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Introduction

Modern electronics rely heavily on switching power supplies.

From mobile devices and industrial controls to electric vehicles and telecommunications equipment, switching regulators provide efficient voltage conversion while minimizing power dissipation.

At the heart of nearly every switching power supply is an inductor.

The inductor stores energy, filters current ripple, improves efficiency, and helps regulate output voltage.

Designing an inductor for a switching power supply requires balancing electrical, thermal, mechanical, and manufacturing considerations.

In this chapter, we will combine the concepts learned throughout this guide and walk through the complete design process used to create practical power inductors.

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The Role of the Inductor in a Switching Power Supply

The primary function of an inductor is to store energy in a magnetic field and release it when needed.

The amount of energy stored is:

As discussed in Chapter 06: Energy Storage in Magnetic Components, both inductance and current determine the energy storage capability of the component.

In switching regulators, the inductor continuously stores and releases energy during each switching cycle.

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Step 1: Define the Design Requirements

Every magnetic design begins with a set of electrical requirements.

Typical inputs include:

• Inductance (L)
• RMS Current
• Peak Current
• Switching Frequency
• Maximum DCR
• Maximum Temperature Rise
• Ambient Temperature
• Size Constraints
• Core Material Preferences
• Safety or Compliance Requirements

Without clearly defined requirements, it is impossible to optimize a design.

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Step 2: Determine Energy Storage Requirements

Before selecting a core, determine how much energy must be stored.

Use:

This equation immediately shows why current has such a significant impact on magnetic design.

Because current is squared, even modest increases in current can dramatically increase the required energy storage.

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Step 3: Select an Appropriate Core Material

As discussed in Chapter 04: Understanding Magnetic Core Materials, the operating frequency largely determines which magnetic materials are suitable.

General guidelines:

Material	Typical Frequency Range
Ferrite	20 kHz – Several MHz
Powdered Iron	Low to Medium Frequency
Amorphous	High Efficiency Designs
Nanocrystalline	High Performance Applications
Silicon Steel	Line Frequency Applications

Most switching power supplies use ferrite materials due to their low core losses at high frequencies.

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Step 4: Select a Core Geometry

The next step is choosing a core geometry.

As discussed in Chapter 08: Selecting the Right Core Geometry, geometry influences:

• Winding window area
• Cooling capability
• Saturation performance
• Manufacturing complexity
• Cost

Common choices include:

• EE Cores
• ETD Cores
• EFD Cores
• PQ Cores
• RM Cores
• Toroids

Higher power designs often benefit from ETD and PQ geometries due to their larger winding windows and improved power density.

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Step 5: Determine Required Turns

Inductance is related to turns by:

A more useful design relationship is:

Where:

• L = Inductance
• AL = Core Inductance Factor
• N = Number of Turns

Rearranging:

This equation provides an initial estimate of the required number of turns.

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Step 6: Verify Flux Density

After estimating the turns count, verify that the core will not saturate.

Magnetic flux density is:

The relationship between magnetic field strength and flux density is:

As explained in Chapter 07: Understanding Magnetic Saturation, operating too close to saturation dramatically increases losses and reduces reliability.

Most designers maintain a safety margin below the material’s saturation limit.

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Step 7: Design the Air Gap

For energy-storage inductors, the air gap is often the most important design parameter.

As discussed in Chapter 05: Understanding Air Gaps and Energy Storage, the air gap:

• Reduces permeability
• Increases saturation current
• Stores most of the magnetic energy

In many power inductors, the majority of stored energy resides within the air gap rather than the core itself.

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Step 8: Select the Winding Conductor

The conductor must safely carry the required current while fitting within the available window area.

As discussed in Chapter 09: Choosing the Correct Wire and Winding Method, common choices include:

• Round Magnet Wire
• Parallel Conductors
• Litz Wire
• Copper Foil

The resistance of the winding is:

Where:

• ρ = Resistivity
• l = Length
• A = Cross-Sectional Area

Larger conductors reduce resistance but consume more winding space.

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Step 9: Calculate Copper Losses

Copper losses are:

As explained in Chapter 11: Understanding Copper Losses, current has a squared effect on winding losses.

Reducing resistance is often one of the most effective ways to improve efficiency.

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Step 10: Estimate Core Losses

Core losses depend on:

• Material
• Frequency
• Flux Density

A common approximation is the Steinmetz Equation:

Where:

• K, α, β are material-specific constants

Manufacturers typically provide loss curves and Steinmetz parameters for their materials.

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Step 11: Evaluate Temperature Rise

Once losses have been estimated, thermal performance must be verified.

Total loss is:

Temperature rise can be estimated using:

Where:

• ΔT = Temperature Rise
• P = Total Power Dissipation
• θ = Thermal Resistance

As discussed in Chapter 12: Thermal Design of Inductors, acceptable temperatures depend on insulation class, ambient temperature, and reliability goals.

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Step 12: Verify Manufacturability

A design that works mathematically may still be impossible to manufacture.

Verify:

• Turns fit within the window
• Fill factor is reasonable
• Lead termination is practical
• Insulation requirements are met
• Assembly processes are feasible

Many otherwise excellent designs fail during this stage.

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Step 13: Optimize the Design

Optimization often requires several iterations.

Possible improvements include:

Reduce Temperature Rise

• Larger core
• Lower losses
• Better cooling

Reduce Size

• Higher operating frequency
• Higher flux density
• More compact geometry

Improve Efficiency

• Larger conductors
• Lower-loss materials
• Improved winding layout

Every improvement usually introduces trade-offs elsewhere.

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Design Trade-Offs

There is no perfect magnetic design.

Increasing one performance metric often impacts another.

Examples:

Improve	Often Requires
Lower Temperature	Larger Core
Lower DCR	More Copper
Higher Efficiency	Higher Cost
Smaller Size	Higher Frequency
Lower Cost	Reduced Performance

Successful designs balance these competing objectives.

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How Modern Design Software Helps

Traditional magnetic design often required hours or days of manual calculations.

Modern design tools can automate:

• Core selection
• Air gap calculation
• Wire selection
• Loss calculations
• Thermal estimation
• Manufacturability checks

This allows engineers to evaluate multiple design options quickly and identify the best solution for a given application.

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What You’ve Learned

In this chapter you learned:

• The role of inductors in switching power supplies
• How to define magnetic design requirements
• How core materials and geometry are selected
• How turns count is determined
• Why air gaps are required
• How conductors are selected
• How copper and core losses are estimated
• How temperature rise is evaluated
• Why manufacturability must be verified
• How optimization is performed in practical designs

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Continue Reading

Chapter 14: Common Inductor Design Mistakes

Even experienced engineers occasionally make mistakes when designing magnetic components. In the next chapter, we will examine the most common design errors, why they occur, and how to avoid them before they become costly problems.

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