Transformer leakage inductance is an important indicator of a transformer. It refers to the portion of magnetic flux that leaks out during the coupling process between the primary and secondary windings. This flux originates from the magnetic lines of force generated by the coils. However, because some primary (secondary) flux does not couple back to the core through the secondary (primary) winding but instead returns to the primary (secondary) winding through the air, a closed magnetic circuit is formed in the air, resulting in leakage flux.
The magnitude of leakage inductance is related to several factors, such as the winding coefficient, winding width, winding insulation thickness, and the thickness of all windings. Furthermore, leakage inductance is also affected by the operating frequency; as the operating frequency increases, the leakage inductance also increases.
Leakage inductance has a significant impact on the performance of switching power supplies. The presence of leakage inductance can generate a back electromotive force when the switching device is turned off, easily causing overvoltage breakdown. Leakage inductance can also form an oscillating circuit with the distributed capacitance in the circuit and the distributed capacitance of the transformer coil, causing the circuit to oscillate and radiate electromagnetic energy, resulting in electromagnetic interference.
Methods to reduce leakage inductance include optimizing the transformer structure, such as using a sandwich winding method, and selecting high-quality, high-performance, low-loss magnetic materials. Additionally, adjusting circuit characteristics can reduce the frequency components of the circuit, resulting in a more uniform distribution of current and magnetic field within the transformer.
Transformer leakage inductance has a significant impact on rectifier circuits. This article will explain in detail the concept of transformer leakage inductance, its causes, and its effects on rectifier circuits.
The leakage inductance[2] is caused by some primary (secondary) magnetic flux not being coupled to the secondary (primary) through the magnetic core, but instead returning to the primary (secondary) through the closed air circuit. The conductivity of the conductor is about 109 times that of the air, while the permeability of the ferrite core material used in the transformer is only about 104 times that of the air. Therefore, when the magnetic flux passes through the magnetic circuit formed by the ferrite core, some of it will leak into the air, forming a closed magnetic circuit in the air, thus generating leakage flux. Moreover, as the operating frequency increases, the permeability of the ferrite core material used will decrease. Therefore, this phenomenon is more obvious at high frequencies[2].
Leakage inductance is an important indicator of switching transformers and has a great impact on the performance of switching power supplies. The presence of leakage inductance will generate back electromotive force when the switching device is turned off, which can easily cause the switching device to break down due to overvoltage. Leakage inductance can also form an oscillating circuit with the distributed capacitance in the circuit and the distributed capacitance of the transformer coil, causing the circuit to oscillate and radiate electromagnetic energy outward, resulting in electromagnetic interference [2].
I. The concept of transformer leakage inductance
Leakage inductance refers to the external chain inductance caused by the self-inductance and magnetic coupling of transformer windings in a transformer. It is directly related to the performance of the winding joints and insulation space between the magnetically coupled conductors. Transformer leakage inductance can be divided into two forms: mutual inductance and self-inductance. Mutual inductance is mainly related to the magnetic circuit length between windings, while self-inductance is mainly related to the properties and structure of the windings themselves.
II. Causes of transformer leakage inductance
Leakage is mainly caused by the following reasons:
The magnetic path lengths differ between windings. Differences in the magnetic path length between different windings can lead to leakage inductance.
The insulation space between the windings and the core. A certain insulation space exists between the transformer windings and the core, and this space can cause leakage inductance.
Different winding structures. Transformers may have different winding structures, such as multi-layer windings and single-layer windings. Their structural characteristics can lead to leakage inductance.
III. The Impact of Transformer Leakage Inductance on Rectifier Circuits
Leakage inductance reduces the efficiency of rectifier circuits.
In rectifier circuits, leakage inductance leads to leakage current between transformer windings, thereby reducing the efficiency of the rectifier circuit. This leakage current wastes energy and causes power loss in the transformer by reducing power transmission.
Leakage inductance affects the stability of the rectifier circuit.
Leakage inductance can cause changes in the winding inductance of a rectifier circuit, thus affecting the circuit's stability. This is especially true in high-frequency rectifier circuits, where leakage inductance can lead to resonance, causing instability in the circuit's operation.
Leakage inductance affects the power factor of the rectifier circuit.
Leakage inductance reduces the power factor of the rectifier circuit, thereby decreasing its power factor correction. This leads to increased grid load and reduced grid efficiency.
Leakage inductance affects the output waveform of the rectifier circuit.
Leakage inductance can cause instability in the output waveform of a rectifier circuit, generating harmonic components and voltage spikes. This can cause interference to other electrical equipment and may damage components in the rectifier circuit.
Leakage inductance affects the overcurrent protection of the rectifier circuit.
When an overcurrent occurs in the rectifier circuit, the leakage inductance may generate an additional, brief current, triggering the overcurrent protection mechanism. This could cause the rectifier circuit to disconnect prematurely, affecting the normal operation of the equipment.
IV. Solutions for Leakage Inductance in Rectifier Circuits
Optimize transformer design
Leakage inductance can be effectively reduced by improving the winding structure and insulation space of the transformer. For example, using a denser winding structure and increasing the thickness of the insulation material can effectively reduce leakage inductance.
Use a high-efficiency rectifier circuit
Choosing a suitable rectifier circuit, such as a fully controlled rectifier circuit or a grid-free filter rectifier, can reduce the impact of leakage inductance on the rectifier circuit.
Adopt appropriate leakage inductance compensation measures
By introducing leakage inductance compensation devices, such as parallel inductance compensation coils or leakage inductance compensation capacitors, the impact of leakage inductance on the rectifier circuit can be reduced.
In summary, the leakage inductance of a transformer has a significant impact on rectifier circuits. Understanding the concept and causes of leakage inductance, as well as its effect on rectifier circuits, can help us take appropriate measures to reduce its impact and improve the efficiency and stability of rectifier circuits.
