A little thought on magnetics can save a lot of energy

Andrew Adams
Technical Manager
advanced design and innovative materials bring new possibilities for custom magnetic components
Electromagnetic components like transformers and chokes are considered unglamorous, so frequently insufficient thought is given to their selection. Engineers will often simply re-use a device or even a power supply from a previous project, ignoring the fact that new and better components are available, allowing them to save weight, improve efficiency or reduce size. This article reviews some recent developments that are creating new optimised solutions across a range of applications.

Innovative magnetic materials

For the electromagnets used in inductors, transformers, DC-DC converters and the like, designers look for core material that deliver high permeability and maximum flux density. Iron and alloys like SiFe are the traditional starting point.

Ferrites - ceramic, homogeneous materials are composed of various oxides. Those with iron oxide as their main constituent exhibit excellent EMI protection against common mode and differential conducted noise, since their insertion loss is proportional to frequency - thereby showing no attenuation to signals, but high impedance to high frequency noise.

For power conversion applications, working temperature, flux density and frequency are the key parameters to select the proper material: from standard 60 to 100 deg C in handheld converters to -20 to 100 or -60 to 140 deg C for automotive or industrial applications. Converters operate at a wide switching frequency range, depending on voltage, power and cost constraints. Specific materials enable components to operate from just a few kHz through to hundreds of kHz and even MHz, providing high efficiency, compact converters.

Powder cores are distributed air gap cores that are primarily used in power inductor applications, specifically in switched-mode power supply (SMPS) output filters, also known as DC inductors. Other power applications include differential inductors, boost inductors, buck inductors, and flyback transformers.

Different core materials have particular advantages for certain applications. For lowest loss, core loss is the key factor, whilst designs requiring minimum core size, such as a DC bias dominated design, should use materials with highest flux capacity. Saturation is another property to consider, with available materials providing trade-offs between low losses and reasonably high saturation (0.8T) at a low cost, up to higher priced, high saturation material (1.6 T). High saturation is advantageous where inductance under load is critical.

 An advance on powder technologies, new amorphous and nanocrystalline magnetic cores allow smaller, lighter and more energy-efficient designs in many high frequency applications for inverters, adjustable speed drives and power supplies. Amorphous metals are produced by using special technology where molten metal is cast into thin solid ribbons. Since the material has no crystalline magnetic anisotropy, amorphous magnetic metal has high permeability.

When compared with conventional crystalline magnetic materials, amorphous magnetic cores have superior magnetic characteristics, such as lower core loss. These cores offer superior design alternatives when used as the core material.

Nanocrystalline alloys offer a unique combination of high permeability with large flux density and low losses at high frequencies. Operational temperatures of up to 180°C are possible. Materials like this enable construction of chokes and transformers in much smaller dimensions than is possible using ferrite based assemblies. Common-Mode chokes, in particular, benefit from the high permeability, because the amount of copper wire can be reduced, thus reducing copper losses and component size.

Why wire is important

Another development facilitating smaller form factors is the introduction of triple insulated wire. This allows windings to be laid on top of each other, giving mains isolation in a smaller form factor. This technique meets all the leading international safety codes and enables transformers to be manufactured to meet safety isolation standards without the need for margins and tape barriers. With conductor diameters ranging from 0.2 mm to 1 mm, the increased winding space permits smaller transformers to be designed and reduces manufacturing time and cost,

Advances in magnetic materials, wire, winding techniques and equipment, can be fed back rapidly through accurate simulation tool and 3D CAD solutions, to create new optimised solutions across a range of applications, from standard products, through custom designs for signal transformers, LAN & xDSL Modules, Planar Transformers, Inductors and Modules, and CANbus Chokes.

Conclusion

Magnetic and electromagnetic components like transformers and inductors are everywhere but (to the frustration of those like me to whom they are a livelihood) they are considered dull and unglamorous. I hope that I have convinced the reader to keep abreast of developments in the field, rather than sticking with older, bulkier and less efficient solutions.

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