Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 2

This section presents Transformer T1 Structural Design [Part 2]. Whereas [Part 1] described Steps (1) to (4) in the procedure given below, [Part 2] covers Steps (5) to (7).

Part 1
(1) Selecting a bobbin
(2) Verifying the effective winding frame
(3) Determining a winding wire configuration
(4) Creepage distance and barrier tape

Part 2
(5) Selecting wiring materials
(6) Connection diagram, Layer construction,
     Wire specifications
(7) Determining transformer specifications

(5) Selecting a wire material

With regard to the materials with which coils are made, polyurethane enameled copper wires (UEW) and polyester enameled copper wires (PEW) are commonly employed. However, in situations where the required creepage distance is difficult to attain, such as the inside of a small transformer, three-layer insulated wires should be utilized.

Since the degree of coupling can be increased by winding the wire to the full extent of its winding width, select a wire diameter that permits winding to the full extent of the winding width.

In terms of wire diameter, the smaller the wire diameter, the lower the parasitic capacitance and the lower the effect of the skin effect, at the expense of an increase in current density. As a rule of thumb, a wire diameter in the range of 4 to 8 A/mm2 current density should be selected.

An example for the current density calculation is shown below. Calculation results at ①, ② and ③ of “Transformer Design (Calculating numerical values)” are used.

According to the maximum duty cycle, Duty(max)=0.424; the primary side maximum current, Ippk＝2.32A and the secondary side maximum current, Ispk＝12.5A, the calculation results for the primary side effective current, Iprms and the secondary side effective current, Isrms are shown as follows:

Where, if the current density is 6A/mm², the wire diameter is calculated by the following formula.

In this example, the secondary side windings consist of 2 layers with 2 parallel rows, then 4 windings exist

* Note that the above calculations aren’t considered on a proximity effect and the skin effect.

Then Calculating the current density with a selected wire diameter, and to confirm that the current density is within 4 to 8A/mm² as the target figure.

These calculations are exclusive of the proximity and skin effects. The proximity effect refers to the phenomenon in which the current, flowing through nearby conductors, is affected by the excited magnetic fields and fails to uniformly flow through the conductors. In the skin effect, at high frequencies the current tends to concentrate in the outer edges of the conductor.

For the organization of coils, in [Transformer Design (Structural design) ? 1] see the sandwich coil configuration in [(3) Determining a winding wire configuration] and the succeeding items, [(6) Connection diagram, Layer construction, Wire specifications].

If a wire diameter that precisely fills your needs is not available or in cases where the properties need to be improved further, it would be helpful to utilize Litz wires. A Litz wire consists of thin wire materials twisted together, and it minimizes the skin effect that would accrue from the use of thin wires. When used in combination, multiple Litz wires can increase the cross-sectional area.

Finally, check for any increase in the temperature of the transformer, and perform adjustments as necessary.

(6) Connection diagram, Layer construction, Wire specifications

The kinds of wiring and layer organizations to be attained should be diagrammed. Coil specifications should be clearly documented in the form of tables. These items are necessary as part of design drawings in requesting for prototyping of the transformer to be created.

The wiring diagram shown below (in the left section) describes which signal lines are to be connected to which pins in the power supply circuit. Because the type of wiring impacts the board layout, it should be set up in connection with the board layout design task.

The layer configuration diagram (in the lower right) describes the type of organization that has been decided upon. In this design, with importance attached to the properties to be attained, the sandwich winding is selected that increases the degree of coupling.

Coil specifications: As mentioned above, select a wire diameter that takes up the full extent of the winding width. Also, make sure that the thickness direction of the winding frame fits within the allowable range.

(7) Determining transformer specifications

After calculating the numerical values and designing the structure, finally we create the type of transformer specifications described below.

Required information includes:

• Wiring
• Structure
• Specifying a core and a bobbin
• Inductance, turns, and wire diameter
• Isolation performance and assembly instructions

In creating a transformer, once these items are fixed, most manufacturers of transformers would be happy to fabricate a trial unit. Some transformer manufacturers would be willing to undertake prototyping based on a simpler set of specifications, such as input/output voltages and frequencies. Ask each manufacturer the minimum set of specifications that needs to be spelled out explicitly before a trial fabrication can be undertaken.