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

EC, ETD AND EER CORES

The double E core, while matching the standardization of the industry, also carries with it the inherent restriction that the core center leg is rectangular. It was soon realized that pressed ferrite cores need not be restricted by a square center leg. Therefore, E cores with round center legs were developed (Figure 4). These cores have the advantage of easier winding on a cylindrical coil, and better control in production over the leakage inductance and self-capacitance. These shapes offer a cross between E core and pot cores. Like E cores, they provide a wide opening on each side. This gives adequate space for the large size wires required forlow output voltage switchmode power supplies. It also allows air flow that keeps the assembly cooler. One of the advantages of the round center post is that the winding has a shorter path length around it than the wire around a square center post with equal area. This reduces the losses of the windings by approximately 13% and enables the core to handle a higher output power. The round center post also eliminates the sharp bend in the wire that occurs when winding on a square center post.

TOROIDS

Toroids (Figure 5) are the least costly of all comparable core shapes. While no bobbin is required, accessory and assembly costs vary widely. The simplest method of mounting is the self-leaded style where the magnet wire extends beyond the core winding to provide the device leads for insertion into the circuit board. There are a limited assortment of bases available from several sources for surface mount applications. Assembly costs associated with them also vary.

Figure 6 shows several different mounting styles API Delevan uses for toroids. Windings are placed on the cores by either a manual process, whereby a predetermined length of wire is cut to length and then the wire is thread through the center of the core one turn at a time, or by using a toroid winding machine. The machine is required to load, or prewind, a fixed length of wire unto a shuttle prior to start of the winding. Each method has its advantages, typically the hand wind is used for heavier wire and fewer number of turns. As produced, the toroid core has sharp edges and must be tumbled in its manufacture to prevent magnet wire insulation from being cut through during the winding process.

To provide higher voltage insulation between the windings toroid cores typically will be insulated by use of parylene or epoxy coatings. A few suppliers can grind a small gap in the core to provide an air gap for applications that require a dc bias that would otherwise saturate the core. A more common method of attaining higher saturation levels is to use materials other than ferrite, such as molypermalloy or Kool-Mµ. These materials are only available in a toroid configuration, although the Kool-Mµ has just recently been introduced in an E-coreshape. The toroid shape provides excellent shielding.

SPECIFICATION

To illustrate an example of each of these core types; a widely-used power transformer will be designed. The transformer has a push-pull primary and a center-tapped secondary. Also assume a 1.0 V diode drop (Vd) on the output. Its specs are:

Input Voltage, Vin= 25 V

Output Voltage, Vo= 25 V

Output Current, Io= 1.375 A

Frequency, f = 21 kHz

Efficiency, η= 0.9875

Temperature Rise = 25°C

Flux Density, Bm= 0.2 T

Core configuration constants given in McLyman's book[1] can guide selection of the proper core geometry. This process is simply a guideline for proper core selection. By modifying parameters after this initial core determination, further optimization can be achieved.

Table 1

lists the constants for E cores (EE as well as round center leg E cores, Pot cores, and toroids.

Using the allowable core loss calculations, a suitable ferrite material can be selected from a vendor catalog. The transformer designer is faced with a set of constraints that must be observed in the design of any transformer. As described above, some of these constraints are output power (Po), minimum efficiency (η), and permissible temperature rise. Others are pad layout, profile height, and EMI concerns. The transformer engineer's problem is only partially indicated by the variety of materials whose mechanical, electrical, magnetic, dielectric, thermal, and environmental characteristics he must know.

Transformer designers have used various approaches to use all these parameters to arrive at a satisfactory design. The engineer would like to produce the final design by the direct application of general principles to the specifications before him, but in practice the design procedure is often influenced by previous proven designs.

When a designer is faced with specifications not indicating core preferences, the final design is typically guided into the easiest to manufacture. These designs have a history of performance and the manufacturing process has been designed around them. Thus, the design process may involve some cut and try of an existing design in which iterative calculations yield a satisfactory solution.

Depending on the application, certain geometric constraints will dominate. Some factors affecting these constraints can then be modified as necessary to achieve the more practical designs. It is not possible to optimize all the parameters in a design because of their interaction and interdependence. For example, if lower volume and weight are important, reductions in both can often be achieved by operating the transformer at a higher frequency but the cost will be lower efficiency. When the frequency cannot be increased, weight and volume reductions may still be possible by selecting a more efficient core material, but at increased cost. By knowing the application and design history, a cost-effective transformer can be produced.

Table 3

from the Magnetics' ferrites catalog shows the relative merits of the four core types discussed.

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