Leakage Inductance: An Introduction

Key Takeaways

• Leakage inductance is an inherent characteristic of transformers caused by imperfect magnetic coupling between windings, reducing signal transfer efficiency.

• Various factors—including winding and core geometry, the number of turns, and gaps between windings—significantly impact the level of leakage inductance in a transformer or inductor.

• While leakage inductance is often considered undesirable, it has practical applications, such as in resonant converters, RF transformers, magnetic amplifiers, and inductive sensors. 

Leakage inductance, a result of magnetic leakage flux, is shown in the transformer diagram above.

In an ideal transformer, the complete magnetic energy transfers seamlessly from the primary to the secondary windings, but real-world imperfections cause a reduction in the induced signal within the secondary windings. This reduction can be modeled electrically as a self-inductance component in series with the primary windings, and this component is referred to as "leakage inductance.” 

Methods for Modifying Leakage Inductance

How Leakage Inductance Arises in Transformers

When an alternating current (AC) passes through the primary winding of a transformer, it generates a magnetic field. This magnetic field induces a voltage in the secondary winding, leading to the desired energy transfer. However, not all of the magnetic flux generated by the primary winding links with the secondary winding. Some of it escapes or "leaks" into the surrounding environment, resulting in a self-inductance within each winding. This self-inductance is what we refer to as leakage inductance.

In a transformer system, each winding behaves as a self-inductance in series with the winding's respective ohmic resistance constant. These four winding constants (primary and secondary winding self-inductance and ohmic resistance) also interact with the transformer's mutual inductance. Leakage reactance is usually the most crucial element of a power system transformer due to power factor, voltage drop, reactive power consumption, and fault current considerations.

Factors Affecting Leakage Inductance

Leakage inductance depends on the core's geometry and the windings. The voltage drop across the leakage reactance often results in undesirable supply regulation with varying transformer load. But it can also be useful for harmonic isolation (attenuating higher frequencies) of some loads. Leakage inductance is fundamentally influenced by several key factors.

• Winding Geometry: How windings are structured and arranged.
• Core Geometry: The shape and characteristics of the core material surrounding the windings also impact leakage inductance.
• Number of Turns: The quantity of turns in the windings affects how magnetic flux couples between them.
• Inter-Winding Gap: The distance between the primary and secondary winding layers can affect the degree of magnetic coupling and leakage inductance.
• Inter-Layer Gap: The gap between consecutive layers within the same winding also contributes to the leakage inductance.
• Inter-Turn Gap: The spacing between individual turns within a single winding layer.
• Overlapping Height of Windings: The extent to which the windings overlap can impact the magnetic coupling and, thus, the level of leakage inductance.

Calculating leakage inductance analytically can be challenging due to the intricate interplay of these parameters. Obtaining precise results through analytical methods can be complex. Nevertheless, understanding the influence of these factors on leakage inductance is essential, as it enables engineers and designers to effectively control and manage this phenomenon in their electrical circuits and systems.

Leakage Inductance in Practice 

Leakage inductance can have favorable and unfavorable characteristics, primarily influencing voltage fluctuations under varying loads. However, in many instances, it controls current flow within a transformer and its load without dissipating power, excluding the typical non-ideal transformer losses. Transformer designs often target specific leakage inductance values, which generate a predetermined leakage reactance at the desired operating frequency. Interestingly, the critical parameter in this context isn't the leakage inductance itself but rather the short-circuit inductance value. While leakage inductance is generally considered an undesirable characteristic, it has practical applications in various real-world scenarios:

• Resonant Converters: In resonant power converters, leakage inductance can be deliberately utilized to create soft switching, reducing switching losses and improving efficiency.
• RF Transformers: Radio-frequency (RF) transformers often employ leakage inductance to achieve impedance transformation while isolating the primary and secondary windings.
• Magnetic amplifiers used in control systems and signal processing rely on varying leakage inductance to achieve amplification.
• Inductive Sensors: Some inductive sensors utilize variations in leakage inductance to detect changes in position, proximity, or material properties.
• In specific applications involving negative resistance, like neon signs, high leakage reactance transformers come into play. Transformers are chosen for situations where both voltage amplification (due to transformer action) and current limitation are necessary. Typically, in such cases, the leakage reactance amounts to 100% of the full load impedance. This unique feature ensures that even if the transformer is short-circuited, it remains undamaged. The presence of leakage inductance is vital in preventing gas discharge lamps, with their inherent negative resistance characteristics, from conducting excessive current, thereby safeguarding them against potential damage.

Reducing Leakage Inductance

Minimizing leakage inductance is essential in transformer and inductor design to optimize performance. In situations where a higher level of leakage inductance is desired, options include inserting a magnetic shunt between layers or employing fractional turns. Methods of decreasing leakage inductance include the following:

• Increase Core Material Permeability: Using core materials with higher permeability can help concentrate magnetic flux, reducing its escape into the surroundings.
• Improve Winding Techniques: Techniques such as interleaved or bifilar windings can enhance magnetic coupling and reduce leakage.
• Utilize Magnetic Shields: Magnetic shields, typically made from materials like mu-metal, can be placed strategically around the windings to confine the magnetic flux.
• Reduce Air Gaps: Minimize air gaps within the core, which can lead to higher leakage inductance.
• Optimize Core and Winding Geometry: Careful design of the transformer or inductor's physical geometry can help reduce leakage inductance by ensuring that magnetic flux lines are concentrated within the core. Examples include 
• Decreasing the number of turns and layers.
• Reducing the thickness of the insulator layer.
• Shortening the mean turn length.
• Widening the core window.
• Lowering the core window height.

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