Chapter 12: Thermal Design of Inductors

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Introduction

Every inductor generates heat.

No magnetic component is perfectly efficient.

Core losses and copper losses convert a portion of the electrical energy into thermal energy, causing the component temperature to rise above ambient conditions.

Proper thermal design ensures that an inductor:

• Operates safely
• Meets efficiency goals
• Achieves its expected lifetime
• Remains reliable under all operating conditions

In this chapter, we will explore how inductors generate heat, how temperature rise is estimated, and the design techniques used to improve thermal performance.

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Why Temperature Matters

Temperature affects nearly every aspect of magnetic component performance.

As temperature increases:

• Copper resistance increases
• Efficiency decreases
• Core properties change
• Insulation ages more rapidly
• Reliability decreases

Excessive temperature is one of the most common causes of magnetic component failure.

Good thermal design helps ensure long-term reliability.

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Sources of Heat in an Inductor

Most heat comes from two primary sources:

Copper Losses

Current flowing through the winding resistance produces heat.

Copper loss is:

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Core Losses

Changing magnetic fields generate losses inside the magnetic material.

Core losses increase with:

• Frequency
• Flux density
• Material characteristics

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Total Power Dissipation

The total heat generated by the inductor is:

This total loss becomes the primary source of temperature rise.

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Ambient Temperature

Inductor temperature is always measured relative to ambient temperature.

For example:

Ambient	Temperature Rise	Inductor Temperature
25°C	40°C	65°C
50°C	40°C	90°C
70°C	40°C	110°C

This is why thermal design must always consider the actual operating environment.

A design that works perfectly in a laboratory may overheat inside a sealed enclosure.

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Thermal Resistance

Thermal resistance describes how effectively heat flows from the inductor into the surrounding environment.

It is commonly expressed as:

The relationship is:

Where:

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

This equation is often the first estimate used during magnetic design.

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Example Temperature Rise Calculation

Suppose an inductor produces:

• Copper Loss = 0.8 W
• Core Loss = 0.4 W

Total Loss:

If the thermal resistance is:

The temperature rise becomes:

At an ambient temperature of 40°C:

This simple approach provides a useful first-order estimate.

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Copper Temperature Effects

Copper resistance increases as temperature rises.

The relationship is approximately:

Where:

• RT = Resistance at Temperature T
• R25 = Resistance at 25°C
• α ≈ 0.00393/°C for copper

As the winding becomes hotter:

• Resistance increases
• Copper losses increase
• Additional heat is generated

This positive feedback effect is one reason thermal analysis is important.

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Core Temperature Effects

Magnetic materials also change with temperature.

Common effects include:

Reduced Permeability

Some materials lose permeability as temperature increases.

Increased Losses

Certain materials experience higher losses at elevated temperatures.

Curie Temperature

At sufficiently high temperatures, ferrite materials lose their magnetic properties.

This temperature is known as the Curie Temperature.

Although most designs never approach this limit, it represents an important material boundary.

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Surface Area and Cooling

Larger inductors often dissipate heat more effectively.

This occurs because:

• Surface area increases
• Heat transfer improves
• Thermal resistance decreases

This is one reason larger magnetic components often operate cooler despite handling more power.

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Natural Convection Cooling

Most inductors rely on natural convection.

Heat is transferred through:

1. Conduction inside the component
2. Convection to surrounding air
3. Radiation from exposed surfaces

Natural convection is:

• Simple
• Low cost
• Highly reliable

Many power supplies depend entirely on natural convection cooling.

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Forced Air Cooling

Higher power systems frequently use airflow.

Advantages:

• Lower thermal resistance
• Higher power capability
• Reduced component temperature

Disadvantages:

• Increased system cost
• Noise
• Reduced reliability due to moving parts

Forced-air cooling is commonly used in:

• Server power supplies
• Industrial drives
• Telecom equipment

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Thermal Design Guidelines

Engineers typically improve thermal performance by:

Reducing Copper Losses

Lower resistance produces less heat.

Reducing Core Losses

Lower flux density and better materials reduce thermal loading.

Increasing Core Size

Larger surface area improves cooling.

Improving Airflow

Better airflow reduces thermal resistance.

Using Lower-Loss Materials

Material selection directly affects heat generation.

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Thermal Classes and Insulation Ratings

Insulation systems are assigned temperature classes.

Class	Maximum Temperature
A	105°C
B	130°C
F	155°C
H	180°C

Operating well below these limits generally improves reliability and lifetime.

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Reliability and Temperature

Temperature strongly influences product lifetime.

A common engineering rule states:

Every 10°C reduction in operating temperature can approximately double component lifetime.

Although the exact relationship varies, cooler designs are generally more reliable.

This is why thermal design is often considered as important as electrical design.

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What You’ve Learned

In this chapter you learned:

• Why thermal design is important
• The primary sources of heat in inductors
• How temperature rise is estimated
• The role of thermal resistance
• How copper resistance changes with temperature
• Why magnetic materials are affected by heat
• The difference between natural and forced-air cooling
• Practical methods for improving thermal performance

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Continue Reading

Chapter 13: Designing Inductors for Switching Power Supplies

Now that we understand magnetic materials, winding methods, losses, and thermal performance, we are ready to combine these concepts into the complete design process used to create inductors for modern switching power supplies.

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