The magnetic lines of force generated by the coil cannot pass through the secondary coil, so the inductance that causes magnetic leakage is called leakage inductance. Refers to the part of the flux that the primary and secondary of the transformer are missing during the coupling process.
Leakage inductance is an important indicator of switching transformers. It has a great influence on the performance of switching power supply. The existence of leakage inductance, when the switching device is turned off, will generate back electromotive force, which is easy to over-voltage breakdown of the switching device; leakage inductance can also be The distributed capacitance in the circuit and the distributed capacitance of the transformer coil form an oscillating circuit, causing the circuit to oscillate and radiate electromagnetic energy outward, causing electromagnetic interference.
So sometimes we need to use as many ways as possible to reduce the harm caused by leakage.
How to reduce the leakage inductance of the transformer
1. Reduce the number of turns NP of the primary winding;
2. Increase the width of the winding (for example, select the EE core to increase the width of the skeleton b);
3. Increase the height and width ratio of the winding to reduce the thickness of the winding and increase the height of the winding;
4. Increase the degree of coupling between the windings.
5. Minimize the number of turns of the winding, and select magnetic materials with high saturation magnetic induction and low loss;
6. Reduce the insulation thickness between the windings as much as possible.
7. Step-down transformer, the secondary winding is sandwiched between the primary windings; the step-up transformer, the primary winding is sandwiched between the secondary windings.
8, double or multi-line main winding
9, the sandwich winding method, to improve the leakage is more obvious
10, reduce the air gap (if you can not reduce, you can use the larger magnetic core), to improve the leakage is more obvious
11, the secondary corresponds to the middle of the primary, at least not to deviate, the leakage is also obvious
12, reduce the initial and secondary turns ratio, have some impact on improving leakage inductance
13. If the secondary is not full, it can evenly wrap around a layer, which has a little impact on improving leakage.
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 coil is divided too much, the winding process is complicated, the interval between the coils is increased, the filling factor is lowered, and the shielding between the primary and secondary is difficult.
In the case where the output and input voltage are relatively low, the leakage inductance is required to be very small. For example, the driving transformer can be wound with two wires, and the magnetic core with a relatively large window height and height, like can type, RM type, 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.
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