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The magnetic field lines generated by the coil cannot all pass through the secondary coil, so the inductance that produces the leakage magnetic field is called leakage inductance. Refers to the part of the magnetic flux that is lost during the coupling process of the primary and secondary transformers.
Definition of leakage inductance, causes of leakage inductance, harm of leakage inductance, several factors affecting leakage inductance, main methods to reduce leakage inductance, measurement of leakage inductance, difference between leakage inductance and magnetic flux leakage.
Leakage Inductance Definition
The leakage inductance is the part of the magnetic flux that is lost during the coupling process of the primary and secondary of the motor. The leakage inductance of the transformer should be that the magnetic lines of force generated by the coil cannot all pass through the secondary coil, so the inductance that produces the magnetic leakage is called leakage inductance.
Cause of leakage inductance
Leakage inductance occurs because some of the primary (secondary) flux is not coupled to the secondary (primary) through the core, but returns to the primary (secondary) through air closure. The conductivity of wire is about 109 times that of air, while the 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 it will leak into the air, forming a closed magnetic circuit in the air, resulting in magnetic leakage. And as the operating frequency increases, the permeability of the ferrite core material used decreases. Therefore, at high frequencies, this phenomenon is more pronounced.
The danger of leakage inductance
Leakage inductance is an important indicator of switching transformers, which has a great impact on the performance indicators of switching power supplies. The existence of leakage inductance will generate back electromotive force when the switching device is turned off, which is easy to cause overvoltage breakdown of the switching device; leakage inductance can also be related to The distributed capacitance in the circuit and the distributed capacitance of the transformer coil form an oscillation circuit, which makes the circuit oscillate and radiate electromagnetic energy outward, causing electromagnetic interference.
Several factors affecting leakage inductance
For a fixed transformer that has already been made, the leakage inductance is related to the following factors: K: winding coefficient, which is proportional to the leakage inductance. For simple primary and secondary windings, take 3. If the secondary winding and the primary winding are alternately wound Then, take 0.85, which is why the sandwich winding method is recommended, the leakage inductance drops a lot, probably less than 1/3 of the original. Lmt: The average length of each turn of the entire winding on the skeleton Therefore, transformer designers like to choose a core with a long core. The wider the winding, the smaller the leakage inductance. It is very beneficial to reduce the leakage inductance by controlling the number of turns of the winding to a minimum. The influence of the inductance is a quadratic relationship. Nx: the number of turns of the winding W: the winding width Tins: the thickness of the winding insulation bW: the thickness of all the windings of the finished transformer. However, the sandwich winding method brings the trouble that the parasitic capacitance increases , the efficiency is reduced. These capacitances are caused by the different potentials of the adjacent coils of the unified winding. When the switch is switched, the energy stored in it will be released in the form of spikes.
The main method to reduce leakage inductance
The interlaced coils 1. Each group of windings should be wound tightly, and should be evenly distributed. 2. The lead-out lines should be well-organized, try to form a right angle, and close to the skeleton wall 3. If one layer cannot be fully wound, one layer should be sparsely wound. 4 The insulating layer should be minimized to meet the withstand voltage requirements and if there is more space, consider an elongated skeleton and minimize the thickness. 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, it is 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 etc., the maximum magnetic field strength is reduced to 1/9. However, the coils are divided too much, the winding process is complicated, the interval ratio between the coils is increased, the filling factor is reduced, and the prohibition between the primary and the secondary is difficult. In the case where the output and input voltages are relatively low, the leakage inductance is required to be very small. For example, the drive transformer can be wound with two wires in parallel. At the same time, a magnetic core with a large window width and height is used, such as pot type, RM type, and PM iron. Oxygen is magnetic, so that the magnetic field strength in the window is very low, and a small leakage inductance can be obtained.
Measurement of leakage inductance
The general way to measure leakage inductance is to short circuit the secondary (primary) winding, measure the inductance of the primary (secondary) winding, and the resulting inductance value is the primary (secondary) to secondary (primary) leakage inductance. A good transformer leakage inductance should not exceed 2~4% of its own magnetizing inductance. By measuring the leakage inductance of the transformer, the quality of a transformer can be judged. The leakage inductance has a greater impact on the circuit at high frequencies. When winding the transformer, the leakage inductance should be reduced as much as possible. Most of the “sandwich” structures of primary (secondary)-secondary (primary)-primary (secondary) are used to wind the transformer. to reduce leakage inductance.
The difference between leakage inductance and magnetic flux leakage
The leakage inductance is the coupling between the primary and the secondary when there are two or more windings, and a part of the magnetic flux is not fully coupled to the secondary. The unit of the leakage inductance is H, which is generated by the leakage magnetic flux from the primary to the secondary. Magnetic flux leakage can be one winding or multiple windings, and a part of the magnetic flux leakage is not in the direction of the main magnetic flux. The unit of magnetic flux leakage is Wb. Leakage inductance is caused by magnetic flux leakage, but magnetic flux leakage does not necessarily produce leakage inductance.