Frankly ferrites

Frankly ferritesFerrites are not known as being the sexiest of passive components, but recent developments undertaken by Siemens Matsushita Components are set to give them renewed importance among designers of electronic equipment. Paul Gregg reports. Ferrites are powdered and compressed materials with high permeabilities, and they have a high resistivity that minimises eddy current losses when used in high frequency applications. Eddy currents are those electric currents induced in the core material that increase inductor I2R losses. Examples of ferrite materials include nickel-cobalt and nickel types. The most significant advantage of using ferrite over laminated and powdered iron is the high resistivity, combined with the fact that it offers a dense, homogeneous magnetic medium with high permeability and stability. It is also capable of operating high temperatures without the high eddy current losses inherent in conventional materials. Ferrite cored windings are used in three main application groups: narrow band, wide band and power. One of the results of a capital funding and know-how deal signed between Siemens and Matsushita 10 years ago has been the strengthening of Siemens’ position as one of the major world producers of ferrite products. Previously Matsushita was the dominant partner in the partnership. Today the situation is reversed and Siemens sells ferrite products to Matsushita. Development undertaken by the two companies is resulting in the production of ferrite materials with reduced core losses (see graph). The material N87 is available now, and the N97 material will be available in the middle of the year. The measurements were taken at f = 100kHz and flux density (B) = 200 milli Tesla. At the same time, permeability is being pushed to 20,000 and switching frequencies are going up to 1MHz and above. In seeking the optimum operating conditions for a particular application, critical attention has to be paid to the selection of the iron oxide used for ferrite production as well as the dopant, and the sintering process. It is at the sintering stage that the magnetic grain structure is formed and the ferrite becomes brittle. Generically named low profile ferrite cores such as E, U, and DE shapes are used in the manufacturer of switch-mode power supplies, DC to DC converters, mobile phones. Ring cores or toroids have applications in EMC filter networks as well as switch-mode power supplies. Pot cores offer a near ideal performance when used as small signal transformers. Planer ferrite cores can be placed both sides of the PCB substrate where space is a critical factor. Symphony in blue… Ring cores from Siemens Matsushita Components Siemens and Matsushita have developed epoxy coated rings that have improved electrical and mechanical characteristics for toroid diameter sizes 40mm and below. Extremely high permeability of 15000 is obtainable for a material like T46. At the same time, the drop in the Inductance Factor (Al) that normally occurs with the conventional polyamide coat, compared to uncoated cores of materials like T38 and T46, is avoided. The epoxy coat enhances both the voltage and mechanical strength. More significantly, the two companies have developed a ferrite polymer composite (FPC) film that has wide ranging applications for designers. Designers have become familiar with ferrite cores as brittle, rigid and often bulky components for high inductance coils and transformers. The performance of traditional ferrite components depends very much on external influences, such as temperature, pressure, electromagnetic fields and frequency. But FPC film (patents pending) is a pliable material. It is a homogeneous mixture of ferrite powder and plastic with outstanding mechanical and magnetic properties. It is rugged, yet it is processed as a thin flexible film. There are so many potential applications, that many of the problems that it can solve have yet to be identified. One immediate application is in the retail trade, where the FPC film can be put into an identity or smart card in order to operate a contactless transponder or electronic article surveillance system. When it is incorporated into customer exit system at a department store it enables the security gates to be placed further apart – useful in the case of wheelchairs. FPC can be used for compensation of deflection yoke coils in TV picture tubes and computer monitors. It is also suitable as a bridge between two U shaped ferrite cores in order to suppress leakage fields or to adjust the biasing curve. In EMC applications for example, it can be used to shield coils against fields or absorb interference at frequencies of 500MHz and above. The latest C351 film material is particularly suitable for high temperature applications up to 200 degrees C, and it is UL94-V0 listed. It is also available with copper coatings of 35 to 100 microns, and in various thicknesses from 0.2 to 0.4mm. FPC film of materials C350 and C351 can also be supplied in self-adhesive versions.


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