7 Inductor Specifications You Need to Know


An inductor also called a coil or choke, is a passive electronic component that is used to store energy or filter high frequencies. It is usually made of magnetic material with a coil of wire winded around the outside. When there is any current change going through the wire, there is a change in magnetic flux. And the magnetic field of the magnetic material will induce an EMF (electromotive force) with a corresponding voltage and current whose magnetic field opposes the initial magnetic field change, based on Lenz’s law. That is to say, inductors will resist any current and magnetic field change through them, and magnetic flux induction makes them energy storage elements. It is important to understand circuit requirements, inductor performance, and inductor specifications before selecting the type of inductor that works best for an electrical part in a project.


What are the 7 important inductor specifications?


  1. Inductance

  Inductance is defined as the ratio of the induced magnetic flux to the initial current, so it is also the ratio of the induced voltage to the initial current change over time. The inductance is the most important specification for an inductor as it directly indicates the inductor’s capability of opposing the change in current flow. The larger the inductance is, the stronger the opposing current is. The unit of inductance is called henry (H). 1 henry inductance happens when 1 ampere current flowing through an inductor and producing 1 weber magnetic flux, generally, microhenry (μH) is the most widely used unit since the inductance of an inductor is not that large. Overall, inductance is affected by multiple factors of the coil, such as the core, the diameter, and the turns counts.

Which is an example of F-L (inductance vs. frequency) curve, the inductance of AMPI4030ED3R3MT from Arlitech is around 3.5 μH, while the inductance of a competitor’s sample is around 2.8 μH at 100 kHz.


  1. Tolerance

  Tolerance refers to the inductance variation when an inductor is affected by external factors like temperature or so. Based on the material, structure, and other parameters, tolerance usually ranges from 20% to 40% of the nominal inductance. Under normal circumstances, tolerance is not wanted, therefore, a balance is always needed between performance and budget.


  1. SRF (Self-Resonant Frequency)

  There is a small capacitance inside an inductor, and there is a frequency happening when the capacitive and the inductive impedance meet each other. This frequency is called the self-resonant frequency (SRF). At the SRF, an inductor is no longer an inductor but a capacitor. Therefore, the SRF of any inductor should be higher than the operating frequency.


  1. DCR (DC Resistance)

  DCR indicates DC resistance. DC resistance can significantly decrease the efficiency of an inductor, so, the lower the DC resistance is, the better the inductor performance is. DC resistance is proportional to the diameter and length of the coil; hence it is impossible to be zero. However, if you can choose a very low DCR inductor, the operating current of it can be higher.


  1. Isat (Saturation Current)

  When we increase the passing through current of an inductor to a certain level, the inductance of it will decrease. This is because the magnetic flux density crossing over the magnetic core material has reached its limit, and there is no more flux that can go across the material. If we keep increasing the current at that time, the inductance will start to fall. The saturation current is defined as the current at which the inductance decreases to a certain level, and the level (usually at the point when the inductance drops 30%) can be differentiated by manufacturers and inductor types.

Which is an example of ΔL/L vs. current curve, you can see the ΔL/L of improved AMPI4030ED3R3MT only drops less than 15% at 6A, while other products are about to saturate.


  1. Irms (RMS Current, Root Mean Square Current)

  With parasitic DC resistance built in an inductor, the internal temperature will rise along with the rising current during operation. Irms is defined as the DC current when the temperature of an inductor rises to a certain level (such as 20oC more or 40oC more), and the level is basically determined by the manufacturer and the inductor quality. This temperature level implies the operating limit of an inductor, hence, Irms indicate the temperature limit of an inductor.


  1. Q Factor (Quality Factor)

  Q factor normally will not appear in the specifications of an inductor. But it is useful to calculate the Q factor in order to know how well an inductor performs. In general, a high Q factor refers to a close-to-ideal inductor with low losses, and according to the formula, the Q factor varies when the frequency changes.

Source: Techdesign Blog