Coercive Force
Magnetic field created by current flow alone. The unit of measure is the Oerstad.
Copper Loss
Also referred to as I2R loss, this is the power dissipated in the coil when current is flowing. As the second term implies, the power dissipated is the load current squared times the DC resistance of the coil. This is valid for low frequencies, such as 50 Hz (European commercial power) and 60 Hz (American commercial power) but gets more complicated as the frequency goes up. The overall effect is still copper loss but there are other effects at higher frequencies. See proximity effect and skin effect.
Core
Material and structure in most magnetic components that concentrate the magnetic field generated by the primary coil and carry it in a circuit to engulf both the primary and all secondary coils. Many different shapes and material types are available, each optimal for a different purpose.
Core Gap
Any air gap that breaks the smooth flow of magnetic flux in a core. It can be a gap formed by the core being split into pieces, or a gap that is artificially built in to control the inductance more precisely.
Core Loss
Power dissipated in the core while being energized. While the core is energized, even if there is no load on the secondary, power is being dissipated. There are two main causes of core losses. Hysteresis loss is that which occurs when the core material is continually being magnetized, demagnetized and re-magnetized in the opposite polarity. The more often it happens, which directly relates to frequency, the greater the losses. The other main source is hysteresis loss. See hysteresis loss for a description..
Curie Temperature
The temperature of a magnetic material that renders it non-ferromagnetic. In other words, the permeability, or magnetic gain of a core can transition abruptly from a high value below the Curie temperature to almost non-existent above the Curie temperature. For ferrite materials it can be from 170° to 350° C depending on the ferrite grade and formulation.
Dielectric Withstand
This goes hand in hand with isolation voltage and is the voltage potential that an insulation system can safely withstand without breaking or arcing over.
Duty-Cycle
In an application where a square-wave is applied to a transformer, the wave may not be exactly 50% positive and 50% negative. There may be a zero-voltage period called the dead-time to adjust the RMS voltage seen on the transformer primary coil. The duty-cycle is the ratio of active (non-zero) time to the whole time. Often the non-zero time is the “on” time, the dead-time called the “off” time and the whole cycle the Period so that “on” + “off” = Period. Duty-cycle is usually defined as “on” divided by Period and sometimes multiplied by 100 to get a percentage. For example, if a switched-mode power supply is a push-pull topology with two power transistors, each transistor responsible for energizing the core in an opposite polarity, each one will have an “on” time and an “off” time. If each transistor is “on” for 3.0µS and “off” for 2.0µS the period is 5.0µS. The resultant duty-cycle is 3.0µS / 5.0µS * 100 = 60.0%.
Eddy Current Loss
A varying magnetic flux can induce a varying voltage, and therefore current in a good conductor, in any material even material that is not ferromagnetic. It can happen in wire within a transformer, but mostly it happens within the core itself. To reduce this effect, and hence increase the efficiency, steel based cores are laminated so that each slice can be insulated from the next. This prevents any large eddy currents from forming.
Excitation Current
Also called magnetizing current or quiescent current, it is the current circulating in the primary coil that maintains the magnetic field in the core. It is also the source of all core losses.
Ferrite
A ceramic-like ferromagnetic material that is a mix of manganese, zinc and nickel pressed and formed into numerous shapes. The type, ratio of materials and size of particles determines the magnetic properties of each material grade.
Ferromagnetic
A term meaning magnetic properties of iron. A material does not have to have iron in it to be ferromagnetic, such as most ferrites. The main property of a ferromagnetic material is its strong affinity for concentrating and magnifying a magnetic field formed by electric current flow into magnetic flux.
Fringe Flux
In a magnetic core that has a gap, such as a standard “E” and “I” lamination or EE ferrite, the flux must jump across the gap to get to the opposing face. It doesn't jump entirely straight but bends around the edges of the gap. The greater the gap, the more fringing there is. The effect is that the local flux density is slightly lower, or put another way, the cross sectional area seems a bit larger. However it is accounted for the result is a little higher inductance than expected based on the gap and the flux jumping straight across. The fringe flux can also interfere with conductors nearby increasing copper losses.
Gauss
Named after one of the most preeminent scientists of the 18th and 19th centuries, the Gauss is a unit of magnetic flux density. The Earth's natural magnetic flux density is about 0.6 Gauss at its poles and less at the equator.
Hysteresis Loss
The power dissipated in a ferromagnetic core due to the process of magnetizing and demagnetizing the core material in an AC cycle. When a core is magnetically energized, there is a small amount of energy required to do it. The more cycles that occur per second the more power is dissipated. Some core material is best suited to low frequency applications, such as silicon steel, while ferrite is well suited to higher frequencies due to lower hysteresis losses.
Inductance Factor(AL)
A standard method for determining the inductance of a particular core material and shape. It states the inductance with a standard number of turns, usually either 1 turn or 1000 turns. With 1 turn standard coils the measurements are in nanohenries while 1000 turn coils are often in millihenries.
Interleave
A method to help reduce eddy current losses in a laminated steel core. The core is split into slices, each slice shaped into an “E” and an “I”. Placing them together forms a complete magnetic circuit with a tiny gap between each “E” and “I”. To help prevent the small amount of flux at the gap from interfering with other lamination slices, the "E"s and "I"s are alternately inserted into the core, sometimes every layer but often every few layers.
Isolation Voltage
This is the voltage requirement in which primary and secondary coils must withstand from each other. It is usually in terms of the application of the transformer or inductor. In other words, independent of the voltage applied to the primary and voltage transferred to the secondary, there is a voltage difference between the primary and secondary that must also be met. It could be a voltage in the same range as the primary or secondary voltage or much, much larger. In medical applications the isolation voltage can be many thousands of Volts even if the primary and secondary voltage is 20 or 30 Volts.
Leakage Inductance
A portion of the magnetic field in a core does not get converted to magnetic flux, and as such does not couple to the secondary winding. Similarly, some of the magnetic flux does not couple to the secondary either. All of these stray magnetic fields are called leakage inductance. For simplicity, many designers prefer to lump all leakage inductance into a single inductive component on the primary winding only and refer to that as the leakage inductance. Normally the leakage inductance in a transformer is minimized as much as possible, but at the expense of dielectric withstand capability. There is usually a trade-off between dielectric withstand (or isolation voltage) and leakage inductance.
Magnetic Flux
Without a magnetic core the magnetic field is the same as magnetic flux. Only with a magnetic core that concentrates and magnifies the magnetic field is the magnetic flux different. The permeability of the material determines how much magnification there is.
Magnetic Flux Density
Closely related to magnetic flux, it is the strength of the flux squeezed into a core. To use an analogy, magnetic flux is like light hitting the Earth and magnetic flux density is like light intensity, or how bright it is in a certain spot. Core material can only hold so much magnetic "intensity" before it runs out of room, then it saturates.
Operating Frequency
The nominal frequency of the AC component in a transformer or inductor. If the operation requires the frequency to vary, then a range between minimum and maximum is needed to properly design the component.
Permeability
This represents the affinity a core material has for concentrating a magnetic field generated by current flow into flux. It is in effect the gain of the core material. In the CGS (Centimeter-Gram-Second) system of units the permeability of air, which is pretty much the same as free space, is normalized to 1. The permeability of core materials is a multiplier of free space. A common permeability of high quality power ferrites is 2000-5000. For high grade silicon steel the permeability can exceed 50,000.
Planar Magnetics
A specially shaped ferrite core that has short legs and a wide body. It is designed so it has a very low profile allowing it to sit lower on a printed circuit board than similar core types. Normal coil winding techniques will not work with a planar core so new thin copper foil techniques have been devised borrowing heavily from printed circuit board technology.
Powdered Iron
A special kind of core material made from iron oxides that are ground into a fine powder and formed into various shapes. The most common shapes are toroids and E cores. The material typically has a low permeability and is used in applications where a significant DC bias is in the coil. It is often used for moderate frequency inductors and flyback transformers.
Primary
Coil in a transformer designated as the input, or as the source. Examples would be the coil plugged into commercial AC, or the coil connected to the drive transistors in a power supply.
Proximity Effect
Copper loss that is above and beyond normal DC current losses, caused by localized magnetic fields in a coil interfering with the field produced by adjacent turns. That is, in a coil of wire consisting of several turns, current flowing through each turn can create local magnetic fields that can interfere with the field produced by a neighbor turn. The effect is more pronounced the higher the operating frequency gets. Interleaving primary turns and secondary turns in alternating layers can reduce this effect.
RMS
Abbreviation for Root-Mean-Square, it is a book-keeping tool for measuring the heating value of current in a wire. Typically it makes calculation of power as easy as using Ohm's Law for AC circuits.
Saturation
It is a condition in a magnetic core when it can no longer sustain a magnetic flux density. It runs out of room. The result is the effective permeability of the core can quickly change from very high to near that of air as the flux density increases. Different materials and shapes can make the transition softer but it happens to all core materials. Similar to a sponge's ability to absorb water, once it has absorbed all it can any more water will simply roll off.
Secondary
Coil in a transformer designated as an output. More than one secondary coil is common and in many different configurations.
Silicon Steel
Special highly pure iron with a very controlled set of impurities, including from 0.5% up to 3.25% silicon. It has exceptional magnetic properties, including very high permeability and low hysteresis loss. It can be fabricated into sheets and stamped into various shapes.
Skin Effect
Copper loss that is above and beyond normal DC current losses, caused by high frequencies tending to push the current towards the outside edge of wire. It is discussed in terms of skin depth, which refers to how deep in the wire the current will travel. The higher the frequency the less wire area is used to pass current. The result is high copper losses due to the current only traveling down a cylinder-like pattern in the wire. To combat this tendency, designers will use several smaller wires in parallel. The depth of penetration is a constant so greater utilization of wire area is possible with smaller wire. The ultimate in this approach is litz wire, German litzendraht meaning woven wire.
Stack Height
In a laminated silicon steel core the core pieces are not changeable but the number of pieces in the core are. The number of pieces determines the stack height. Mechanically there is no real limit to the stack height, but electrically and thermally there is. The greater the stack height the less copper is exposed to the surrounding air so the transformer may tend to run hotter.
Tesla
Named after another one of the most preeminent inventors of the 19th and 20th centuries, Nikola Tesla, the Tesla is a unit of magnetic flux density. Similar to the Gauss, there are 10,000 Gauss in one Tesla. The Tesla is an MKS (Meter-Kilogram-Second) unit of measure.
Toroid
A fancy name for the shape of a donut, the toroid is a common magnetic core shape. It is typically fabricated into one piece so there is no gap and very little fringe flux resulting in excellent coupling and minimum leakage inductance. It is normally reserved for applications requiring a moderate number of turns due to the inherently difficult nature of winding on a closed core. Special winding equipment is often employed to wind coils efficiently.
Turns Ratio
The ratio between one coil to another, often the primary to the secondary. It is an extremely useful number because for a transformer the voltage induced on the secondary is proportional to the turns ratio. There is a similar relationship between primary current and secondary current, but the relationship is a reciprocal one.
Vacuum Impregnation
A technique used to force varnish in every void and space in a magnetic component. The component is placed in and sealed within a chamber and a vacuum developed inside the chamber. Once the vacuum is established the varnish is introduced into the chamber and engulfs the magnetic component. Any air in the component is replaced with varnish. If the varnish has a low thermal resistance, the vacuum impregnated varnish can provide a better path for heat inside the coil to get to the outside. A simplification to the process is to dip the component into the varnish then draw the vacuum. The mechanics of the varnish tank is simplified without sacrificing a lot of performance.
Varnish
A chemical coating applied to many magnetic components that prevents rust in iron based cores, bonds coil to core and core pieces together to reduce audible noise and vibration. Many varnish chemistries are available, solvent based, pre-catalyzed polyester and epoxy.
VPI
Abbreviation for Vacuum-Pressure-Impregnation. It is a variation on vacuum impregnation that adds an extra pressurization step. After vacuum impregnation the vacuum is released then a positive pressure is pumped into the chamber further forcing varnish into any voids.
