What Is Leakage Inductance?

The magnetic lines of force produced by the coil cannot all pass through the secondary coil, so the inductance that generates magnetic leakage is called leakage inductance. Refers to the part of the magnetic flux that the transformer has lost during the coupling process.

Chinese name: Leakage

Unit: H

Definition of leakage inductance

Leakage inductance is the part of the magnetic flux that is lost during the coupling of the first stage of the motor

The leakage inductance of a transformer is that the magnetic lines of force generated by the coil cannot all pass through the secondary coil, so the inductance that generates magnetic leakage is called leakage inductance. ^[1]

Causes of leakage inductance

The leakage inductance ^[2] is due to some primary (secondary) magnetic flux is not coupled to the secondary (primary) through the magnetic core, but returns to the primary (secondary) through air closing.

The electrical conductivity of the wire is about 109 times that of air, and the magnetic permeability of the ferrite core material used in transformers is only about 104 times that of air. Therefore, when the magnetic flux passes through the magnetic circuit formed by the ferrite core, a part of the magnetic flux leaks into the air, forming a closed magnetic circuit in the air, thereby generating magnetic leakage. And as the operating frequency increases, the magnetic permeability of the ferrite core material used will decrease. Therefore, this phenomenon is more obvious at high frequencies ^[2] .

Harm of leakage inductance

Leakage inductance is an important indicator of switching transformers. It has a great impact on the performance indicators of switching power supplies. The existence of leakage inductance can generate back-EMF when the switching device is turned off, which easily breaks down the switching device overvoltage. The distributed capacitance in the circuit and the distributed capacitance of the transformer coil form an oscillating loop, which causes the circuit to oscillate and radiate electromagnetic energy to the outside, causing electromagnetic interference ^[2] .

Several factors affecting leakage inductance

For a fixed and already manufactured transformer, the leakage inductance is related to the following factors:

K: Winding coefficient, which is proportional to the leakage inductance. For simple primary winding and secondary winding, take 3, if the secondary winding is intertwined with the primary winding, then take 0.85, which is why the sandwich winding method is recommended. The leakage inductance has dropped a lot, probably less than 1/3 of the original.

Lmt: The entire winding is wound on the skeleton with an average length of each turn. Therefore, transformer designers like to choose a core with a long column length in the core. The wider the winding, the smaller the leakage inductance. Control the number of windings to a minimum The degree is very beneficial to reduce the leakage inductance. The effect of the number of turns on the leakage inductance is a quadratic relationship.

Nx: number of turns of the winding

W: winding width

Tins: Winding insulation thickness

bW: The thickness of all the windings of the manufactured transformer.

However, the trouble caused by the sandwich winding method is that the parasitic capacitance increases and the efficiency decreases. These capacitances are caused by the different potentials of the coils adjacent to the unified winding. When the switch is switched, the energy stored in them will be released in the form of spikes.

The main way to reduce leakage inductance

Staggered coils

1Each set of windings must be wound tightly and distributed evenly

2 The lead-out line should be quite satisfactory, try to be at right angles, and close to the skeleton wall

3 Those who fail to reach the full level should sparse evenly.

4 Minimize the insulation layer to meet the requirements of withstand voltage and

5If there is more space, consider lengthening the skeleton to minimize the thickness ^[1]

If it is a multi-layer coil, the magnetic field distribution map of more layers of coils can be made in the same way. To reduce leakage inductance, both primary and secondary can be segmented. For example, divided into primary 1/3 secondary 1/2 primary 1/3 secondary 1/2 primary 1/3 or primary 1/3 secondary 2/3 primary 2/3 secondary 1/3 Wait, the maximum magnetic field strength is reduced to 1/9. However, the coils are divided too much, the winding process is complicated, the ratio between the coils is increased, the filling factor is reduced, and the shielding between the primary and secondary is difficult.

When the output and input voltages are relatively low, the leakage inductance is also required to be very small. For example, the drive transformer can use two wires in parallel and use a magnetic core with a relatively large window width and height, such as canister type, RM type, PM iron Oxygen magnetism, so that the magnetic field strength is very low in the window, and a smaller leakage inductance can be obtained. ^[3]

Measurement of leakage inductance

The general method for measuring leakage inductance is to short the secondary (primary) winding and measure the inductance of the primary (secondary) winding. The resulting inductance value is the leakage inductance from the primary (secondary) to the secondary (primary). A good transformer leakage inductance should not exceed 2 ~ 4% of its own magnetizing inductance. By measuring the leakage inductance of a transformer, you can judge the pros and cons of a transformer. Leakage inductance has a greater impact on the circuit at high frequencies. Leakage inductance should be minimized when winding the transformer. Mostly, the primary (secondary)-secondary (primary)-primary (secondary) "sandwich" structure is used to wind the transformer To reduce leakage inductance.

Difference between leakage inductance and magnetic leakage

The leakage inductance is the coupling between the primary and the secondary in the case of two or more windings, and a part of the magnetic flux is not completely coupled to the secondary. The unit of the leakage inductance is H, which is generated by the leakage from the primary to the secondary. The magnetic flux leakage can be one winding or multiple windings, and a part of the magnetic field line leakage is not in the direction of the main magnetic field line. The unit of magnetic leakage is Wb. Leakage inductance is caused by magnetic leakage, but not all leakage