Choking Coil

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Use of the choking-coil/flyback transformer core tables

Choking coils and the transformer of the flyback converter must store energy. The stored energy is stored as magnetic field-energy in the ferrite-core, more exactly: stored in the air-gap.

The energy which has to be stored amounts to:

Eqn 1.0

The values L and Imax have been determined on the simulation side.
In order to choose a suitable core, the following requirements for the core must be considered,

The core table for the selection of suitable cores includes 11 columns, and 12 for the flyback transformer. These are:

The next three columns serve the identification of the core. They are not required for calculations within the program. The next four columns contain data from the data sheet, which are required by the program for the calculations. The next four columns are calculated by the program

The program suggests suitable cores:

Cores can also be added:
Under the core table are seven input-fields. The fields 'core', 'ID' and 'manufacturer', all serve for the identification of the core and are irrelevant for calculations in the program. The fields 'Al', 'Ae', 'le', and 'Amin' must be filled corresponding to the data sheet. To complete your input click "ADD". The inputed core will be added to the table and treated in the same way as the rest of the pre-determined cores in the table.

The Wire-diameters proposed by us as well as the Wire-cross-section is always for a declared current density of 3A/mm2.

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Calculation of choking-coils and flyback transformers

On the simulation side an inductor and the related inductor current proposed by us or chosen by you, was calculated. The inductor and the maximum current determine the election of a suitable core.

Choking-coils shall store energy. The stored energy amounts to: W = ½ L I 2max. This energy is stored in the form of magnetic field energy. In fact, as well as being stored in the ferrite it is also stored in the air-gap of the core (see right of illustration).

The field energy in the storage inductor amounts to:

Eqn 2.0


The magnetic flux-density B is continuous and is approximately the same in the ferrite and the air gap, i.e. BBFeBg. The magnetic field-strength H is not continuous, it is higher in the air-gap rather than the ferrite by a factor of μr. This is introduced in equation (1), which results in B = μ0μr ·H, IFe = lFe ·A and Vg = g ·A:

Eqn 2.1


μr in the ferrite amounts to approximately 1000...4000. The effective magnetic core-length only goes into the energy calculation with lFe/μr. Therefore one can say that with usual core-dimensions energy is stored mainly within the air-gap. Since the energy is stored in the air-gap, one requires a certain air-gap volume in order to store the demanded energy. The maximum capable flux-density within customary ferrite amounts to approximately Bmax = 0.3T The manufacturers of ferrite cores give substitutional values for the term (lFe/μr+g)·A) in equation (2), namely This implies:

Eqn 2.2

The value μe can be calculated by means of magnetic conductance AL:

Eqn 2.3

The magnetic energy storage capacity then amounts to:

Eqn 2.4

Therefore from the table-values Ae, le, AL and Amin, our program first calculates the magnetic energy storage capacity, and from the result of this produces suggestions for suitable cores.

The number of turns N1 are calculated with the help of the magnetic conductance AL:

Eqn 2.5

Calculation of wire-diameter:

The current density S of the winding can be chosen between 2 and 5 A/mm2, (depending on the thermal resistance of the transformer). For this it follows that for the wire-cross-section and the wire-diameter:

Eqn 2.6

The wire-diameters proposed by us are calculated for a current density of 3A/mm2.

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Mathematics used in the program

The columns Wmax, Bmax, and N1 are calculated as follows: Let W be the maximum energy, that the core has to store. This energy amounts to W = ½ L I2. From the data-sheet of the core, the following values are required for further calculation: From this it follows:

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