Quick AnswerCore shape determines winding window, heat path, EMI and mounting. E/EI cores suit open transformers, EFD suits low-profile boards, PQ’s round center leg minimizes EMI at high power, RM/EP cores are compact and shielded, toroids give the lowest stray flux, and planar cores suit high-frequency low-profile designs. Match the shape to your power level, profile and EMI budget.
Once the ferrite material is chosen, the core shape decides how much copper fits, how the heat flows, how much flux leaks and how the part mounts on the board. Here is a practical guide to the common ferrite core shapes and when each one wins.
| Shape | Profile | Winding area | EMI / stray flux | Typical use |
|---|---|---|---|---|
| E / EI | Medium | Large | Moderate (open) | SMPS transformers, inductors |
| EFD | Low | Medium | Low (shielded sides) | Low-profile planar boards |
| ETD / EER | Medium | Large round window | Moderate | Mid-to-high power transformers |
| PQ | Medium | Round center leg | Low (round window) | High-power DC-DC, LLC |
| RM | Compact | Medium | Low (shielded) | Telecom, filters, compact PSU |
| EP | Very compact | Small | Very low (fully shielded) | Filters, small transformers |
| Toroid | Low | Limited (inner hole) | Lowest (no gap) | EMI chokes, power inductors |
| Pot | Round, compact | Medium | Very low (fully enclosed) | RF coils, filters |
| Planar | Ultra-low | PCB winding | Low | High-frequency low-profile converters |
E and EI cores are the workhorses of switch-mode power supplies. The large winding window accepts thick wire or multiple windings, and the open structure makes tooling and assembly simple. EI pairs add a lower I leg for transformers; EE pairs are common for power inductors with a controlled gap in the center leg.
EFD (Economic Flat Design) cores extend the E shape sideways to reduce height, which suits low-profile boards and planar assembly. They keep a relatively large winding window for their height and are widely used in notebook and on-board converters.
PQ cores have a round center leg and a circular winding window, so the copper fills the window more efficiently and the round geometry keeps stray flux low. This makes PQ a preferred shape for high-power LLC converters and DC-DC stages where EMI and copper loss both matter.
RM cores are compact and partially shielded, which reduces crosstalk between adjacent converters on the same board — a key reason they appear in telecom and compact power supplies. EP cores are almost fully enclosed, giving the lowest radiated EMI for sensitive filter and signal applications.
Toroids have no air gap, so they produce the lowest stray field and highest effective permeability of the shapes. They are the default for EMI chokes and current-sense inductors, and their symmetric geometry is friendly to automatic winding — but winding is slower for high-turn-count designs and the inner hole limits wire size.
Planar E and I cores sit flat over PCB copper windings, giving an ultra-low profile and excellent thermal coupling to the board. They are the right choice for high-frequency, high-density converters where height is the hard constraint.
YUTE Magnetics supplies custom ferrite cores across E, EI, EFD, PQ, RM, toroid, pot and planar geometries, with material grade selection, tooling design, AEC-Q200 qualification and volume production. Share your frequency, power and height targets for a free engineering evaluation.
See the Ferrite Core Guide, the Core Selection Guide and the Technical Data tables for material grades and loss curves.
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