to 30 MHz showed:
Low-frequency (100 Hz – 1 kHz) amplitude increased by 4 dB
First resonant peak shifted from 18 kHz to 21 kHz (17% up)
Phase asymmetry: primary measured from A vs. B terminals differed by 8° (previously 1°)
These changes indicated a cracked ferrite core. Disassembly confirmed a hairline crack through the center leg of the EE core. The core was replaced, and FRA returned to baseline. Without FRA, the crack would have expanded, causing excessive magnetizing current and eventual transformer failure.
Testing Protocol for High-Frequency SST Transformers
Adapt standard FRA to MHz-range requirements:
Use an instrument with bandwidth up to 50 MHz (many power FRA units stop at 25 MHz; SSTs may need higher).
Perform FRA with transformer in its final assembly (including busbars, snubbers, and gate drive transformers).
Use short, shielded leads (maximum 1 m) to avoid parasitic capacitance that corrupts >10 MHz measurements.
For three-phase SSTs (common in EV charging and data centers), compare phases—healthy units show CC > 0.90 between phases.
Measuring Leakage Inductance via Low-Frequency FRA
Resonant converters require precise leakage inductance (Lk). FRA provides Lk by fitting the low-frequency roll-off:
From the FRA amplitude slope below the first resonance, calculate Lk using A = 20 × log(2πf × Lk / R_ref).
Compare computed Lk to design value. If Lk deviates >10%, inspect winding spacing or shorted turns.
For solid-state transformer engineers, a high-bandwidth Transformer Frequency Response Analyzer is essential for verifying core integrity, winding alignment, and leakage inductance. FRA diagnostics reduce infant mortality and enable condition-based maintenance in next-generation power electronics.
