Basic
and Useful Magnetic Terms and Definitions
The temperature of the air surrounding a component.
Also known as shielded cores, these core geometries are designed to contain all of the magnetic flux generated from an excited winding within the core. Theoretically, leakage flux outside the structure is zero, although in practice this is not true. The most commonly used closed magnetic path geometries are E- cores, toroidal cores, and pot cores. Most frequently used open magnetic path cores are the drum cores. A good example of a closed magnetic path inductor design is the GBI 4415-G, while a good example of an open magnetic path inductor design is the GBI 3800-G. Both styles have their advantages and disadvantages.
Magnetics dissipate power and this power loss is due to both copper and core losses. Copper loss is a term that describes both the AC and DC losses in a magnetic winding and is solely due to the resistive properties of the winding. Refer to the winding table in the reference section for specific ohmic values of different wire gauges.
Core loss is the term describing the portion of the loss in a magnetic device due to losses in the core. This loss is proportional to excitation frequency and flux density swing. In bipolar excitation applications, the core loss is proportional to the peak-to-peak flux density. In unipolar excitation applications, the core loss is proportional to the peak flux density.
The ability for an inductive element to store energy is limited. Beyond this limit, the permeability of the core drops which causes a drop in inductance. It is standard to identify core saturation when the inductance has dropped 10% from its zero bias inductance level.
Curie temperature is different for different grades of Ferrite material and is the temperature at which the ferrite core losses its magnetic properties. Most data books identify the Curie temperature of a ferrite core as the temperature when the core permeability has dropped by 10%. The Curie Temperature of Ferrite material ranges from 100 to 300 degrees Celsius, depending on the exact make-up of material.
Also known as winding resistance, this term defines the total DC resistance of a winding within a magnetic device.
Electric fields in close proximity to magnetic flux lines cause currents to flow both in magnetic cores, which are electrically conductive, and in windings. These undesirable currents are known as Eddy Currents. Eddy currents cause additional losses in both the windings and the core. Proper magnetic design and construction should always be adhered to minimizing the flow of these currents.
Ferrites are the most important group of ferromagnetic
materials. Ferrites are formed from
alloys of iron oxide combined with other metals such as zinc and
manganese. The substance is made into a
fine powder with an insulating oxide, which is molded or sintered under a
high-pressure process. Since Ferrites
are molded and fired using ceramic type processes, they can be made into
practically any shape. Some of the most
frequently used geometries are EE cores, Pot cores, RM cores, and toroids.
Flux Density
Flux density is simply the Magnetic Flux per cross-sectional area and is given the symbol b.
Magnetic flux is imperceptible to the five senses and thus hard to describe. Magnetic flux is little more than a convenient concept. Flux is known only through its effects. Flux is measured in webers. A uniform change of one Webber per second in flux linking a one-turn winding will generate one volt across it. In electrical circuit theory, a voltage source is thought of as a driving force that pushes current around an electric circuit. The magnetic dual circuit is described by ampere-turns being the magnetic driving force pushing flux around the magnetic circuit.
Magnetizing Force
The driving force that pushes flux around a magnetic circuit is call the magnetizing force and is given the symbol H. This force is sometimes referred to as mmf, or magneto motive force. See the table of Useful Magnetic Formulas to better understand the relationship between flux and the force that moves it around a magnetic circuit.
Permeability
Simply, permeability is the ratio of flux density b to field intensity H. Permeability is given the symbol m and often varies with temperature, flux density, and frequency of excitation.
Skin Effect
An isolated conductor carrying current will generate a concentric magnetic field. With alternating currents, a magnetizing force will exist, generating eddy currents in the conductor. The direction of these eddy currents is such as to add to the current at the surface of the conductor and subtract from the current in the center. The effect is to encourage the current to flow near the surface of the conductor. The majority of the current will flow in an equivalent surface skin thickness or penetration depth. At one skin depth in a conductor the current density will have decreased by 1/e, or 36.8%.
Volume Resistivity
Volume resistivity is a measure of a magnetic cores ability to impede the flow of current through the material or on its surface. When a core comes in contact with one or more of its terminals, a low core volume resistivity can present some problems. Typically, the higher the cores permeability, the lower the cores volume resistivity.