For same 6.8mH common mode inductors, why the performance difference?

During EMC filtering design ,engineers may encounter such problem: two similar size common mode chokes, both with indunctance 6.8mH, rated current 1A, the actual test results are different, one can pass the conduction test, while the other insufficient suppression ability in certain frequency bands, and even has a significant decrease in inductance after operation.

One reason is that the core design is different. The important parameter that determines the magnetic flux carrying capacity of a magnetic core is Ae (Effective Core Area).

Ae represents the equivalent cross-sectional area of magnetic field lines passing through a magnetic core.

Ae determines the magnetic flux density

One of the most important formulas in core design:

B magnetic flux density (T), L inductance (H), I winding current (A), N winding turns, Ae effective cross-sectional area (m ²).

When L, I, and N are all constant: the smaller Ae, the higher the magnetic flux density B. And if the magnetic flux density is too high, it will cause the magnetic core to approach saturation.

What is the relationship between Ae and magnetic flux?

Magnetic Flux Formula:

Φ = B × A

(Φ Magnetic Flux (Wb), B Magnetic Flux Density (T), A Area Through Which Flux Passes (m²)

For the core:Φ = B × Ae

Thus: The larger the Ae, the less magnetic flux per unit area it carries, making the core easier to remain in the linear region.

How are the three parameters Ae, le, and AL related?

In engineering, Ae cannot be considered in isolation. Core design typically focuses on three parameters:

Ae determines the flux-carrying capacity, le determines the magnetic path impedance, and AL (L = AL×N²) determines the relationship between the number of turns and inductance.

Why does a magnetic core saturate?

There are a large number of magnetic domains inside the core. During normal operation, an external magnetic field drives the rotation of these magnetic domains, causing the core to exhibit a high magnetic permeability.

However, as the magnetic field continues to strengthen, the magnetic domains gradually reach their limit—so that even with continued increases in current, the growth of magnetic flux slows down more and more. This is known as magnetic saturation.

After saturation, the following are observed: ① Decrease in inductance ② Increase in current ③ Increase in temperature rise ④ Deterioration of EMI performance.

Real Example:How to calculate a 6.8mH common-mode inductor?

Objective: To make a 6.8mH common-mode inductor. Core parameters: Ae = 40mm², AL = 1700nH/N².

① Calculate the number of turns: N =√(L/AL) =√(6800000÷1700)

① Now calculate the magnetic flux density:B = (L × I) / (N × Ae)

Engineers make judgments based on the results: whether it exceeds the material's Bsat, whether the core needs to be increased in size, and whether the material needs to be adjusted.

Why Can't You Just Look at 'How Many mH' When Choosing a Common Mode Inductor?

Inductance is just the result. What truly determines performance is: core performance = material + Ae + le + winding structure. For the same 6.8mH inductance, different cores can lead to: different impedance curves, different temperature rise, different saturation capabilities, and different EMI suppression effects.

Common-mode inductor selection is not about choosing an inductance value, but rather about selecting a set of core operating conditions.

Conclusion

Ae may seem like just a core size parameter. But it connects to: Faraday's electromagnetic induction theory, Maxwell's electromagnetic field theory, and today's magnetic component engineering design. For common-mode inductors: understanding Ae is understanding why the core can work.

Original information based on https://mp.weixin.qq.com/s/RV3ivi_HhQnH943AnVap5A