How to Choose Soft Magnetic Materials for High-Frequency Applications

You have spent 3 months designing a compact, elegant power supply for a very advanced electronic device. You start it up, run it under stress and it appears OK for approximately 10 minutes. After that, the temperatures rise, the efficiency falls dramatically and the entire system shuts down because of overheating.

What went wrong? You used a traditional magnetic core which could not withstand high frequencies.

As you scale up the operating frequency to make power electronics smaller, lighter and more efficient, standard magnetic cores become miniscule heaters. The selection of the appropriate soft magnetic materials is the most critical in today's high frequency design.

This article will explain the working principles of high frequency magnetic materials, the types of materials available and how to select the right core for your project.

What Are Soft Magnetic Materials?

Let's keep things simple with the basic definition, before we head into high frequencies. A soft magnetic material is a material with a large magnetic susceptibility; it can be magnetized easily through a magnetic field acting on it, but it quickly loses magnetic characteristics when this field is removed.

These materials do not dissipate much energy as heat because they can change their magnetic direction millions of times per second without any resistance. That reset is quick and that's why they are vital for today's alternating current (AC) electronics. You'll see them working silently in the cores of power transformers, electric motors, and high-speed energy storage inductors.

Why Frequency Changes Everything

Standard silicon steel works fine at standard low frequencies, such as 50Hz or 60Hz power coming directly from a wall socket. It maintains a strong magnetic field, has a slow switching rate and remains fairly cool.

However, at higher frequency (hundreds of kilohertz (kHz) or megahertz (MHz)), the core undergoes physical modifications:

• First, there is hysteresis loss. Whenever the magnetic field is reversed, a small portion of energy is trapped and transformed into heat. Such tiny losses accumulate into huge thermal losses when repeated millions of times a second.
• Secondly, high frequencies induce high eddy currents. They are microscopic electrical loops that swirl inside conductive metal cores. Eddy currents are a microscopic built-in heating element that toasts the component inside out instead of letting the energy through it cleanly.

To achieve a cool and efficient electronic system, you must select soft magnetic materials which are specially made to minimize such losses at high frequencies.

Comparing Soft Magnetic Materials for High Frequencies

Each material has its own set of benefits for different frequency bands, dimensions, and power levels. Let's break down the most common material choices.

1. Ferrite Cores (MnZn & NiZn)

Ferrites are ceramic oxides manufactured by sintering iron oxide with other metals such as manganese, zinc or nickel.

• Why choose them: They have an extraordinarily high electrical resistivity due to their ceramic composition. With ferrites, no eddy currents will be generated; therefore, ferrites remain cool at high operating frequencies, up to multi-megahertz frequencies. They are also economical in mass production.
• Trade-offs: Ferrites possess low saturation flux density. They can easily saturate under heavy current loads. Normally, the cores need to be scaled up to prevent saturation, which is a challenge for achieving compact designs.

2. Nanocrystalline Alloys

Nanocrystalline ribbons are prepared by rapidly cooling molten Fe-based alloys into thin ribbons then annealed to achieve a uniform microscopic grain size of approximately 10 nm.

• Why choose them: Nanocrystalline alloys have very low core loss and extremely high magnetic permeability, as well as high magnetic saturation. This combination allows to handle much more power in a core that is up to 50% smaller than a similar ferrite core. They also exhibit good performance at low temperatures, and good performance at high temperatures.
• Trade-offs: Ultra-thin metallic ribbons require additional processing steps, which increases the initial price of the ribbons compared to standard ferrites. But the smaller overall component footprint often results in lower overall system costs.

3. Amorphous Alloys

Amorphous metals are formed by rapid cooling of the liquid alloy, in which no crystal structure has sufficient time to grow. This results in a non-crystalline flexible metallic ribbon.

• Why choose them: They offer better magnetic saturation than ferrites and reduced core loss compared to standard silicon steel. They are ideal for medium frequency use where high current capacity is needed.
• Trade-offs: Core losses grow faster in amorphous alloys than in nanocrystalline alloys or ferrites at frequencies over 100 kHz.

4. Powdered Metal Cores

Powdered cores are created by coating microscopic grains of a metal alloy such as iron-silicon-aluminum or nickel-iron with a forming binder and pressing it into place under a high pressure.

• Why choose them: They have a distributed air gap, which is spreaded throughout the material. This design avoids sudden magnetic saturation on encountering high current spikes, with inductance drooping gradually.
• Trade-offs: Core losses are greater at high frequency compared to ferrites or nanocrystalline alloys, and can lead to thermal issues in operation at high continuous loads.

Step-by-Step Guide: How to Select the Right Core

To select a core in your new high frequency project, this quick checklist will help you:

Step 1: Pinpoint Your Operating Frequency

The switching frequency directly affects which material families are usable. The core losses increase non-linearly as frequency is raised, making it important to match the material to your desired band.

• Less than 50 kHz: Silicon steel or amorphous alloys are used for low cost and high power handling.
• 50-200 kHz: Nanocrystalline cores have an optimal balance of low loss and high power density.
• Over 200 kHz: It is preferable to use Manganese-Zinc (MnZn) ferrites or powdered metal cores due to their high electrical resistivity.

Step 2: Determine Required Saturation ($B_s$)

For application where high peak currents or space restrictions are involved, high saturation flux density ($B_s$) core material is required. The higher the $B_s$ value, the more magnetic energy the core can store without saturation, which means that smaller components can be designed without the concern of circuit failure.

• Medium to High Saturation (1.2 T – 1.6 T): Powdered metals and nanocrystalline alloys.
• Low Saturation (0.3 T - 0.5 T): Standard ferrites.

Step 3: Compare Core Loss ($P_c$) Curves

Always verify the core loss ($P_c$) curves published by the manufacturer under your working temperature and switching frequency. Reduced core loss results in less heat generation and increased operating efficiency. More detailed core loss testing methods can be found at  IEEE Xplore.

Step 4: Verify Thermal & Environmental Stability

The core loss generates heat, which, accordingly, alters the magnetic permeability. Ensure that the core you choose has a uniform magnetic characteristic in your operating temperature range (typically -40° C to +125° C).

Nanocrystalline and high-grade ferrite alloys exhibit better temperature-versus-loss properties compared to iron powder forms at lower cost.

Conclusion

The selection of the right soft magnetic material depends essentially on three fundamental engineering parameters: operating frequency, power density and thermal limits.

With the trend of faster switching speeds to reduce size and weight, the use of dated core technologies will result in excessive heat and energy loss. Knowing the particular strengths of ferrites, nanocrystalline alloys, amorphous metals, and powdered cores allows you to choose a material to keep your system cool, compact, and efficient.

At Transmart, from lightweight nanocrystalline cores offering optimal performance in electric vehicle power applications, to low-loss ferrites ideal for high-frequency signal circuits, choosing the right core material to meet your specific frequency profile guarantees your design will function reliably over the long haul.