One Powerful Decade: Keep Core Geometry in Mind When Designing Transformers

Jul 1, 2010 12:00 PM
Charles R. Wild, PE, API Delevan, East Aurora, New York

Design engineers are constantly faced with the challenge of specifying a transformer that will meet design specifications as well as cost constraints. The designer who draws up the circuit specification is seldom expected to design the transformer. However, the designer is expected to know the basic differences various core shapes offer, if not the techniques used in designing with them.

If core shape is not specified, the task of designing the transformer is left to the transformer vendor. Each vendor has a specialty, or niche product. Previous proven designs dictate the way custom products will be developed. When broadly specified transformers arrive, they are guided into one niche or standard process. In this way, the component is more efficiently and economically produced because the manufacturing process is addressed during the design phase. The end product will be a reliable transformer, but it may be optimized for production, rather than for circuit performance. This physical separation between the circuit designer and the transformer designer often leads to cost effective designs, but sometimes does not address overall system needs.

By comparing and contrasting various ferrite cores, evaluating design equations, looking at manufacturing techniques, and stepping through the design process, a basic understanding of component design should be achieved. Ferrite cores were chosen because they offer a wide range of shape choices, are available as standard product by most vendors, and their performance is well documented by ferrite manufacturers.

FERRITE CORES

When ferrite material was introduced, it became possible to make magnetic circuits of conventional shapes, as well as custom shapes. Excluding toroids, most ferrite transformer cores consist of two like pieces assembled together by proper grinding and polishing techniques so the resultant air gap between the mating surface is virtually eliminated. Although a discrete air gap may need to be introduced into the magnetic structure to allow the core to support higher dc bias levels that would otherwise drive the non-gapped core into saturation. This gap is commonly ground into the center post, or leg, of the core by the ferrite core manufacturer, but placing non-magnetic shims and spacers between the core halves is sometimes used by the transformer manufacturer.

Ferrite transformer cores are made in a variety of different shapes. Each shape has optimum proportions to obtain specific desired characteristics. For example, the pot core is designed for a high degree of magnetic shielding, while ER cores are designed for their low profile. Even with this optimization, it must be remembered that practical considerations must be met. Typically, the termination requirement, land pattern, EMI concerns, and profile height all lead to the proper choice of core shape. The geometric arrangement of the terminals must usually be compatible with the printed circuit grid or land pattern. These considerations lead to the use of a specific coil former having a specified number of terminal pins or pads arranged in a particular pattern. The core selection must often be designed around the termination requirement. In today's industry, height restrictions and EMI shielding are becoming an issue. Therefore, low profile designs and pot cores may need to be considered.

POT CORES

The pot core (Figure 1) is a closed cylinder containing an upright round center post on which a bobbin is placed. Pot cores, when assembled, nearly surround the wound bobbin. This aids in shielding the coil from picking up external EMI sources. Good electromagnetic coupling is readily obtainable with this core configuration. The pot core dimensions typically follow IEC standards so that there is interchangeability between manufacturers. Both plain and printed circuit bobbins are available, as are mounting and assembly hardware.

Mounting on a p. c. board tends to be very straightforward. The main disadvantage of the pot core is that the lead out slots are not wide enough to accommodate a coil former having a large number of fixed terminal pins. Because of its design, the pot core is a more expensive core than other shapes of comparable size. Pot cores for high power applications are not readily available and the bobbin windings have limited exposure to air circulation.

E CORES

The first ferrite transformer cores were made in the form of double E cores to match the standard scrapless lamination shapes that had long been in use for 50/60 Hz transformers. Lamination size E shapes (Figure 2) are available to fit commercially available bobbins. E cores are less expensive than pot cores, and have the advantages of simple bobbin winding plus easy assembly. Gang winding is possible for the bobbins used with these cores. E cores can also be pressed to a different thickness, providing a broad selection of cross sectional areas. Consideration should be given to those core sizes where bobbins are readily available from several bobbin suppliers. E cores can be mounted horizontally or vertically, depending upon whether above board height or board real estate area is most important. Printed circuit bobbins are available for low profile mounting. Figure 3 shows various mounting styles used at API Delevan for E core transformers, including horizontal, vertical and surface mount arrangements. E cores are popular shapes due to lower cost, ease of assembly, ease of winding and ready availability of a variety of hardware. The shielding on E cores is not nearly as effective as the shielding on pot cores.

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