Choosing the right core shape is one of the first decisions in magnetic component design. Each core geometry has trade-offs in winding area, flux path, mounting, cost and performance. The right choice depends on power level, frequency, height constraint and manufacturing volume.
This guide compares the most common core shapes and helps you select the right one.
Shaanxi SiRui Technology Co., Ltd. manufactures transformers and inductors using EE, EI, ETD, toroidal, planar and custom cores. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. EE / EI Cores
The workhorse shapes. Large winding window, easy to wind on bobbins, widest availability and lowest cost. Best for general-purpose transformers, flyback, forward converters and inductors. EI is slightly cheaper than EE. Best for cost-sensitive consumer power.
2. Toroidal Cores
Ring-shaped with closed flux path. Lowest leakage inductance and EMI. No bobbin needed. But harder to wind (requires toroidal machine), harder to terminate, and difficult to add an air gap. Best for common mode chokes, inductors, current transformers and low-EMI applications.
3. ETD / EER Cores
Modified E-core with round center leg and optimized winding area. Better copper utilization than EE, lower profile. Good for high-frequency SMPS, resonant converters and telecom power. Slightly more expensive than EE.
4. Planar Cores
Very low height (3–10 mm), excellent thermal performance, repeatable automated manufacturing. But higher cost (PCB trace windings), limited to lower power. Best for compact DC-DC converters, telecom and point-of-load modules.
5. Comparison Table
| Shape | Height | EMI | Cost | Winding Ease | Power Range |
|---|---|---|---|---|---|
| EE/EI | Medium | Medium | Low | Easy | 10 W–5 kW |
| Toroidal | Med-low | Low | Medium | Hard | 5 W–2 kW |
| ETD/EER | Low | Low-med | Medium | Medium | 50 W–3 kW |
| Planar | Very low | Low | High | PCB traces | 10 W–500 W |
6. How to Choose
- Cost-sensitive, general purpose: EE or EI
- Low EMI, closed flux: toroidal (CMC, inductor)
- High frequency, low profile: ETD or planar
- Tight height constraint: planar
- Need adjustable air gap: EE/EI (gap in center leg)
7. Sirui-Tech Capability
- EE/EI transformers: 5 W–5 kW
- Toroidal: CMC, inductors, current transformers
- Planar: low-profile DC-DC magnetics
- Custom shapes: customer-specified geometries
Conclusion
Core shape selection balances performance, cost, height and manufacturing. EE/EI remain the workhorses. Toroidal wins for low-EMI CMC and inductors. Planar is the answer for compact, low-profile designs. ETD offers an optimized compromise for high-frequency resonant converters.
Need help selecting a core shape? Contact Sirui-Tech at sales@sirui-tech.com with your power level, frequency and height constraint.