Table of Contents
ETD ferrite core
| Stan Zurek, ETD ferrite core, Encyclopedia Magnetica, http://e-magnetica.pl/doku.php/etd_ferrite_core |
| reviewed by Jeff Jones, 2015-01-07 |
ETD ferrite core (acronym of Economic Transformer Design1)) - a name of commonly used family of ferrite cores for transformers, inductors, chokes and electronic lamp ballast devices.2)
The dimensions of ETD sets are defined in industry and international standards like IEC 61185 and IEC 60424.
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S. Zurek, E-Magnetica.pl, CC-BY-4.0
Naming convention
There are several sizes in the ETD series, from ETD19 to ETD59.4)5) The number denotes the width of the "E" part in millimetres, so for instance ETD44 core is around 44 mm wide (disregarding mechanical tolerances).
The full names include further numbers, defining width, length, and thickness of the core half, respectively. Since the dimensions are standardised usually only the first to digits are used for description, eg. just ETD59 instead of the full ETD59/31/22.
- ETD59 ⇒ ETD59/31/22 22)
Size and shape
The ETD cores are normally assembled of two identical halves. When assembled, the outline of the core is almost square.
The central limb of the core has a circular cross-section. This allows having the smallest possible turn length and thus reduces the copper loss as compared to cores with rectangular area - hence the name “economic”. Additionally, the outer limbs have characteristic concave shapes so as to accommodate a coil with greater outer diameter (than it would be possible for a simple rectangular core).
The ETD geometry gives a good compromise between the volume of ferrite (pertinent to core loss), the volume of copper (copper loss) and surface area (thermal resistance). The additional benefit is that the core has almost constant cross-section area along the magnetic path.23) As a result, the core volume is magnetised in a more uniform way and the core weight is optimised.
The ETD part description (ETD19/14/8 for example) defines only the cores physical size, the actual cores may be constructed from one of several different types of magnetically soft ferrite material, the material would be selected to suite a particular application, operating frequency, cooling requirements etc.
If an air gap is required then it should be made in the central leg of the core, for all E-type cores this assures optimum performance. Some manufacturers provide standardised gap lengths (e.g 0.2 mm or 0.5 mm). For smaller gaps thin shims can be inserted in the side legs.24)
Bobbins and clips
Different manufacturers use various approaches to the bobbins and retaining clips. In some versions the clips hold the core halves to the bobbin. In others, the clip holds the core halves together without touching the bobbin.
Also, there are available horizontal and vertical, as well as split bobbins (multiple parts).
The bobbins can be supplied without pins, as well as partially or fully pinned, with square or round pins. Partial pinning might be useful for so that the bobbin will fit into a PCB only one way thus avoiding assembly mistakes.
Practical use
Most manufacturers of magnetically soft ferrites used for energy conversion applications produce ETD cores. These manufactures will typically produce ETD core accessories such as bobbins and the core retaining clips.
The ETD cores are widely used for transformers and inductors in higher power applications like power supplies 25)26)27)28) from tens to thousands of watts.29)
However, the ETD cores offer less magnetic shielding in configuration with a large gap. In such cases distributed air gap or toroidal powder cores can be more effective.
