Ferrite Core Shapes Guide: E, EI, PQ, RM & Toroid

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.

1. Core shape comparison at a glance

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

2. E and EI cores

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.

3. EFD cores

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.

4. PQ cores

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.

5. RM and EP cores

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.

6. Toroid cores

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.

7. Planar cores

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.

How to choose a shape

• Height-limited board → EFD or planar
• Highest power in a round window with low EMI → PQ
• Compact multiple converters on one board → RM or EP
• EMI choke or current sense → toroid
• General SMPS transformer → E / EI / ETD

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 support on request 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.

Frequently Asked Questions

Which ferrite core shape should I choose?

It depends on the binding constraint. E and EI cores suit general transformers, EFD and planar shapes suit height-limited boards, PQ cores handle high power with low EMI, RM and EP cores are compact and shielded, and toroids give the lowest stray flux.

What is the difference between E and PQ cores?

E cores have a rectangular center leg and window, which is inexpensive to tool and easy to wind. PQ cores have a round center leg and round winding window, so copper fills the window more efficiently and stray flux is lower, which suits high-power converters.

Why are toroidal cores used for EMI chokes?

A toroid has no air gap, so it produces the lowest stray magnetic field of the common shapes and provides high effective permeability. That makes it a natural fit for common-mode chokes and current-sense applications.

When should I use a planar core?

Use a planar core when component height is the hard constraint and the converter runs at high frequency. The winding is formed in the PCB rather than from wire, giving an ultra-low profile and good thermal coupling to the board.

How does core shape affect EMI?

Shapes that partially or fully enclose the winding, such as RM, EP and pot cores, contain the magnetic field and reduce radiated interference. Open shapes such as E and EI cores radiate more, which matters when sensitive circuits sit nearby.