High Frequency Transformer Design

Main page | | Use of the tables | | Calculation of primary turns | | Tips |

Top of page | | Design of HF-Transformer | | Calculation of wire-diameter | | Literature Notes |

The program makes suggestions for

- very well-suited cores (
**Green writing**), whose volume lies between the value which was calculated by us to be suitable for the required power transfer, and 50% over that value. This volume is chosen such that the transformer temperature rise during operation is under 30K and the coil with a current density*S*= 3A/mm^{2}fits into the available winding area. - well suited cores (
**Brown writing**), whose volume lies between 50% and 100% over the value recommended by us, - suitable cores (
**Black writing**), whose volume is greater than 100% over the value recommended by us (thus being uneconomically large), - inappropriately small cores (
**Gray writing**), whose volume is below the value recommended by us. However, this does not mean that the core would be unsuitable. By reducing the primary number of turns*N*_{1}you can adapt the magnetic flux density and the winding area to your request. However in this case they will have a higher temperature rise than the cores indicated in green.

The wire-diameter proposed by us as well as the wire-cross-section is always calculated for a current density of *S* = 3A/mm^{2}. If you change the number of primary turns, it can happen that the wire cross-section proposed by us no longer fits into the winding area, especially if you choose a smaller core (**Gray writing**), than the one suggested by us.

Data books for appropriate cores provide information about the possible transfer power for various cores.

The first step to calculate a high frequency transformer is usually to choose an appropriate core with the help of the data book which provides certain tables for this purpose. Another way to choose an appropriate core is described in
[1] and [2] where at first a core-weight or core-volume is determined depending on the transfer power and switching frequency.

In the second step, the primary number of turns is calculated because this determines the magnetic flux-density within the core. Then the wire-diameter is calculated, which is dependent on the current in the primary and secondary coils.

Illustration 1: Voltages and currents of the transformer |

The square-wave voltage at the input of the transformer causes a triangular shaped magnetising current *I*_{M} which is almost independent of the secondary current (see also the equivalent circuit). The magnetising current is approximately proportional to the magnetic flux Φ i.e. to the magnetic flux density *B*. The input voltage *V*_{1} determines the magnetic flux in the transformer core corresponding to Faraday's Law *V* = N · d(Φ)/dt (see illustration 2).

Illustration 2: Input voltage and magnetic flux density of the transformer |

- The change Δ
*B*of flux-density depends on the frequency*f*= 1/*T*and the number of turns*N*_{1}. The higher the frequency and the number of turns the lower the change of flux density.

- In general the following applies: the smaller the change in flux-density Δ
*B*, the smaller the hysteresis losses.

(Where *A*_{min} is the minimum core cross-section. This determines the maximum flux density. *A*_{min} is given in the data-sheet)

**Note:**

With single transistor forward converters, the core is magnetised in one direction only, while with the push pull converter it is magnetised in both directions. If the core is used up to the saturation level, the maximum change in flux density with the push-pull converter may be 0.6T and may amount to 0.3T for the single transistor forward converter, if usual ferrites are used.

The wire-diameter depends on the respective r.m.s. value of the coil current. This can be calculated from the coil power. If the losses are neglected and it is assumed that with

- For the Single Transistor Forward Converter:

- For the Full Bridge Push-Pull Converter:

- For the Half Bridge Push-Pull Converter:

Usual cores are designed such that the above calculated coil fits into the available winding area. Primary and secondary windings both need an equal amount of the winding area.

**Note:**

For high frequencies and large wire-diameters, the skin-effect must be taken into account. It is recommended to use copper-foil or HF-wire for frequencies > 20kHz and wire-cross-sections > 1mm^{2}.

- Do not alter the turns ratio
*N*_{1}/*N*_{2}. - A reduction in the number of turns
*N*_{1}will cause an increase in Δ*B*and a quadratic increase of hysteresis losses. - Cores, whose effective core volume
*V*_{e}lie marginally below the value suggested by us, can be suitable if one allows a higher temperature. However the resulting core temperature can only be determined properly in an experiment. - Pay attention not to exceed the saturation levels of Δ
*B*when varying the number of turns (Δ*B*_{max}= 0.3T for the Two Transistor Forward Converter and Δ*B*_{max}= 0.6T for the Push-Pull Converter). - The number of turns
*N*_{2}can only be altered on the simulation side by altering the turns ratio*N*_{1}/*N*_{2}. An alteration, which would prevent the required output voltage from being reached for*V*_{in}=*V*_{in_min}, will be rejected by the program.

Main page | Use of the tables | Calculation of primary turns | Tips Top of page | Design of HF-Transformer | Calculation of wire-diameter | Literature Notes