Quick AnswerHigh-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.
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.
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.
| 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 |
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.
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. Send your frequency, power and height targets for a free engineering evaluation.
Our engineering team responds within 24 hours with a free technical evaluation.
Request A Quote