-E, E-I, EFD, EEM, ER, and ETD Powdered and Ferrite Cores did not exist In the early development of transformers and inductors. Cores consisted of stacks of laminations; patterns cut or stamped out of thin sheets of electrical steels. Most applications required a lamination pattern (or patterns) that would form a closed magnetic loop when assembled together. Early patterns included rings for toroids, L shapes, U shapes, E shapes and I shapes (used with the E and the U). Patterns were sought that were easy to assemble, could be interleave to minimize gap effects, and would minimize waste. E shapes used in E-E and E-I combinations became popular choices. Scrapless E-I patterns were developed. The electrical steel stamped out of two adjacent E laminations (placed leg end to leg end) to form the winding window area became the two I laminations to be placed across the leg ends of the E laminations.
In the typical E lamination, the center leg (one of three legs) is twice the width of either outer leg. In theory, magnetic flux flowing out of the center leg divides equally and flows into the outer two E core legs. Since the outer legs handle half the flux they only need to have half the cross-section that the center leg has. An E core structure occupies two outer sides of the coil. This constitutes a shell type core structure (not explained in detail here). In contrast, a U core or C core structure (which has two core legs) only occupies one side of a coil placed over one of its legs. The E core structure provides better self-shielding than the U core structure (but neither provides good shielding). E type cores are easily gapped. For the typical E laminations this requires a butt stacked core. There is no interleaving of laminations.
Since E cores have two open coil sides, they provide substantial room to bring high current lead wires out from the coil. This also permits good heat dissipation but not as good as a toroid. In contrast, the standard pot core has a much more restricted space in which to bring out lead wires and restricts heat flow. It is easier to achieve high voltage electrical isolation with an E core than with a pot core.
Because the core stack is a stack of laminations the typical stack has core legs of rectangular cross-section. Typically the inductor or transformer coil is placed over the center core leg. To minimize winding resistance (hence also minimize winding losses) it is desirable to have a round center leg. A round center leg also eliminates the sharp bend encountered when winding wire around a rectangular leg; consequently a round center leg permits use of larger wire. Achieving round center legs with laminations is possible but very impractical. With the development of powdered cores and ferrite cores it became practical to have a round center leg. EC and ETD are examples of type E cores with round center legs. The combined cross section of the two outer legs should equal or exceed that of the center leg. EC, EER, and ETD type ferrite cores were developed for higher power higher frequency switching transformers.