Magnetic cores are doughnut-shaped magnetic materials that are used with inductors, transformers, and electromagnets. They are also used as computer memory elements. Magnetic cores are made of metal or ceramic materials that produce a flux field when a current-carrying drive line is wound through the center hole. The current’s direction determines whether the flux line runs in a clockwise or counterclockwise direction. In computer memory applications, the forward direction is 0 and the reverse direction is 1. Multiple drive lines are used for read/write, inhibit, and sense functions. Often, these drive lines are arranged in a grid array to achieve coincident core switching. Because only one core is switched at a time, disturb and strobe pulses are used to eliminate interference from adjacent cores. A disturb pulse drives a signal from one point to another on the hysteresis loop. A strobe pulse selects the mid-amplitude portion of the sense pulse.

Specifications

Product specifications for magnetic cores include:

• permeability
• saturation
• core loss
• materials of construction

Permeability is a measure of a material’s suitability as a path for a flux field. Saturation is the maximum magnetic induction at a given field strength. Core loss is the amount of power lost while the flux field passes through the magnetic core. Possible causes include hysteresis loss, Eddy current loss, and the movement of magnetic domains. Hysteresis losses increase at higher frequencies. Eddy current losses increase at lower core resistances. The normal movement of magnetic fields causes some domains to grow and others to shrink. Both types of changes absorb energy. In terms of materials of construction, most magnetic cores are made of powdered iron or ferrite ceramics. Carbonyl iron is used in broadband inductors for high-power applications. Hydrogen-reduced iron is used in low-frequency chokes for switched-mode power supplies. Ferrite ceramics are designed for high-frequency applications.

Standards

Like other magnetic components, magnetic cores comply with guidelines from the International Electrotechnical Commission (IEC). Technical Committee 51 (TC51) prepares standards for parts and components with magnetic properties, test measurements and methods, and ferrite materials. Magnetic cores that are sold in Europe bear the CE mark to indicate compliance with relevant health and safety regulations.

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Image credit:

Dexter Magnetic Technologies, Inc.