Design of a flyback transformer for an IC switching power supply (Part 2)

4) Transformer structure

　　There are two main design approaches for the flyback transformer structure, which are:

　　1〕Margin Wound Method - This method is to leave space at the edge of the frame to provide

　　Required leakage and safety requirements.

　　2) Triple Insulated - The secondary winding wire is made of three layers of insulation

　　So that any two layers combined can meet the electrical strength requirements.

　　Safety requirements, leakage and electrical strength requirements are listed in appropriate standards, such as UL1950 in the United States and EN60950 (IEC950) in Europe for ITE. A leakage distance of 5-6mm is usually sufficient, so a space of 2.5-3mm is usually left between the primary and secondary in edge applications. Figure 5 shows the edge gap method structure and the three-layer insulation method structure. The edge gap method structure is the most commonly used type. The edge gap method structure has a high cost-effectiveness due to its low material cost. The volume of the three-fold insulation method structure transformer can be made very small because the winding can use the full width of the frame and no gap is required at the edge, but the material cost and winding cost are relatively high.

　　Figure 5 a) shows the edge gap method structure. In this example, the edge space is realized by a tape cut to the desired edge width. This tape usually requires 1/2 creepage distance (such as 3mm for 6mm creepage distance). The number of layers of edge tape is matched to the height of the winding. The core should be selected so that the available winding width is at least twice the required creepage distance to maintain good coupling and minimize leakage inductance. The primary winding is the first winding in the frame. The starting end of the winding (closely connected to the primary) is the end connected to the drain pin of the IR40xx. This allows the maximum voltage swing point to be protected through other windings. This minimizes EMI that can be coupled to other components on the printed circuit board.

　　If the primary winding has more than one layer, a basic insulation layer (cut to fill the remaining width between the two sides) should be placed between the two winding layers to reduce the possible breakdown between the two layers and also reduce the capacitance between the two layers. Another insulation layer is placed on top of the primary winding, and the auxiliary winding is placed on top of this insulation layer. Three layers of tape (cut to fill the entire frame width) are placed on the auxiliary winding to meet the insulation requirements between the primary and secondary.

　　On top of this layer, place another edge gap and the secondary is wound between them, so there is an effective creepage distance of 6mm and full voltage insulation between the primary and secondary. Finally, wrap 3 layers of tape (the entire width of the bobbin) on the secondary winding to tighten the secondary winding and ensure insulation.

　　                    Figure 5 Transformer structures of edge gap method and three-layer insulation method types

　　Figure 5 b) shows the structure of the three-layer insulation method. It can be seen that the primary fills the entire width of the frame, and there is only one layer of tape between the primary and the auxiliary winding. A layer of tape is wrapped around the auxiliary winding to prevent damage to the three times insulation layer of the secondary winding wire. The secondary winding is wrapped on top of it, and finally wrapped with a single layer of tape for protection. Pay attention to the insulation not being damaged during winding and welding.

　　4.1) Transformer materials

　　Iron Core

　　There are many manufacturers of iron cores that can be used as flyback transformers. The following materials are suitable for use:

　　TDK-PC40 or PC44 material

　　Philips-3C85, 3C90 or 3F3

　　Siemens - N27 or N67

　　There are many core shapes available but the flyback transformer generally uses the E-shaped core because it is low cost and easy to use. Other core types such as EF, EFD, ETD, EER and EI are used in special applications with height requirements. RM, toroid and pot cores are not suitable for use due to safety insulation requirements. EFD is better for low profile design, ETD is better for high power design, and EER is better for multiple output design.

　　skeleton

　　The main requirements for the skeleton are to ensure that the safe creepage distance, the distance requirements for the primary and secondary pins passing through the core, and the distance requirements for the primary and secondary winding areas are met. The skeleton should be made of materials that can withstand welding temperatures.

　　Insulation tape

　　Polyester and polyester film are the most common forms of insulating tape, which can be customized to the required basic insulation width or primary and secondary full insulation width (such as 3M#1296 or 1P801). Edge tape is usually thicker and only requires a few layers. It is usually a polyester tape such as 3M#44 or 1H860.

　　Excitation wire

　　The preferred sheath for the field wire is nylon/polyurethane, which becomes flame retardant when in contact with molten solder, allowing the transformer to be immersed in a solder pot. Standard enameled wire is not recommended because the insulation must be stripped before soldering.

　　3-layer insulated wire

　　In the three-layer insulation structure, the secondary winding wire uses a three-layer insulated wire. Similar to the excitation wire, the main wire is a single core, but it has three different insulation layers. Even if any two of the three layers are in contact, the insulation requirements are met.

　　jacket

　　The beginning and end of the winding of the edge gap structure transformer need a sheath. The sheath must be certified by the relevant safety agency to have a wall thickness of at least 0.41mm to meet the insulation requirements. Due to the thermal resistance requirements, heat shrink tubing is usually used to ensure that it will not melt at the welding temperature.

　　Dip paint

　　Varnishing is often used to lock the space between the winding and the core to prevent noise and moisture from entering the transformer. It helps to improve the withstand voltage and thermal conductivity. However, this is a very laborious step.

　　4.2) Winding method

　　C-type winding

　　This is the most common winding method. Figure 6) shows a C-type winding with 2 layers of primary winding. C-type winding is easy to implement and low cost, but it leads to increased capacitance between primary windings. It can be seen that the primary goes from one side of the frame to the other side and then back to the starting side. This is a simple winding method.

　　Z-winding

　　Figure 7 shows a Z-type winding method with 2 layers of primary winding. It can be seen that this method is more complicated and more expensive to manufacture than C-type winding, but it reduces the capacitance between the windings.

　　4.3) Winding sequence

　　The primary winding is usually wound on the innermost layer to minimize the length of each turn and reduce the primary capacitance. As discussed earlier, placing the primary winding on the innermost layer protects it from the other windings and reduces the noise coupling to other components on the printed circuit board. The coupled noise can also be reduced by making the beginning of the winding (the innermost end of the primary) the end connected to the drain of the IR40xx, which is also protected by the other windings (with the largest voltage fluctuations). Wrapping a layer of tape between the two layers of the primary winding has a great effect on the capacitance of the primary winding (which is one of the four elements that should be minimized).

　　The placement of the auxiliary winding and the secondary winding depends on the regulation method used. If the secondary side is regulated, the secondary winding is the outermost layer, otherwise the auxiliary winding is the outermost layer. When designing the edge clearance, the secondary winding is placed as the outermost layer in order to reduce the number of required edges and insulation layers. If the auxiliary winding is placed as the outermost winding, the coupling to the primary will be weakened, the coupling to the secondary will be strengthened, and the output regulation performance will be improved. At the same time, the peak charging current of the auxiliary source capacitor is reduced through the leakage inductance.

　　4.4) Multiple outputs

　　For high power multi-output designs, the secondary should be closed relative to the primary winding to reduce leakage inductance and ensure optimal coupling. The secondary should fill the available winding width as much as possible, which makes it easier to make multi-way secondary as discussed earlier, and it also improves wire utilization at high frequencies.

　　Using the secondary stacking technique described above can improve the load regulation performance of the auxiliary output and reduce the total number of secondary turns and the number of bobbin pins.