No transformer is perfect.

In an ideal transformer, all magnetic flux generated by the primary winding couples directly into the secondary winding. In real transformers, however, some magnetic flux does not link both windings.

This uncoupled magnetic flux creates what engineers call leakage inductance.

Leakage inductance is one of the most important non-ideal characteristics of transformers and can significantly affect efficiency, EMI, voltage stress, and overall converter performance.

This guide explains what leakage inductance is, why it occurs, and how engineers manage it in practical transformer designs.

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What Is Leakage Inductance?

Leakage inductance is inductance created by magnetic flux that does not couple between windings.

Ideally:

• All primary flux links the secondary.
• All secondary flux links the primary.

In reality:

• Some flux remains localized.
• Some magnetic energy is not transferred.

This uncoupled flux behaves like an unwanted inductance in series with the winding.

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Why Leakage Inductance Exists

Leakage inductance occurs because magnetic fields occupy physical space.

Factors that contribute include:

• Winding separation
• Insulation thickness
• Bobbin geometry
• Core geometry
• Winding arrangement

No practical transformer can completely eliminate leakage inductance.

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Coupling Coefficient

Transformer coupling is often described using the coupling coefficient:

k=\frac{M}{\sqrt{L_1L_2}}

Where:

• k = Coupling coefficient
• M = Mutual inductance
• L₁ = Primary inductance
• L₂ = Secondary inductance

Perfect transformers have:

k = 1

Real transformers typically have:

k < 1

The lower the coupling coefficient, the higher the leakage inductance.

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Why Leakage Inductance Matters

Leakage inductance affects:

• Efficiency
• Voltage spikes
• Switching stress
• EMI
• Thermal performance

In high-performance power supplies, even small amounts of leakage inductance can create significant problems.

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Leakage Inductance in Flyback Converters

Leakage inductance is especially important in flyback converters.

👉 Related Guide: Flyback Transformer Design Basics

When the primary switch turns off:

• Stored leakage energy cannot transfer to the secondary.
• Voltage spikes occur.
• MOSFET stress increases.

Without proper protection, these voltage spikes can damage switching devices.

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Voltage Spikes and Ringing

Leakage inductance often causes:

• Overshoot
• Ringing
• High-frequency oscillations

These effects increase:

• EMI
• Switching losses
• Component stress

Designers frequently observe these effects using an oscilloscope.

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Snubber Circuits

To manage leakage inductance, engineers often use snubbers.

Common examples include:

RC Snubber

• Simple
• Low cost

RCD Clamp

• Common in flyback converters

Active Clamp

• Improved efficiency
• Higher complexity

These circuits absorb or recycle leakage energy.

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Winding Arrangement Matters

One of the most effective methods of reducing leakage inductance is improving winding placement.

Common techniques include:

Primary Over Secondary

Simple construction but higher leakage.

Interleaved Windings

Excellent coupling.

Lower leakage inductance.

Higher manufacturing complexity.

Sectional Windings

Used in higher-power designs.

The winding arrangement often has a larger impact than the core itself.

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Core Geometry Effects

Core geometry influences magnetic coupling.

Popular transformer core styles include:

• EE
• ETD
• EFD
• PQ

Each geometry provides different winding windows and coupling opportunities.

Proper core selection can help reduce leakage inductance.

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Leakage Inductance vs Isolation

Reducing leakage inductance is not always the only objective.

Isolation requirements often increase:

• Winding spacing
• Insulation thickness
• Creepage distance

These requirements may increase leakage inductance.

Transformer design always involves balancing competing requirements.

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Leakage Inductance and EMI

Leakage inductance is a common source of EMI problems.

It can generate:

• High-frequency ringing
• Radiated emissions
• Conducted emissions

Good transformer design often improves EMI performance significantly.

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Thermal Effects

Leakage inductance indirectly affects temperature rise.

Additional switching losses increase:

• MOSFET heating
• Transformer heating
• System temperature

👉 Related Guide: Inductor Temperature Rise Explained

Reducing leakage inductance often improves overall thermal performance.

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Measuring Leakage Inductance

Engineers commonly measure leakage inductance using:

• LCR meters
• Impedance analyzers
• Specialized transformer test equipment

Measurements are often performed by shorting one winding while measuring the other.

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Practical Design Guidelines

To reduce leakage inductance:

• Minimize winding separation
• Use interleaved windings
• Optimize bobbin geometry
• Select appropriate core structures
• Reduce unnecessary insulation thickness where permitted

Perfect coupling is impossible, but significant improvements are often achievable.

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Modern Design Software

Modern magnetic design tools can evaluate:

• Coupling coefficient
• Leakage inductance
• Core losses
• Saturation margin
• Thermal performance

This helps engineers optimize designs before building prototypes.

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Conclusion

Leakage inductance is an unavoidable characteristic of real transformers.

While it cannot be eliminated entirely, understanding its causes and effects allows engineers to improve efficiency, reduce EMI, lower switching stress, and build more reliable power supplies.

Proper winding design, core selection, and magnetic optimization remain the most effective tools for controlling leakage inductance.

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Looking Ahead

Future SolidMagnetics platform releases are planned to include transformer design tools capable of evaluating coupling, leakage inductance, thermal performance, and manufacturability alongside automated CAD generation.