Selection principles and winding methods of transformer core and structure

The core and construction parameters of the transformer are determined by the core type and winding technique used in the assembly. When selecting a core, physical height and cost are usually the most important. This is particularly important for switching power supplies in AC grid converters.

When the height of the application component allows for smaller dimensions, low-cost BE or EI cores (such as products from Japan's TDK and TOKIN, or products from Europe's PHILIPS, SIEMENS and THOMSON) can be used.

When the design application requires a smaller core cross-sectional area, you can choose a BPD core product. If you want to design a multiple output power supply, the PER core provides a large window area, it requires fewer turns, and the true winding frame has more available pins. When space is not a problem, the ETD core is usually used for higher power. The PQ core is more expensive, but it occupies less printed circuit board space and requires fewer turns than the E core. For occasions with high safety insulation requirements, pot cores and RM cores should be selected. Ring cores are usually not suitable for flyback switching power supply transformers.

When the flyback transformer is wound, insulation measures should be added between the primary and secondary. For example, communication technology equipment must meet the requirements of the electrical insulation standards of IEC950 in Europe and UL1950 in the United States. These documents also detail the creepage and spacing distances of the insulation system used in the transformer structure. Usually, a creepage distance of 5 to 6 mm is required between the primary and secondary of the transformer (in compliance with specifications and requirements). The electrical insulation index is usually a test that specifies the electrical strength, applying a typical value of 3000 V AC high voltage for up to 60 s without breakdown. If the electrical strength of each insulation barrier does not meet the requirements of the specification, two insulation layers can be used between the primary and secondary of the transformer, one basic and the other supplementary. If the combination of two insulation layers still does not meet the electrical strength requirements, three insulation layers with reinforcement can also be used.

Figure 1 shows the limiting technique used on both sides of the winding edge of most flyback transformers. Usually, the edge limiting is to use tape as a barrier, and the width of the tape slits requires a margin to wrap the package with enough barrier to match the height of the winding. In general, the insulation limit on one side of the winding is half the leakage distance from the primary winding to the secondary winding (usually 2.5 mm). The core frame should be selected to be large enough, and in fact the insulation width of the winding is at least twice the total leakage distance. Pay attention to maintaining the coupling of the transformer and reducing the leakage inductance. The primary winding is wound within the frame. In order to reduce the barrier voltage breakdown caused by insulation wear, improve the insulation between layers, and reduce distributed capacitance , the barrier of the primary winding should be insulated and separated by at least one layer of polyester film tape (3M1298) required by UL specifications, and the tape should have a suitable width between the frames.

Figure 1 Schematic diagram of two different edge winding methods for transformer bobbins

Varnish or epoxy impregnation can also improve the insulation and electrical strength between the layers, but it does not reduce the distributed capacitance. The bias winding can then be wound on the primary winding. Supplementary or reinforced insulation consists of two or three layers of UL-compliant polyester film tape cut to the full width of the skeleton and then wrapped around the primary and bias windings. The edges also need to be wound again to isolate. The secondary winding is wound within the boundary. In addition, two or three layers of

Tape is used to fix the winding. Insulating sleeves are often used when the wires are across all windings to ensure that the creepage distance requirements are met where the wires cross.

Nylon or Teflon sleeves with a minimum wall thickness of 0.41 mm should be used to ensure that the windings meet safety insulation requirements. Considering that the transformer core is an isolated, voltage-free metal material, that is, the core is conductive but no part of it touches the circuit , it is safe. The distance from the primary winding (or where the wire passes) to the core, and the added distance from the core to the secondary winding (or where the wire passes), must be equal to or greater than the creepage distance required by the specification.

When the primary winding has multiple insulating layers, Figure 1 shows the primary Z-shaped winding method and C-shaped winding method. Note that the primary winding connected to the drain is buried under the second layer, which can shield itself and reduce electromagnetic interference EMI (common mode conduction radiation current). The Z-shaped winding reduces the distributed capacitance of the transformer, which reduces the high-frequency alternating loss and improves efficiency, but the winding is more difficult and the cost is higher. The C-shaped winding is easier to implement and the winding cost is lower, but it has higher losses and lower efficiency.

Figure 2 shows a new process: double-insulated wire or triple-insulated wire is used in the secondary to eliminate the required edge restrictions (the specifications of the insulated wire can be found in the relevant information). In double-insulated wire, each insulation layer can usually meet the safe electrical strength requirements; in triple-insulated wire, each two layers have an insulation effect and should usually meet the electrical strength requirements. In the process of winding and welding the transformer frame, special attention should be paid to preventing damage to the insulation layer, and the actual production process and techniques should be carefully summarized.

Figure 2 Schematic diagram of triple insulated wire wound around transformer frame

The above process reduces the size of the transformer and reduces the amount of work required to add margins, but the material cost is high and the cost of the winding is increased. The primary winding is wound over the full width of the frame edge, and the bias winding can be considered to cover the primary winding. A layer of tape is usually required between the primary or bias winding and the secondary winding to prevent wear of the insulated wire. In order to fix the insulated winding, an additional layer of tape is required.

Figure 3 also shows the alternate winding position of the bias winding, which directly covers the secondary winding, improving coupling with the secondary winding and reducing leakage inductance (i.e. improving load regulation in the bias winding feedback circuit). Note that because the bias winding is part of the primary circuit, an additional insulation barrier must be added between the secondary winding and the alternate bias winding at the edge of the transformer to supplement or enhance the insulation performance.