Please note! Don’t ignore these inductor parameters!

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Please note! Don’t ignore these inductor parameters!

Posted Date: 2024-01-25

Author:Alan Yang

During the design process of inductor circuits, we often only pay attention to the inductance value, but easily ignore that the inductance value also changes with frequency; we find the DC resistance of the inductor, but neglect to pay attention to its AC resistance characteristics. This article attempts to share some key parameters of inductors that are easily overlooked.

1. What is an inductor?

An inductor consists of a coil of wire usually wound around a solid material that is typically ferromagnetic or has high magnetic permeability. In fact, the electromagnetic field is generated in a specific way based on the internal windings and the direction of current flow.If you want to understand how electromagnetic fields occur, it is recommended to read this post: A brief introduction to the basics of electromagnetism

Inductor kickback characteristics and direction

A key characteristic of inductors is kickback. The magnetic field generated in and around the inductor stores energy and, in the event of a rapid loss of power, can release the energy at a very high voltage on the connected circuit.

Figure 1 Inductor current direction and magnetic field direction

As the voltage across the inductor decreases, the magnetic field breaks down and stores the energy needed to generate high voltages (which is critical to many circuit designs and can damage sensitive components). The coil inside the inductor is always the same whether you flip it or not. A coil that turns clockwise upward will still turn clockwise upward when inverted. If you look from top to bottom, it rotates clockwise. Likewise, if you turn the coil upside down, it will rotate counterclockwise from top to bottom.

Regardless of the direction and orientation of the inductor windings, the polarization of the magnetic field is independent of backlash because the current caused by the voltage generated by the inductor across the connected circuit always flows in the opposite direction. This is why individual inductors have no polarity. The real question is how to apply the current correctly according to the desired circuit characteristics. Here is a quick animation demonstrating the backlash phenomenon:

Figure 2 Inductor backlash phenomenon

2. What inductor parameters are easily overlooked?

2.1 Inductance value and frequency

Let’s take Würth Elektronik’s 744901115 as an example

The data sheet gives an inductance value of 15nH, which is under 500MHz test conditions and can achieve a tolerance of +/-2%.

Figure 3 Inductance value (picture from Würth Elektronik)

However, the inductance value is not static and changes with frequency. If you want to know the inductance value at a specific frequency, you can look at the inductance vs. frequency curve. Usually listed in the data sheet.

Figure 4 Inductance value and frequency (picture from Würth Elektronik)

Generally speaking, it is very important that the inductor value frequency curve is as flat as possible.

DigiKey inductance unit conversion online tool

The Inductance Converter helps you convert inductance measurements between different range units, from picohenries (pH) to kilohenries (kH), as well as convert between these units.

Figure 5 DigiKey inductance unit conversion online tool

2.2 Impedance of inductor

DC resistance (DCR) of the inductor

DC resistance (DCR) represents the resistance value of the inductor when the signal frequency is close to 0Hz.Generally, often

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