How to choose the materials for a magnetic core

Selecting the right material for a magnetic core is a critical decision in the design of transformers, inductors, and other electromagnetic devices. The core material directly affects efficiency, size, cost, and operating frequency. This guide explains the key factors to consider and the most common core materials available today.

1. Understand Your Operating Frequency

The frequency of the current passing through the winding is the first and most important parameter.

• Low frequency (50/60 Hz, up to a few hundred Hz):

Ideal for power transformers in utility grids. Here, silicon steel (electrical steel) is the standard choice. It offers high saturation flux density (around 1.7–2.0 T) and relatively low loss at these frequencies.

• Medium frequency (1 kHz – 100 kHz):

Common in switch-mode power supplies, wireless chargers, and automotive electronics. Options include ferrite, amorphous alloy, and powder cores. Ferrites are popular due to very low eddy current losses, but they have lower saturation (0.3–0.5 T).

• High frequency (100 kHz – several MHz):

Used in RF circuits, high-density DC-DC converters, and resonant converters. Only ferrites and nanocrystalline materials work well here. Their high resistivity keeps core losses manageable.

2. Consider Saturation Flux Density (Bₛ)

Saturation is the maximum magnetic flux density the core can handle before it stops behaving like a magnetic material. Exceeding Bₛ causes a sudden drop in inductance and may lead to overheating or circuit failure.

• High Bₛ (≥1.5 T): Silicon steel, amorphous alloy, nanocrystalline.

These allow smaller core cross-sections, reducing size and weight for low-frequency applications.

• Low Bₛ (≤0.5 T): Ferrites, powder cores (iron powder, sendust, etc.).

Require larger cores to avoid saturation, but they excel at high frequencies where low loss is more important than size.

• Practical rule: For power applications below 10 kHz, prioritize high Bₛ. For high-frequency power conversion, accept lower Bₛ to gain low loss.

3. Evaluate Core Losses (Iron Loss)

Core loss consists of hysteresis loss (from magnetic domain realignment) and eddy current loss (from circulating currents inside the core). Higher frequency and higher flux swing increase both losses.

• Silicon steel: Low loss at 50/60 Hz, but losses rise rapidly above 400 Hz. Thin laminations (0.23–0.35 mm) reduce eddy currents.
• Amorphous alloy: Very low hysteresis loss, excellent for medium frequencies (up to ~20 kHz) with Bₛ around 1.5 T. However, it is brittle and sensitive to mechanical stress.
• Ferrite: Extremely low eddy current loss because it is a ceramic insulator. Suitable for frequencies from 20 kHz to several MHz. Losses increase if temperature exceeds 100–120°C.
• Powder cores (iron, sendust, high-flux, Kool-Mu): The magnetic particles are insulated from each other, spreading eddy current loss over a wide frequency range. Good for energy storage inductors (e.g., boost converters) because they tolerate DC bias well.

4. Check Permeability (μ)

Permeability determines how easily the core concentrates magnetic flux. Higher μ means fewer turns are needed for a given inductance, reducing copper loss and size.

• High μ (2000–10,000+): Ferrites, silicon steel, nanocrystalline.
• Best for transformers where you want to maximize mutual coupling and minimize magnetizing current.
• Low to medium μ (10–200): Powder cores (e.g., iron powder μ≈20–90, sendust μ≈60–125).
• These are less sensitive to DC bias – they do not saturate easily when a large DC current flows. This makes them ideal for output chokes and PFC inductors.
• Trade-off: High μ materials usually have lower saturation Bₛ and worse DC bias performance. Low μ materials are harder to saturate but require more turns.

5. Consider Temperature Stability

Some core materials change properties significantly with temperature.

• Ferrites have a Curie temperature typically above 200°C, but their loss minimum often occurs around 80–100°C. Above 120°C, losses can increase sharply.
• Silicon steel is stable up to 200–400°C, limited only by insulation coatings.
• Powder cores are generally stable over –40°C to +150°C, making them suitable for automotive and industrial environments.

If your device operates in a hot enclosure or outdoors, verify the material's performance at the maximum expected temperature.

6. Compare Cost and Availability

• Silicon steel: Inexpensive and widely available. Best for large power transformers.
• Ferrite: Low cost for common sizes (e.g., E, ETD, PQ cores). Tooling for custom shapes is expensive.
• Amorphous and nanocrystalline: More expensive than silicon steel but cheaper than some powder cores. Used where efficiency is critical (e.g., high-efficiency power supplies).
• Powder cores: Moderate to high cost, depending on composition (sendust is cheaper than high-flux). Often sold as toroids.

7. Match the Material to Your Application

Here is a quick reference table:

Application	Recommended core material	Key reason
Mains transformer (50/60 Hz)	Silicon steel (grain-oriented)	High Bₛ, low cost
High-efficiency power transformer (1–20 kHz)	Amorphous alloy	Low loss, medium Bₛ
SMPS transformer (20–200 kHz)	Ferrite (e.g., MnZn)	Very low high-frequency loss
Common-mode choke	High-μ ferrite or nanocrystalline	High impedance at small size
DC/DC output inductor (with DC bias)	Powder core (sendust, high-flux)	Soft saturation, stable inductance
PFC choke	Iron powder or sendust	Handles large AC + DC current
RF transformer / EMI filter	NiZn ferrite	High resistivity, low loss at MHz range

Summary – A Step-by-Step Decision Process

1.What is the operating frequency?

<1 kHz → silicon steel

1–100 kHz → amorphous, ferrite, or powder

100 kHz → ferrite or nanocrystalline

2.Is there a large DC bias current?

Yes → powder core

No → ferrite or amorphous

3.What is the allowable core loss?

Very low loss needed → amorphous or nanocrystalline

Moderate loss acceptable → ferrite or silicon steel

4.What is the budget and volume?

High volume, low cost → silicon steel or ferrite

Premium efficiency → amorphous or nanocrystalline

By answering these four questions, you can narrow down the choices to one or two materials that will give you the best balance of performance, size, and cost for your magnetic core.

For more information, please contact us at sales@xfullstar.com