Text: MAGNETICS & FERROMAGNETICS MATERIALS Magnetic materials are used in applications such as power supply transformers, audio transformers, AC and RF Filter inductors, broadband and narrow band transformers, damping network, EMI/RFI suppressors, etc. The basic characteristic of magnetic materials is the permeability (µ). It is a measure of how superior a specific material is than air as a path for magnetic lines of force. Air has a µ of 1. Another characteristic of magnetic material is saturation. It is the maximum value of magnetic induction at a specified field strength. When a material saturates, it losses its linearity. Magnetic materials are available in many different types and sizes. IRON POWDER CORES (Suggested Reading) These cores are composed of finely defined particles of iron which are insulated from each other but bound together with a binding compound. The iron powder and binding compound are mixed and compressed under heavy pressure, and baked at high temperature. The characteristics of the cores are determined by the size and density of the core, and the property of the iron powder. Powdered iron cores do not saturate easily, and have high core temperature stability and Q. However, it is only in low permeability (below µi = 75). The high temperature stability makes it suitable for applications such as narrow band filter inductors, tuned transformers, oscillators and tank circuits. See Section I for more details on materials, shapes and sizes. FERRITES (Suggested Reading) Ferrites are ceramics materials that can be magnetized to a high degree. The basic component is iron oxide combined with binder compounds such as nickel, manganese, zinc or magnesium. Two major categories of ferrites are manganese zinc (MnZn), and nickel zinc (NiZn). Ferrites are manufactured by homogeneously mixing the iron oxide with the binder, and calcinated (heating mixture to 1000°c). This causes partial decomposition of the carbonates and oxides. The mixtures are dry pressed into a core configuration, and finally sintered. This is done by gradually raising the temperature up to 1500°C in a kiln. Typically the cores will shrink by 10 to 20% of its original size after sintering. Ferrites can be manufactured to permeability of over 15, 000 with little eddy current losses. However, the high permeability of the ferrite makes it unstable at high temperatures, and saturates easily. It is suitable for applications such as DC to DC converters, magnetics amplifiers, etc. It must be noted that driving ferrites with excessive current may cause permanent damage to the core. Ferrites are widely used as attenuators of unwanted high frequency signals. These ferrites are known as EMI/RFI suppressors. They are typically available as beads, split cores, flat ribbon core and toroidal cores. Ferrite tiles are also available for use in anechoic chambers. Another application of ferrites are in transformers, inverters and inductors in the 5KHz to 100 KHz range. It is cheaper than tape wound cores and are used in applications where high flux density and high temperature stability are not critical. Typical applications: 1. Inverter power supplies: 5KHz to 500 KHz, and under 50 watts at 10 KHz. For high power application, use tape wound core as saturating core and ferrite core as output transformers. 2. Fly back transformers 3. High frequency power supplies (1 Kw) Ferrite cores can be gapped to avoid saturation under DC bias conditions. LAMINATED OR TAPE WOUND CORES These cores are manufactured by using different steel grades with different widths and thickness, wound in circular manner. Tape wound cores have very high permeability and are used primarily in power transformers, reactors in 60 Hz to 400 Hz, DC to DC converters, and current transformers. It provides very high flux densities and good temperature stabilities. It is also the most costly core to manufacture. http://www.bytemark.com/amidon/material.htm

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