EE vs ETD Ferrite Cores: When to Use Each
EE cores use a rectangular center leg and window — cheap tooling, easy winding and fine up to a few hundred watts. ETD cores use a round center leg and round window, which fills with round magnet wire more efficiently, cutting copper loss and supporting higher power. Choose EE for cost-sensitive low-to-mid power; choose ETD when efficiency, power density or high frequency matters.
EE and ETD are two of the most common ferrite core families for transformers, and the difference between them comes down to geometry. This guide compares the two shapes and gives a practical rule for when each one wins.
1. The geometry difference
An EE core has a rectangular center leg and a rectangular winding window. An ETD (Economic Transformer Design) core has a round center leg and a round winding window. That single change drives most of the performance difference: round wire fits a round window much better than a square one.
2. Copper fill and copper loss
In an EE core, the square window leaves unused corners around round magnet wire, so the same window area carries less effective copper. The ETD round window maximizes fill, which means lower DC resistance and lower copper loss for the same footprint — the reason ETD dominates higher-power designs where every percent of efficiency counts.
3. Power range and thermal behavior
EE cores are economical for low-to-mid power transformers and inductors (roughly tens to a few hundred watts). ETD cores, with better copper fill and a larger effective window, carry into the kilowatt range and dissipate heat more evenly across the round structure.
4. EMI and stray flux
Both are open shapes that radiate more stray flux than shielded RM/EP or round PQ cores. If EMI is the hard constraint, consider PQ for high power or RM for compact designs; if cost and simplicity win, EE or ETD with careful layout is fine.
EE vs ETD at a glance
| Property | EE | ETD |
|---|---|---|
| Center leg / window | Rectangular | Round |
| Copper fill | Lower (wasted corners) | Higher (round wire fills window) |
| Typical power | Low–mid | Mid–high |
| Tooling cost | Lower | Moderate |
| Copper loss | Higher for same window | Lower |
| Typical use | SMPS transformers, chokes | High-power SMPS, inverters |
When to use which
- Cost-sensitive low-to-mid power, simple assembly → EE
- Higher power with efficiency targets → ETD
- Height-limited or planar boards → EFD or planar cores instead
- LLC at high power with EMI concerns → PQ (round window, low stray flux)
Both families are part of the standard ferrite shape set covered in the Ferrite Core Shapes Guide, and the High-Frequency Transformer article walks through the material and loss side of the same decision. Loss data for these grades lives in the Technical Data tables.
YUTE Magnetics manufactures FeNi powder cores, MnZn/NiZn ferrite cores and custom power inductors, with ISO 9001, IATF 16949 and ISO 14001 management systems in place. AEC-Q200 is assessed per part number — ask for the qualification status of the specific reference you need. Share your frequency, current and size targets and an application engineer will review them.
Frequently Asked Questions
What is the difference between EE and ETD cores?
Both are E-type geometries, but ETD cores have a round center leg and round winding window while EE cores have a rectangular leg and window. The round window fills more efficiently with round magnet wire, which lowers copper loss at higher power.
Which core shape is better for higher power?
ETD-type cores are generally better at higher power because the round window accommodates more copper with lower resistance and the geometry handles heat better. EE cores remain a good fit for lower-power designs where cost and winding simplicity dominate.
How does core geometry affect copper loss?
A window that matches the winding shape fills more completely, allowing a larger conductor cross-section for the same turns. Larger cross-section means lower resistance and therefore less copper loss at the same current.
Is there a cost difference between EE and ETD cores?
EE cores are generally less expensive to tool and easier to wind, which makes them attractive at lower power. ETD cores cost more but recover that in efficiency and thermal performance as power rises.
How do I choose between them?
Match the core to the power level and the height allowed. Use EE where power is modest and simplicity or cost matters, and ETD where higher power, lower copper loss and better thermal behaviour are worth the additional cost.