ETD



ETD (Economic Transformer Design ) - a name of commonly used family of ferrite cores for transformers, inductors, chokes and electronic lamp ballast devices. The dimensions of ETD sets are defined in industry and international standards like IEC 61185 and IEC 60424.

Naming convention


There are several sizes in the ETD series, from ETD19 to ETD59. 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".


 * ETD19 = ETD19/14/8
 * ETD24 = ETD24/15/9
 * ETD29 = ETD29/16/10
 * ETD34 = ETD34/17/11
 * ETD39 = ETD39/20/13
 * ETD44 = ETD44/20/15
 * ETD49 = ETD49/25/16
 * ETD54 = ETD54/28/19
 * ETD59 = ETD59/31/22

Size and shape of the ETD cores


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 losses are 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. As a result, the core volume is magnetised in a more uniform way and the core weight is optimised.

The ETD standards define only physical size. The actual cores can be made in several different materials, depending on application, operating frequency, cooling, etc.

If an air gap is required then, as it is the case for all E-type ferrite cores, for an optimum performance it should be made in a central leg of the core. Some manufacturers provide standardised gap length (e.g 0.2 mm or 0.5 mm). For small gaps small shims can be inserted in the side legs.

Bobbins and clips


Different manufacturers use various approaches to the bobbins. In some versions the clips hold the core halves to the bobbin. In others, the clip holds the core halves, without touching the bobbin.

Also, there are available horizontal and vertical, as well as split bobbins (multiple parts).

The bobbins can be designed so the clips can be attached directly to the bobbin. This helps in mechanically fixing the core. In alternative solution, the clips can be just holing the core halves together.

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 and importance
Most manufacturers of magnetically soft ferrites for power applications produce ETD cores, and sometimes also bobbins and clips.

The ETD cores are widely used for transformers and inductors in higher power applications like power supplies from tens to thousands of watts.

However, the ETD cores offer less shielding in configuration with large gap. In such cases distributed air gap or toroidal powder cores can be more effective.