Core of Magnetic Cores: Thickness, Frequency & Permeability

Apr 24, 2026

In electromagnetic devices such as transformers, inductors, and motors, the magnetic core serves as the fundamental framework. Its performance directly determines the efficiency, temperature rise, power density, and even service life of the equipment. ribbon thickness, operating frequency, and magnetic permeability are the three key parameters affecting core performance, which are often confused during component selection.

To put it simply: the core performance of an magnetic core is mainly evaluated by core loss (heat loss), excitation current, saturation capability, and temperature rise. These three parameters do not act independently but interact and cooperate with each other. Only by choosing the right matching relationship can the magnetic core achieve optimal performance.

1. Ribbon Thickness: Determines Heat Resistance, Critical for High

Frequencies

Magnetic cores are mostly laminated or wound from thin magnetic ribbons. Although thickness seems insignificant, it is critical to eddy current loss-one of the main causes of core heating, especially at high frequencies. Thickness directly determines whether the core will overheat and fail.

Core Rule

The thinner the ribbon, the lower the eddy current loss and the better the high-frequency performance, but the higher the cost.

Simple Explanation

When current passes through the core, eddy currents (small internal currents) are generated. Larger eddy currents cause more severe heating. Thinner ribbons reduce the size and lengthen the path of eddy current loops, greatly decreasing eddy current loss.

A key law:

Eddy current loss is proportional to the square of ribbon thickness.

Doubling the thickness increases eddy current loss by a factor of 4.

Example

Power frequency (50Hz): e.g. 220V transformers, industrial motors.

Commonly used strips: 0.35mm, 0.27mm grain-oriented silicon steel.

Moderate thickness avoids excessive heating while controlling cost.

High frequency (50kHz–200kHz): e.g. phone chargers, switching power supplies.

Must use ultra-thin amorphous or nanocrystalline ribbons of 20–30μm.

A typical 65W fast charger uses 25μm nanocrystalline ribbon to suppress high-frequency eddy current loss. Using 0.35mm silicon steel would cause rapid overheating, overheating protection, or even failure.

Drawback

Thinner ribbons increase manufacturing difficulty and inter-layer gaps, reducing effective magnetic cross-sectional area. This may require a larger core volume and higher cost.

2. Operating Frequency: Defines the Core's Operating Limit

Frequency is the lifeline of the magnetic core. Performance changes drastically across different frequencies. Most selection failures stem from mismatching frequency and core material.

Core Rule

Higher frequency → higher core loss → weaker performance → requires specialized matching.

Three Major Effects at High Frequency

Sharply increased heating

Total core loss (hysteresis loss + eddy current loss) rises with frequency:
Hysteresis loss ∝ frequency
Eddy current loss ∝ frequency²
Doubling frequency causes exponential loss growth, overheating, and severe efficiency drop.

Reduced effective permeability

At high frequencies, magnetic domains cannot keep up with field changes, so effective permeability decreases. Larger excitation current is required to establish sufficient magnetic field.

Derated operation required

To control temperature rise, the working flux density must be reduced.
Power-frequency transformers: ~1.5T
High-frequency switching supplies: only 0.2–0.5T
Overdriving power without adjusting flux density or strip thickness leads to immediate overheating and burnout.

3. Magnetic Permeability (μ): Determines Field-Building Efficiency

Permeability is the key index of soft magnetic materials, measuring how easily the core conducts and establishes a magnetic field. It directly affects no-load loss and field efficiency.

Core Rule

Higher permeability → easier field establishment → lower loss, but must match frequency.

Four Key Effects

Lower excitation current

Higher permeability reduces required magnetomotive force (excitation current × turns), lowering no-load loss and improving efficiency.
For example, on-board transformers in electric vehicles use high-permeability nanocrystalline cores (μ = 10⁵–10⁶), cutting no-load loss by over 60%.

Lower hysteresis loss

High-permeability materials (nanocrystalline, permalloy) have low coercivity and narrow hysteresis loops, reducing hysteresis loss.

Higher space utilization

Higher permeability achieves higher flux density with the same winding, allowing smaller core size and more compact design.

Important warning

High permeability ≠ universal performance.
Conventional high-permeability materials have poor high-frequency stability and drop sharply at high frequencies.
For high-frequency use:

Nanocrystalline > Amorphous > Grain-oriented silicon steel

(For GHz ultra-high frequency: ferrite is preferred.)

4. Coupling Logic: Practical Selection Principles

In real applications, the three parameters must be matched.

01 High-frequency applications (tens of kHz to MHz)

Use ultra-thin strip + high-frequency-stable permeability material
Thin strip suppresses eddy current loss; stable permeability compensates for high-frequency drop.
Examples: fast chargers, high-frequency telecom transformers, 5G base station transformers.

02 Low-frequency / power-frequency (50Hz–1kHz)

Use thicker strip + medium permeability material
Thickness has little effect on eddy current loss; balances performance and cost.
Examples: distribution transformers, motors for home appliances. 03 Thickness–frequency matching

03 Thickness–frequency matching