ETD (Economic Transformer Design) is preferred because of easier winding, but still EE is well spread and sold.
ETD coilformers have a circular cross section which means a reduced length per turn and lower winding resistance compared to the square cross section of the E core. Layering of windings is also more uniform with the circular cross section, which makes for a better winding quality and reduced build. ETD coilformers tend to be very robust, allowing termination of larger conductors without breaking problems. They are also very suited to automatic winding processes and are designed to maximise safety isolation distances between the pins and ferrite cores.
E cores of equivalent magnetic volume have higher cross-sectional area and higher AL factor, reducing the number of turns required for a particular inductance. With larger air gaps it is easy to halve the gap with a spacer in each core leg and reduce winding losses arising from fringing flux.This is not so easy on an ETD core because the core is usually intended for assembly with clips attached to the coilformer. The clips will not fit if spacers are added to the outer limbs, so large gaps must be located entirely in the centre leg of the core.
E cores/bobbins are also available in smaller sizes which are not practical for the ETD construction, so there is a greater variety. They also tend to be the cheapest, hence their enduring popularity.
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