Due to some future projects, I feel I have to get more accustomed to winding my own transformers/inductors, so I have been reading about ferrite cores lately. One thing I can't really develop a feeling for is the core shape nomenclature.

I understand the basic core design such as E-I, E-E, U-I, L-L etc. where the letters represent the core shape schematically. But my question is about e.g. the following shapes:

EFD: enter image description here PM: enter image description here PQ: enter image description here

From all I can see, these core shapes are not manufacturer-specific but are being used across different manufacturers universally.

My question can be split into two aspects:

  1. Do these letters still refer to some aspect of the shape, that I am too dumb to see? Or maybe to some kind of performance metric? Or what background do they have?

  2. Given two different shapes, but otherwise same permeance/reluctance, am I right to assume that these will give identical inductance values (for same no. of turns), but potentially slightly different coupling values when used in a multi-winding assembly like a transformer? Is the choice for different core designs then mainly motivated by the shape of the construction volume and ease of winding or do these shapes have important performance differences of the kind that make or break their usability for certain applications?

  • 2
    \$\begingroup\$ EFD: Efficient/Economic Flat Design, derived from Easy-To-Drive (round centre, from E(shape), square centre). Not so sure: Power Mantel (shell type) and Power sQuare/Quatratic (or Powder/Pulver?). \$\endgroup\$
    – greybeard
    Commented Jan 2, 2023 at 8:09
  • \$\begingroup\$ 2. Yes, but please also compare mean length of turn (copper losses), minimum core cross section area (saturation limit) and core volume (core losses) for the two examples and you'll find more trade-offs to be made. \$\endgroup\$
    – winny
    Commented Jan 2, 2023 at 13:06

3 Answers 3


The round shell styles derive from 'P' ("Pot") style, of which there are low-profile or planar (not pictured), wide opening (PM), sQuare (PQ), etc. available.

The tangential shape (how the core is arranged at angular positions around the generally circular winding) isn't very important for magnetic design purposes. If the winding areas have similar area and aspect ratio, and the core cross section (\$A_e\$) and length (\$l_e\$) are comparable, then the cores overall are comparable.

Inductance depends on area, length and effective permeability, while saturation depends on just area.

For a transformer (read: ideally very high magnetizing inductance), air gap is minimal, a high-mu material is chosen (typically over 2000), and cross-sectional and winding areas are chosen to fit the required wire with enough turns to avoid saturation at the highest amplitude and lowest frequency used.

For an inductor, things are a bit looser, with the air gap being a free variable; as you adjust core size, air gap and turns, you will find a minimum size that respects all the ratings (core loss, Isat). Larger cores can be used, with fewer turns and less air gap, or similar turns and much more air gap, but the larger core will generally incur higher core or copper losses, and obviously costs more.

What's left is more subtle aspects, like stray fields, leakage inductance (the part that literally leaks out of the assembly, not just what's measured between windings), thermal performance (say, an EE or toroid core exposes much more winding surface area to free air than a P does) and so on.

Pot cores were/are popular for LF-HF filters because the nearly total coverage shields the winding, giving very low coupling to nearby coils. (Types are available with a hollow center peg, into which a threaded insert can be placed, for a threaded ferrite/powdered iron slug to trim inductance.)

PQ, RM and ELP cores are quite popular these days, for high power density and low profile (including planar magnetics) applications.

Rounded E styles are also quite popular (ETD, ER, EC, etc.), for lower cost, and often higher winding area, trading ferrite for copper. As mentioned, the exposed winding can have good thermal performance; the downside is a ton of leakage that can make EMI problems. (Often a foil shield is wrapped around the assembly to short out some of these fields; this is more important for types like this, than PM or PQ, and hardly important at all for P.)

And there are hybrids, like the EP shape, surrounded on three sides by ferrite so it has excellent shielding and compact size. These aren't available in large sizes, perhaps because thermal performance isn't so great (other types like PQ being preferable), but serve along with other compact types like EFD for SMT transformers.


Just want to add my grain of sand to the discussion, as Etymology is always useful :)

PM stands for Power Module

PQ stands for Power and Quality (thanks to TDK in the 90s)

There is more about all the shapes here: The Etymology and Economy of Shapes

  • \$\begingroup\$ Excellent reference. \$\endgroup\$
    – Marla
    Commented Jan 7, 2023 at 16:43

Cores with the same AL, regardless of shape, are intended to give you the same nominal inductance. Some of the second order parameters, especially those due to winding shape, like coupling (for low permeability cores), cooling, ratio of copper/core losses, SRF, might be slightly different, but the cores should be broadly magnetically interchangeable.

The letters might have meant something in one manufacturer's system (are we bothered about the history?), but it's safest to regard the letters as being pulled out of a random bag. Different manufacturers might have intentionally gone for the same letters to ease the choice of their products as a second source for a competitor's design they have seen to be successful. However, don't rely on it. Work to dimensions and to electrical parameters.

The different physical shapes are all to do with packaging. For instance, the EFD core appears to be intended to sit flat on a height-constrained PCB, perhaps assembled after the windings bobbin has been mounted. PQ is essentially the same as PM, with a lot of side access for lead-outs compared to 4-return-leg designs, but will use the space between two walls in a space constrained design more efficiently.

Rather than the manufacturers brainstorming what shapes would sell, it's more likely that from time to time, a large customer has come to them and said 'I would buy 10 million if you can make it fit in this space, and 10% smaller on that dimension, with the same power throughput'. Hence the plethora of slightly different shapes, as all the specials have ended up in the catalogue.

  • \$\begingroup\$ FWIW I'd guess Mullard / Philips Gloeilampen Fabrieken / ... \$\endgroup\$
    – Russell McMahon
    Commented Jan 2, 2023 at 10:00

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