Transformer leakage inductance fully analyzed: from principle to application ??

Transformer leakage inductance is often considered a “hidden critical factor” in high frequency switching power supplies and general power electronics design. In this article, we'll take a closer look at what leakage inductance is, the factors that influence it, and how it can be utilized or suppressed to improve overall performance.??

1.What is leakage sensing?

Leakage inductance refers to the transformer, due to incomplete coupling of magnetic flux and the resulting leakage interference. Simply put, it is the primary and secondary coupling in the coupling process “missed” part of the magnetic flux, so called leakage inductance.

• Impact on efficiency and performance: excessive leakage inductance will lead to reduced efficiency, noise and radiation interference, but also in the switching moment to trigger the reaction electromotive force, may lead to short-circuit overvoltage breakdown.
• The scope of this paper: we often talk about leakage inductance, mainly refers to the main structure of the leakage inductance, the following content also centers on this part.

2. Effects of leakage inductance and grinding ??

1. High Frequency Increase: In a high frequency operating environment, the leakage inductance will increase with frequency, which may affect the overall stability of the power supply in severe cases.
2. Electromagnetic interference and noise: leakage inductance will not only bring EMI radiation interference, but also may increase the circuit noise.
3. Risk of instantaneous overvoltage breakdown: The reaction electromotive force generated by the switching of the transformer increases as the leakage inductance rises, which can easily cause overvoltage damage to the relevant pins.

Therefore, the size of the leakage inductance needs to be carefully reduced or controlled in the design.

3、The main factors affecting leakage inductance

3.1 Core Structure

• Closed core: If the core itself does not require an air gap, try to use a closed structure such as ring or square, which can minimize the escape of leakage flux.
• Depth and Width of Air Gap: When an air gap is opened in the center column of the core, leakage inductance is generally reduced. However, if the design requires a larger air gap, the change in leakage inductance needs to be weighed against the actual requirements.

3.2 Winding structure and shape

• Curvature, turns ratio, width, number of layers: these all directly affect leakage inductance.
• Increase the peripheral width: Reduce the leakage inductance by using “even winding” in the number of secondary turns;
• Sandwich winding method: Leakage inductance can be controlled within 1% of the inductance by the layered layout of primary-secondary-primary.

3.3 Other tools in the design

• Segmented Air Gap: With an enlarged air gap, the use of a segmented air gap allows for better heat dispersion and reduces the concentration of leakage sensing.
• Coil design: adopting double-slot coil, utilizing maze to separate the primary and secondary coils, which can effectively increase or control the leakage inductance while ensuring the safety distance.

4. When do I need to “increase” leakage? ??

Although the smaller leakage inductance is desirable in most situations, in LLC resonant transformers we sometimes substitute resonance by increasing leakage inductance to simplify the circuit and reduce cost.

• Double slot structure: primary and secondary coils are separated, increasing the distance between the most primary, naturally increasing the leakage inductance.
• Add a retaining wall design: where a greater sense of leakage is required, a retaining wall can be added to the side of the baffle, thus increasing the distance between the structures.
• Increase the number of main arc turns: also a common way to effectively increase leakage inductance.

5. Conclusion

Leakage inductance in transformer design can be both “fear” and “sharp”. How to reasonably control leakage inductance under the premise of ensuring system efficiency and safety is a challenge that every designer must face. Mastering the key technologies of core structure, leading-edge design and air gap optimization will enable you to be at ease in power supply design and protect your system!

Meanwhile, as a professional electronic component supplier, LPEMA is committed to providing you with high-performance and high-reliability transformers and related products. Whether you are looking for optimized design solutions or professional technical support, we look forward to providing first-class solutions and services.

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