Ferrite Cores for High-Frequency Transformers
High-frequency transformers run from ~100 kHz to 1 MHz in LLC, flyback and resonant converters, where MnZn ferrite delivers the lowest core loss per volume. Size the core by loss at the switching frequency, keep the flux swing below saturation (0.3–0.5 T), and pick the shape by power and profile: E/EI for general use, ETD for high power, PQ for low stray flux and LLC, planar for low profile.
A transformer that switches at 100 kHz–1 MHz+ is limited by core loss long before copper loss becomes critical. Ferrite remains the standard material in this window because it combines low loss, high permeability and low cost. This guide covers the selection method and the material and shape choices.
1. Why frequency decides the material
Core loss grows steeply with frequency and flux swing. MnZn ferrite has its loss minimum in the 100 kHz–1 MHz range, which is why it dominates LLC, flyback and forward converters. Above ~1 MHz, NiZn ferrite or metal-powder materials are needed; below ~50 kHz, nanocrystalline or tape-wound cores can beat ferrite on size.
2. Material grade selection
Ferrite suppliers publish loss curves per material grade. Select the grade whose loss curve is lowest at your switching frequency and operating flux swing (ΔB), not at a single datasheet point. Power grades for SMPS are MnZn with permeability 1000–3000 typical; high-permeability grades are for signal transformers and EMI.
3. Core shapes for high-frequency transformers
| Shape | Why it fits HF transformers |
|---|---|
| E / EI | Large winding window, easy assembly, good for low-to-mid power |
| ETD / EER | Round window reduces copper losses at higher power |
| PQ | Round center leg, low stray flux — preferred for LLC |
| Planar E/I | Ultra-low profile with PCB windings for high current density |
| RM / EP | Compact and shielded for board-mounted converters |
4. Flux density and turns
Keep the peak flux swing below the ferrite saturation limit (0.3–0.5 T) with margin, and size the core so the loss-generated temperature rise stays within the thermal budget. More turns reduce flux but increase copper loss — the optimum balances both at the actual frequency.
5. Design method
- Define frequency, power, input/output voltage and height
- Pick the material grade from loss curves at the switching frequency
- Choose the shape by power level and EMI requirements
- Iterate turns and core size until loss and temperature are in budget
- Prototype and verify efficiency and hot-spot temperature
For the loss and permeability data behind these choices, see the Technical Data tables and the Ferrite Core Guide. If your transformer sits at the boundary where ferrite saturation becomes limiting, the High Flux Cores Guide covers the powder-core alternative.
YUTE Magnetics supplies custom ferrite cores for high-frequency transformers, including tooling design and AEC-Q200 qualification support on request. Send your frequency, power and height targets for a free engineering evaluation.
Frequently Asked Questions
What ferrite is used in high-frequency transformers?
Manganese-zinc ferrite grades are standard for transformers operating from roughly 100 kHz to 1 MHz in LLC, flyback and resonant converters, because they deliver the lowest core loss per volume in that range.
How do I size a high-frequency transformer core?
Size it by core loss at the switching frequency, keeping the flux swing safely below saturation. The available winding window and the isolation distances the design requires also constrain the choice, so electrical and mechanical limits are usually settled together.
Why is flux swing limited in ferrite transformers?
Ferrite saturates at roughly 0.3–0.5 T, and the saturation level falls as temperature rises. Keeping the flux swing below the saturation limit with margin prevents the inductance collapse and current spike that occurs if the core saturates.
Which core shapes are used for high-frequency transformers?
E and EI cores are common for general designs, PQ and ER-type cores suit higher power with lower stray flux, and EFD or planar shapes suit height-constrained boards.
Should the core be gapped?
It depends on the topology. Transformers that transfer energy with symmetrical flux are usually ungapped, while resonant designs may use a defined gap to set the required inductance precisely.
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