Help for the High Frequency Transformer Design

Help for th the High Frequency Transformer De Design

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High Frequency Transformer Design Main page | page | Use of the the tables table s | Calculation of primary turns | turns |Tips Tips Top of page | Design of HF-Transform HF-Tra nsformer er |Calculation | Calculation of wire-diameter | | Literature Notes

Use of the transformer-core-tables The high-frequency high-frequency transformers are calculated ca lculated with with the help of the effective effe ctive core volume volume V e and the minimum core-cross-section Amin. For a requi re quired red power output Pout switching out = V out out · I out out and a chosen switching dete rmined. Then an optimal Δ B is selected depending on the frequency f a suitable core volume V e must be determined. chosen switching switching frequency and also a lso regarding regarding the temperature rise of the transform t ransformer. er. (see [ 2], [3 [3]). The program makes suggestions for very well-suited well-suited cores ( Green writing ), whose volume volume lies between the value which was calculated by us to be suitable for the required re quired power transfer, and 50% over that value. This volume volume is chosen such that the transformer transformer temperature te mperature rise during during operation is is under 30K and the coil with with a current density 2 S = = 3A/mm 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 suitable cores c ores (Black writing ), whose volume 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 t he core would would be unsuitable. By reducing the primary 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 t hey will will have a hig higher her temperature rise than the cores c ores indicated indicated in green. green. You can change c hange the suggested suggested value va lue for the primary number of turns N 1 according to your desires (the modification must be concluded with "return"). In each case a new value for Δ B will be displayed in the corresponding column. This also results in a change of the number of secondary turns N 2 such that the ratio N 1/ N 2 will not be affected. affec ted. The turns ratio N 1/ N 2 can only be changed on the simulation side. The wire-diameter wire-diameter proposed by us as well as the wire-cross-section wire-cross-section is always calculated for a current density 2 of S = = 3A/mm . If you change the numb number er of primary turns, it can happen that t hat the wire cross-section cross-section proposed proposed by us no long longer er fits into into the winding winding area, especiall espec iallyy if you you choose a smaller smaller core ( Gray writing ), than the one suggested by us. Top of page

Design of HF transformers High High frequency transform tra nsformers ers transfer electric e lectric power. The physical size size is dependent on the power to be transfered as well as the operating operat ing frequency. frequency. The higher the frequency the smaller smaller the physical physical size. Frequencies are usually usually between 20 and 100kHz. Ferrite is mainly mainly used as the core material. Data books for appropri a ppropriate ate cores provide information information about the possible possible transfer power for various cores.

Help for the High Frequency Transformer Design


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. Top of page

Calculation of the minimum number of primary turns:

Illustration 1: Voltages and currents of the transformer It is assumed that there is a square-wave voltage V 1 at the primary side of the transformer. This causes an input current I 1, which consists of the back transformed secondary current I 2 and the magnetising current I M (see illustration 1). A core without an air-gap is used in order to keep the magnetising current as small as possible. 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 For the transformer on the right in the diagram above, the following applies:

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. Now the minimum number of turns N 1 can be calculated to ensure that a certain change of flux-density Δ B is not exceeded. The saturation flux density of +/-0.3T, (which means Δ B = 0.6T) cannot be used normally for high frequency transformers. In Push-Pull converters going around the hysteresis loop with every clock cycle would cause unacceptable losses, i.e. heat generation. If no further information concerning core losses and thermal resistance is available, Δ B should be limited to Δ B = 0.3...0.2T with usual frequencies (20kHz to 100kHz). Further information concerning the selection of Δ B can be found in [1] and [2]. In general the following applies: the smaller the change in flux-density Δ B, the smaller the hysteresis

losses. From this a suitable number of turns for N 1 results:

(Where Amin is the minimum core cross-section. This determines the maximum flux density. Amin 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. Top of page

Calculation of the wire-diameter:

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 V in_min the maximum duty cycle is achieved, it follows that: For the Single Transistor Forward Converter:



For the Full Bridge Push-Pull Converter:

For the Half Bridge Push-Pull Converter:

In the above calculations the magnetising current can be neglected. The current density S is chosen between 2 and 5 A/mm2, depending on the thermal resistance. The wire cross-section Awire and the wire diameter d wire can be calculated as follows:

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. Top of page

Tips 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 Bmax = 0.3T for the Two Transistor Forward Converter and Δ Bmax = 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

Help for the High Frequency Transformer Design

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