Chapter 10: Understanding Core Losses

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Introduction

Whenever a magnetic component operates with a changing magnetic field, energy is lost within the core material itself.

These losses are known as core losses.

Unlike copper losses, which occur in the winding conductors, core losses occur directly inside the magnetic material.

Core losses convert electrical energy into heat and can significantly affect:

• Efficiency
• Temperature rise
• Reliability
• Maximum operating frequency

Understanding core losses is essential when designing inductors and transformers for switching power supplies.

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What Causes Core Losses?

Core losses occur because magnetic materials are not perfectly efficient.

As magnetic flux repeatedly increases and decreases, the magnetic domains inside the material must continually realign themselves.

This process consumes energy.

Core losses are generally divided into two primary categories:

1. Hysteresis Loss
2. Eddy Current Loss

The total core loss is approximately the sum of these two mechanisms.

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Hysteresis Loss

Magnetic materials contain millions of microscopic magnetic domains.

When a magnetic field is applied, these domains attempt to align with the field.

As the field changes direction, the domains must repeatedly rotate and realign.

This process is not perfectly efficient.

The energy required to continually move these domains appears as heat.

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Understanding the B-H Curve

The relationship between magnetic field strength and magnetic flux density is represented by a B-H curve.

The area enclosed by the B-H loop represents energy lost during each magnetic cycle.

The larger the loop area:

• The greater the hysteresis loss
• The more heat generated
• The lower the efficiency

Magnetic materials with narrow B-H loops generally exhibit lower core losses.

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Hysteresis Loss Factors

Hysteresis losses increase with:

Frequency

More magnetic cycles occur every second.

Flux Density

Larger magnetic swings require more domain movement.

Material Selection

Different materials exhibit different hysteresis characteristics.

Ferrites generally have much lower hysteresis losses than silicon steel at high frequencies.

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Eddy Current Loss

A changing magnetic field induces voltage inside conductive materials.

This voltage creates circulating currents within the core.

These circulating currents are called:

Eddy Currents

Eddy currents produce resistive heating inside the magnetic material.

Just like current flowing through a wire, these currents generate heat and waste energy.

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Why Eddy Currents Occur

According to Faraday’s Law, changing magnetic flux induces voltage.

Although this equation is normally used to describe transformer action, the same principle creates unwanted voltages within the core material itself.

These induced voltages drive eddy currents.

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Reducing Eddy Current Losses

Engineers use several methods to reduce eddy currents.

Ferrite Materials

Ferrites possess extremely high electrical resistance.

This resistance greatly limits eddy current flow.

This is one reason ferrites dominate modern switching power supplies.

Laminated Steel

Low-frequency transformers often use thin laminated steel sheets.

The laminations interrupt the current paths and dramatically reduce eddy currents.

Powdered Materials

Powdered iron and similar materials consist of insulated particles.

These particles naturally limit eddy current formation.

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Frequency and Core Losses

Core losses increase rapidly with frequency.

As switching frequency rises:

• More magnetic cycles occur
• Hysteresis losses increase
• Eddy current losses increase

This is why magnetic material selection becomes increasingly important at higher frequencies.

A material that performs well at 20 kHz may perform poorly at 500 kHz.

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Flux Density and Core Losses

Core losses also increase rapidly with flux density.

Flux density is:

As flux density increases:

• Domain movement increases
• Hysteresis loss rises
• Eddy current loss rises
• Temperature rise increases

Operating near saturation often causes losses to increase dramatically.

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The Steinmetz Equation

Manufacturers commonly use the Steinmetz Equation to estimate core losses.

Where:

• Pcv = Core loss per unit volume
• K = Material constant
• f = Frequency
• Bmax = Peak Flux Density
• α = Frequency exponent
• β = Flux density exponent

The values of K, α, and β are provided by the material manufacturer.

The equation allows engineers to estimate losses before building hardware.

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Comparing Common Core Materials

Manganese Zinc Ferrite (MnZn)

Advantages:

• Very low losses
• Excellent high-frequency performance
• Most common power ferrite

Typical Range:

• 20 kHz to several MHz

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Nickel Zinc Ferrite (NiZn)

Advantages:

• Lower losses at very high frequencies
• Excellent EMI applications

Typical Range:

• Hundreds of kilohertz to tens of megahertz

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Powdered Iron

Advantages:

• Distributed air gap
• Good DC bias capability

Disadvantages:

• Higher losses than ferrite

Typical Range:

• Below approximately 1 MHz

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Amorphous Materials

Advantages:

• Extremely low hysteresis loss
• Excellent efficiency

Disadvantages:

• Higher cost

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Silicon Steel

Advantages:

• Excellent at line frequency

Disadvantages:

• Excessive losses at switching frequencies

Typical Range:

• 50 Hz to several kilohertz

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Core Loss and Temperature Rise

Core losses appear directly as heat.

Higher core losses produce:

• Higher temperatures
• Lower efficiency
• Reduced reliability

A design that looks electrically correct may still fail because excessive core losses create unacceptable temperatures.

Thermal analysis should always include both:

• Core losses
• Copper losses

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How Designers Minimize Core Losses

Engineers commonly reduce core losses by:

Lowering Flux Density

Operate farther from saturation.

Selecting Better Materials

Choose materials optimized for the operating frequency.

Reducing Frequency

Lower frequency reduces magnetic cycling losses.

Increasing Core Size

A larger core often reduces flux density.

Reviewing Manufacturer Loss Curves

Core manufacturers provide loss curves that should always be consulted during design.

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What You’ve Learned

In this chapter you learned:

• What core losses are
• The difference between hysteresis and eddy current losses
• Why B-H loop area affects energy loss
• How frequency influences core losses
• Why flux density affects efficiency
• How the Steinmetz Equation estimates losses
• The strengths and weaknesses of common magnetic materials
• Practical methods for reducing core losses

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Continue Reading

Chapter 11: Understanding Copper Losses

Core losses are only one source of heat within a magnetic component. In the next chapter, we will examine copper losses, DC resistance, AC resistance, skin effect, proximity effect, and how conductor selection influences efficiency and temperature rise.

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