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Palomar Engineers Guide to Mitigating RFIEMI with Ferrite Selection

2025-11-13

Electromagnetic interference (RFI/EMI) can significantly degrade the performance of carefully designed circuits, often rendering them nonfunctional. This technical challenge represents not just an engineering problem but also a substantial waste of time and resources.

Two Primary Ferrite Families, Each with Distinct Advantages

Ferrite materials are typically divided into two main categories, each optimized for different frequency ranges and performance characteristics:

1. Nickel Zinc (NiZn) Ferrites – Mix 43, 52, 61: The Ideal Choice for High-Frequency Applications
  • Low permeability range (20–850 µ): Ensures greater stability at high frequencies with reduced saturation risk
  • High resistivity: Minimizes eddy current losses for improved efficiency
  • Moderate temperature stability: Reliable performance across operational temperature ranges
  • High Q factor: Delivers sharper resonance peaks in tuned circuits
  • Optimal frequency range: 500 kHz–100 MHz, making them perfect for high-frequency applications

Applications:

  • Low-power, high-inductance resonant circuits
  • Broadband transformers
  • Baluns and ununs (unbalanced-to-unbalanced transformers)
  • High-frequency RFI/EMI suppression

Performance benefits: NiZn ferrites demonstrate optimal performance between 2 MHz and several hundred MHz, making them the preferred choice for most baluns, ununs, and high-frequency RFI/EMI suppression applications.

2. Manganese Zinc (MnZn) Ferrites – Mix 31, 73, 75, 77: The Powerhouse for Low-Frequency Applications
  • High permeability values (typically above 850 µ): Provides greater impedance at low frequencies for more effective noise suppression
  • Lower resistivity: Suitable for applications requiring higher current handling
  • Moderate saturation flux density: Capable of handling significant power levels
  • Exceptional low-frequency performance: Outstanding RFI/EMI suppression in the low-frequency spectrum
  • Optimal frequency range: 1 kHz–1 MHz, specifically designed for low-frequency applications

Applications:

  • Switch-mode power transformers (20–100 kHz)
  • Low-frequency RFI/EMI suppression
Quick Selection Guide
  • NiZn (Mix 43, 52, 61): Best for broadband, high-frequency applications including baluns, ununs, and high-frequency RFI/EMI suppression
  • MnZn (Mix 31, 73, 75, 77): Ideal for low-frequency, high-impedance RFI suppression and power line filtering, including common-mode chokes and power line noise suppression
Technical Specifications of Ferrite Materials
Mix # Material Initial Permeability RFI/EMI Suppression Range Tuned Circuits Broadband Transformers
31 MnZn 1500 1-300 MHz – 1:1, <300 MHz
43 NiZn 800 25-300 MHz <10 MHz 3-60 MHz
52 NiZn 250 200-1000 MHz <20 MHz 1-60 MHz
61 NiZn 125 200-1000 MHz <100 MHz 1-300 MHz
73 MnZn 2500 <50 MHz <2 MHz <10 MHz
75/J MnZn 5000 150 kHz–10 MHz <0.75 MHz 0.1-10 MHz
What Are Ferrites?

Ferrites are ceramic materials with unique electromagnetic properties. They are rigid and brittle, with colors ranging from silver-gray to black. Their electromagnetic characteristics can be affected by operating conditions including temperature, pressure, field strength, frequency, and time.

There are two fundamental types of ferrites: "soft" ferrites that don't retain significant magnetization, and "hard" ferrites with permanent magnetization characteristics. The materials discussed in this article are all "soft" ferrites.

Ferrites have a cubic crystal structure with the chemical formula MO·Fe 2 O 3 , where MO represents a combination of divalent metal oxides (such as zinc, nickel, manganese, and copper). Varying these metal oxide combinations creates materials with properties tailored for specific applications.

Historical Background

The history of ferrites (magnetic oxides) dates back centuries before Christ with the discovery of naturally magnetic stones. The most abundant deposits were found in the Magnesia region of Asia Minor, giving rise to the name magnetite (Fe 3 O 4 ).

Early applications included lodestones used by navigators to locate magnetic north. Scientific understanding progressed through contributions from William Gilbert, Hans Christian Ørsted, Michael Faraday, James Clerk Maxwell, Heinrich Hertz, and others.

Modern ferrite development began in the 1930s in Japan and the Netherlands, with J.L. Snoek at Philips Research Laboratories achieving the first commercially viable "soft" ferrites in 1945. Today, ferrites serve three primary electronic applications: low-level signal processing, power applications, and electromagnetic interference (EMI) suppression.