Basic Knowledge of Inductors (2)

The structural example has two wound wires on the primary and secondary sides and therefore can be regarded the same as a transformer. Passing current to the primary-side wound wire will generate a magnetic field that will generate current on the secondary-side wound wire. This is due to electromagnetic induction, which is referred to as mutual induction in the case of transformers. Through this action, conversion to a desired voltage is possible based on the ratio of the number of turns between the primary- and secondary-side wound wires.

This is an example of an inductor in a DC/DC converter. Passing current through the inductor by turning the switch on will generate a magnetic field, causing the inductor to store energy in the form of magnetic energy. Stopping the flow of current through the inductor by turning the switch off will discharge the stored magnetic energy (the magnetic field changes), causing current to flow. This is also due to electromagnetic induction, which is referred to as self-induction in the case of inductors that consist of a single wound wire.

Inductors can be combined with capacitors to create low-pass, high-pass filters, etc. This makes use of their characteristic whereby the ease of passing AC is altered due to changes in impedance in accordance with frequencies. Impedance characteristics will be described later.

Magnetic saturation characteristics

Inductors become magnetically saturated when the flowing current exceeds the maximum value of the magnetic saturation allowable current (DC bias allowable current), resulting in a decrease in inductance. As shown by the impedance equation above, when inductors are saturated, impedance becomes small and the current flowing through them becomes abnormally large. As a result, for example, DC/DC converters may suffer lower efficiency and malfunction. Magnetic saturation allowable current is an important characteristic of inductors.

AC Resistance (ACR)(ACR)

Although only the DC resistance (DCR) was explained earlier in the impedance section, practical inductors also include resistance components that generate eddy current losses at the core and resistance components of conductive wires that increase due to skin and proximity effects. These components are referred to as AC resistances (ACR). AC resistances (ACR), whose value increases in proportion to frequencies, have a significant impact on power losses and increases in component temperature at high frequencies and therefore need to be taken into consideration in practical use. (Eddy current losses, skin effects, and proximity effects will be described later.)

Other Characteristics

Other characteristics of inductors and related terms are summarized below.

Q factor (quality factor)

: Q factor, which refers to the ratio of the inductive reactance to the resistance of an inductor at a specific frequency, is an index of inductor performance. The higher its Q factor, the closer an inductor is to an ideal inductor. The value obtained by dividing inductive reactance XL (= ωL = 2ΠfL) by ACR indicates the magnitude of a loss with respect to the frequency. The equation shows that Q is high when ACR is small.

Copper losses

: Losses due to resistance components when current flows through a conductive wire is referred to as a copper loss.

Iron losses

: Losses that occur in a core when a magnetic flux passes through it (hysteresis and eddy current losses) are referred to as iron losses.

Skin effect

: Increasing the frequency of current flowing through a conductor causes the current to flow only through the surface of the conductor, resulting in a higher current density at the surface section and thereby an increase in the resistance value. This is referred to as a skin effect.

Proximity effect

: When multiple conductive wires are in proximity to each other, the magnetic field formed by each wound wire induces an eddy current. At high frequencies, the current flow in each conductive wire concentrates in a narrow region that contacts an adjacent conductive wire, resulting in a higher current density at the proximity section and thereby an increase in the resistance value. This is referred to as a proximity effect.

Eddy current losses

: A magnetic field that changes due to electromagnetic induction generates an eddy current in the core of the conductor. The energy that generates this current is converted to heat as a loss due to the electric resistance of the core material. This is referred to as an eddy current loss.

Hysteresis losses

: Changing or reversing the magnetic field in a core will cause it to return to its original state with a hysteresis (hysteresis loop shown in BH diagrams of core materials). The energy consumed by this hysteresis action is lost as heat. This is referred to as a hysteresis loss, which is proportional to the area of the hysteresis loop.