Ferrite Cores for SMPS: Transformer & Inductor Guide
SMPS magnetics run from ~50 kHz to 1 MHz+, where MnZn ferrite offers the best balance of low core loss, high permeability and cost. Choose the material grade by switching frequency, the shape by power and height (E/EI for general transformers, PQ for high-power LLC, RM for compact shielded designs), and check the operating flux stays well below 0.3–0.5 T saturation. For high-current output inductors, a FeNi powder core with Bs ≥1.5 T often beats ferrite.
Almost every switch-mode power supply (SMPS) contains magnetic components: a transformer for isolation and voltage scaling, an inductor for energy storage, and often an EMI choke. This guide covers how to select the ferrite cores for those parts, and when a different material is the better engineering call.
1. What SMPS magnetics ask of the core
SMPS cores must handle a defined frequency, a power level, a peak current and a thermal budget. The two dominant constraints are core loss at the switching frequency and saturation at the peak current. Ferrite wins the loss battle below ~1 MHz; powder cores win the saturation battle at high current.
2. Material: why MnZn ferrite dominates SMPS
MnZn ferrite combines high permeability (1000–15000) with very low core loss in the 100 kHz–1 MHz range, which is exactly the SMPS operating window. NiZn ferrite (µ 10–500) takes over above ~1 MHz for EMI and RF roles, where its high resistivity limits eddy-current loss. Above the ferrite frequency window, metal-powder cores (FeNi, Sendust, MPP) become competitive for inductors.
3. Core shapes for SMPS
| Shape | Typical SMPS role |
|---|---|
| E / EI | General-purpose transformers and inductors up to a few hundred watts |
| ETD / EER | Mid-to-high power transformers with a large round window |
| PQ | High-power LLC / DC-DC with low stray flux |
| RM / EP | Compact, shielded designs for telecom and board power |
| EFD | Low-profile transformers for height-constrained boards |
| Toroid | EMI chokes and filter inductors with the lowest stray field |
4. Transformer vs inductor design
A transformer is sized mainly by core loss and turns ratio; an inductor is sized by stored energy (½·L·I²) and saturation. Ferrite transformer cores are usually gapped lightly or not at all, while inductor cores need an air gap — discrete for ferrite, distributed for powder cores — to store energy and keep inductance stable under DC bias.
5. Selection method
- Define switching frequency, output power, peak current and height limit
- Pick the material grade from the loss curves at your frequency (see [Technical Data](/engineering/technical-data/))
- Choose the shape from power, profile and EMI constraints
- Calculate the operating flux B = L·I/(N·Ae) and keep it below 0.3–0.5 T for ferrite
- Verify temperature rise and inductance tolerance with samples
When the output inductor carries high DC current, compare ferrite against a FeNi powder core with Bs ≥1.5 T — the smaller core and stable bias behavior often lower the total cost of the converter. See the High Flux Cores Guide and the Core Selection Guide for the full comparison.
YUTE Magnetics supplies custom ferrite cores and FeNi powder-core inductors for SMPS with IATF 16949 quality management, plus AEC-Q200 stress testing per part number on request. Share your frequency, power and height targets for a free engineering evaluation.
Frequently Asked Questions
What ferrite cores are used in SMPS?
Manganese-zinc ferrite grades are the standard choice for switch mode power supplies, because they combine low core loss, high permeability and reasonable cost across the typical 50 kHz to 1 MHz switching range.
How do I select a ferrite grade for a switching power supply?
Select by switching frequency first, then check core loss at the actual flux swing and operating temperature. Permeability, saturation and Curie temperature matter too, but the loss curve at your frequency is usually what separates candidate grades.
What core shapes suit SMPS transformers?
E and EI cores are common for general-purpose transformers, PQ cores suit higher-power designs with lower EMI, and EFD or planar shapes suit height-limited boards. The choice follows from power level, available height and winding requirements.
Do SMPS inductors and transformers use different cores?
They often do. The transformer transfers energy and is usually ungapped, while an inductor stores energy and may be gapped or use a powder core. Selecting one core type for both functions generally produces a compromise.
Should I specify core loss at my operating frequency?
Yes. A single datasheet loss figure measured at one frequency and flux level does not describe how the material behaves at your operating point, so ask for loss data at the real switching frequency, flux swing and temperature.