The Circuit Designer's Companion - diagramas.diagram...

Passive components 101material, ferrite, is in fact a type of ceramic, and is produced in at least as many varietiesas are the capacitor ceramic materials. Ferrites are a metal-oxide ceramic made of amixture of Fe 2O 3and either manganese-zinc or nickel-zinc oxides pressed or extrudedinto a range of core shapes. Every manufacturer offers a wide variety of shapes and willusually also offer a custom service for large volumes, but for most uses a selection froma relatively small range of standard types is adequate, and offers the benefit of sourcingfrom different suppliers. Two of the most popular core types are the RM series toIEC 60431 and the E, EP and EC series for medium-power high frequencytransformers.Manganese-zinc ferrites have a high permeability but also their losses increaserapidly with frequency, making them more suitable to low-frequency applications.Nickel-zinc ferrites are of lower permeability, but their lower high-frequency lossesmake them useable up to about 200MHz. Their resistivity is higher by several orders ofmagnitude, and their Curie point temperature is higher. The ratio of manganese to zincor nickel to zinc in either case determines the grade of material.Iron powderThe other core material in wide use is iron powder. Such cores are pressed from a veryfine carbonyl iron powder mixed with a bonding material. Eddy current losses in thecore are minimised by creating an insulating layer on the surface of each particle beforepressing, but this introduces minute gaps in the magnetic circuit, which restricts thepermeability of the material to a maximum of around 30. The same effect makes ironpowder cores very hard to saturate. The main uses for iron powder are for highfrequency tuned circuit cores, and suppressor chokes where low saturability is moreimportant than high inductance.3.4.2 Self-capacitanceGenerally, don’t wind directly onto the core. Apart from mechanical instability, thevery high dielectric constant of the ferrite will increase the self-capacitance C p of thewinding several times if the two are in close contact. The bobbin serves to keep thewinding well spaced from the core and minimises self-capacitance. The way thewinding is built up also influences self-capacitance; a single layer has the lowestcapacitance, but if multiple layers are necessary then two possibilities exist to reduce it:• “scramble” or “wave” wind rather than build up in discrete layers. This canreduce C p by around 20% over a layer winding, but uses more windingspace;• use a multi-section former for a single winding. Again this uses more space,but for example a two-section former can reduce C p by a factor of 3.3.4.3 Inductor applicationsThere are three major applications for inductors: as frequency determining componentsin tuned (resonant) circuits, as energy storage components, usually in power supplies,and as filter components in suppression circuits. Each application emphasises differentinductor characteristics and calls for a different approach to inductor design.Tuned circuitsSignal tuned circuits demand predictable inductance values and high Q (low losses).They do not normally see high bias currents, so core saturation and hysteresis loss is nota problem. For low frequencies, ferrite pot cores are the most popular, but for RF use