Resistivity (ρ) – The Eddy Current Killer and Selection Bible for Magnetic Cores

In the world of magnetic components, we often focus on the resistance of copper wires while overlooking the inherent resistance of magnetic cores-resistivity (ρ). Do not be fooled by their seemingly insulating appearance. In high-frequency applications, the resistivity (ρ) of magnetic cores is a "hidden killer" that suppresses eddy current loss and determines temperature rise. Today, we uncover the practical role of ρ.

 

Eddy Current: The Root Cause of Internal Loss at High Frequencies

 

When an alternating magnetic field passes through a conductor (including the magnetic core material itself), it induces closed circular currents known as eddy currents.

Hazard: Eddy currents act like short-circuit rings inside the magnetic core, converting magnetic energy into heat, causing device overheating, sharp efficiency drops, and even thermal runaway.

Defense by ρ: The higher the resistivity (ρ), the more obstacles in the eddy current path, the smaller the eddy current is restricted, and the lower the loss.

In short: In high-frequency design, the resistivity (ρ) of magnetic cores is your first line of defense!

 

Resistivity (ρ) Ladder of Magnetic Materials: A Hard Metric for Material Selection

 

Requirements for ρ vary drastically across applications. Blind material selection leads to overheating or even component failure. Below is a practical comparison of mainstream magnetic materials with complete resistivity ranges:

 

Material Category	Typical Representatives	Resistivity (ρ) Range (Ω·m)	High-Frequency Performance & Typical Applications
Metallic Magnetic Materials	Silicon Steel Sheets	1×10⁻⁷ ~ 1×10⁻⁶	Extremely low ρ, very high eddy current loss. Only for low-frequency (kHz-level) power transformers.
Alloys	Amorphous / Nanocrystalline	1×10⁻⁶ ~ 1×10⁻⁵	Moderate ρ, eddy currents suppressed by ultra-thin strips. Suitable for medium-high frequency (10 kHz–1 MHz) high-power applications.
Ferrites	MnZn Ferrite	1 ~ 1×10³	High ρ, excellent eddy current suppression for mid-to-high frequencies. Widely used in 100 kHz–1 MHz switching power supplies and power transformers.
Ferrites	NiZn Ferrite	1×10² ~ 1×10⁶	Extremely high ρ, natural "insulator" against eddy currents. Preferred for MHz-level high-frequency use, e.g., RF transformers, high-frequency inductors.
Advanced Dielectrics	Microwave Ceramics	1×10¹⁰ ~ 1×10¹⁴	Insulation champion, extremely low loss at ultra-high frequencies. Core material for GHz-level communication resonators and microwave devices.

 

Material Selection Rules

 

Low-frequency high-power (<100 kHz): Prioritize saturation flux density; choose silicon steel sheets regardless of low ρ.

Medium-high frequency high-efficiency (100 kHz–500 kHz): Balance flux density and loss; choose nanocrystalline.

High-frequency / RF (>500 kHz): Prioritize low loss; must choose ferrites with as high ρ as possible!

 

Avoid These Pitfalls: Three Common Misconceptions About ρ

 

Misconception 1: Lower conductor ρ is always better, so gold or silver plating is best.

Truth: Gold/silver plating for connectors mainly resists oxidation and maintains contact to avoid increased contact resistance. Simply pursuing lower conductor ρ brings negligible performance improvement in most circuits but drastically raises cost.

 

Misconception 2: Materials with higher ρ are better.

Truth: ρ is only one metric. Although ferrites have high ρ, their low saturation flux density makes them unsuitable for high-power low-frequency transformers. Selection must comprehensively consider frequency, power, and flux density.

 

Misconception 3: Temperature has no effect on ρ.

Truth: ρ of metals increases with temperature (positive temperature coefficient), worsening eddy current loss; semiconductor-type materials show the opposite trend. ρ variation over the full temperature range must be considered in design.

Notes on Resistivity Values

 

All values are industry-standard typical ranges for commercial-grade magnetic materials, with unit unified as ohm-meter (Ω·m) for consistent comparison.

Resistivity of ferrite materials varies significantly by material grade and formulation: high-permeability MnZn ferrites have lower ρ, while low-loss NiZn ferrites have orders of magnitude higher ρ.

Amorphous/nanocrystalline alloys achieve lower eddy current loss not only by higher ρ than silicon steel, but also by being processed into ultra-thin strips to further limit eddy current paths.