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Introduction

One of the most surprising concepts in magnetic design is that engineers intentionally make magnetic cores less efficient.

At first glance, this seems counterintuitive. Previous chapters explained that magnetic materials provide a low-reluctance path for magnetic flux, improving inductance and magnetic performance.

However, many power inductors and energy-storage magnetic components intentionally include an air gap within the magnetic circuit.

This air gap reduces inductance but dramatically improves the component’s ability to store energy and resist magnetic saturation.

In this chapter, we will examine why air gaps are used and how they affect magnetic performance.

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What Is an Air Gap?

An air gap is a small non-magnetic separation intentionally introduced into a magnetic circuit.

Air gaps may be created by:

• Machining the core
• Using spacers
• Purchasing pre-gapped cores
• Distributing gaps throughout powdered materials

Although the gap may be only a fraction of a millimeter, its impact on magnetic performance can be significant.

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Why Add an Air Gap?

Without an air gap, magnetic flux travels almost entirely through the high-permeability core material.

This produces high inductance but allows saturation to occur at relatively low current levels.

Introducing an air gap increases the reluctance of the magnetic circuit.

As a result:

• Inductance decreases
• Saturation current increases
• Energy storage capability increases

This tradeoff is fundamental to power inductor design.

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Air Gaps Increase Reluctance

Magnetic circuits behave similarly to electrical circuits.

In an electrical circuit:

Resistance limits current flow.

In a magnetic circuit:

Reluctance limits magnetic flux.

Magnetic reluctance is:

Where:

• Rm = Magnetic Reluctance
• l = Magnetic Path Length
• μ = Permeability
• A = Cross-Sectional Area

Because air has much lower permeability than ferrite, even a small air gap contributes a large portion of the total reluctance.

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The Effect on Inductance

As air gap length increases, inductance decreases.

The relationship is approximately:

Where:

• L = Inductance
• g = Air Gap Length

This means:

• Larger gap → Lower inductance
• Smaller gap → Higher inductance

Engineers must carefully balance these competing requirements.

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Why Energy Is Stored in the Gap

Many engineers assume energy is stored inside the magnetic core.

In reality, most of the energy in a gapped power inductor is stored within the magnetic field surrounding the air gap.

The energy stored in an inductor remains:

Where:

• E = Stored Energy
• L = Inductance
• I = Current

The air gap allows the magnetic field to support much higher current levels before saturation occurs, enabling greater total energy storage.

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Air Gaps and Saturation Current

Increasing the air gap raises the current required to saturate the magnetic circuit.

This allows:

• Higher load current
• Larger transient currents
• Greater energy storage
• Improved overload capability

For this reason, nearly all energy-storage inductors contain an air gap.

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Distributed Air Gaps

Not all air gaps are visible.

Powdered iron and some composite materials contain thousands of microscopic air gaps distributed throughout the material.

These distributed gaps provide:

• Improved DC bias performance
• Better energy storage
• Reduced fringing effects

This is one reason powdered iron remains popular in many power applications.

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Fringing Flux

Air gaps introduce another important phenomenon known as fringing flux.

As magnetic flux crosses the air gap, it expands outward beyond the physical dimensions of the core.

This expanded magnetic field can:

• Increase nearby copper losses
• Generate EMI
• Affect nearby components

Good magnetic designs account for fringing effects when positioning windings and selecting core geometries.

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Comparing Gapped and Ungapped Cores

Characteristic	Ungapped Core	Gapped Core
Inductance	Higher	Lower
Saturation Current	Lower	Higher
Energy Storage	Lower	Higher
Reluctance	Lower	Higher
Fringing Flux	Lower	Higher

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What You’ve Learned

In this chapter you learned:

• What an air gap is
• Why air gaps are intentionally added
• How air gaps affect inductance
• Why energy is stored in the gap
• How air gaps improve saturation performance
• What distributed air gaps are
• Why fringing flux occurs

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Continue Reading

Chapter 06: Energy Storage in Magnetic Components

In the next chapter, we will examine how inductors store energy, how energy density affects core selection, and how engineers calculate the energy storage requirements of real-world power electronics systems.

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