Hello everyone, I am Rose. Today I will introduce inductance to you. Inductance is a property of a closed loop and a physical quantity. It is a circuit parameter that describes the induced electromotive force effect in this coil or in another coil due to changes in coil current. Inductance is a general term for self-inductance and mutual inductance. A device that provides an inductance is called an inductor.

What is the definition of inductance? It's a magnetic element that can store electrical energy by converting it to magnetic energy. A magnetic core and windings are the most common structures.

Figure. 1

When a constant current passes through the coil, as indicated in the diagram, a static magnetic field is created in the direction illustrated in the diagram, according to the right-hand spiral rule. In the inductor, alternating current flows, and the magnetic field generated is the alternating magnetic field. The induced electromotive force on the coil provides an induced current, and the changing magnetic field generates an electric field:

The magnetic field becomes stronger as the current increases, and the direction of the magnetic field change is the same as the original magnetic field. The induced magnetic field is opposing the direction of the magnetic field change, the induced current is opposite the original current direction, and the inductive current decreases, according to Lenz's law.

The magnetic field weakens as the current decreases, and the direction of the magnetic field change is the polar opposite of the original magnetic field. According to Lenz's law, the induced magnetic field is in the opposite direction as the magnetic field change, and the induced current is generated in the same direction as the original current, resulting in a bigger inductive current.

To summarize, the inductance will obstruct the flow of current through the change, i.e., the inductance has a high impedance to alternating current. For the same inductance, the higher the current rate of change, the greater the induced current generated, and the higher the impedance presented by the inductor; for the same current rate of change, different inductance, the greater the induced current, the higher the impedance presented by the inductor; for the same current rate of change, different inductance, the greater the induced current, the higher the impedance presented by the inductor.

As a result, when a DC signal passes through the coil, the resistance is equal to the resistance voltage drop of the wire itself; when an AC signal passes through the coil, a self-induced electromotive force is generated at both ends of the coil, and the direction of the self-induced electromotive force is opposite the direction of the applied voltage, obstructing AC transmission. As a result, the inductor's properties are to pass DC while blocking AC. The coil impedance increases as the frequency rises.

As a result, the inductance's impedance is determined by two factors: one is frequency, and the other is the inductance's inherent qualities, namely the value of the inductance, also known as the inductance. The inductance formula for the cylindrical coil is as follows, according to the theoretical derivation:

Figure. 2

Inductors, also known as chokes, reactors, and dynamic reactors, are devices that prevent current from changing. If the inductor has no current flowing through it, it will stop current flow when the circuit is turned on; if the inductor has current flowing through it, it will keep current when the circuit is turned off.

Inductors can be classified into three categories of processes:

1. Winding Inductance

To make a coil, copper wire is twisted around a magnetic core. The cylindrical winding method (Round Wound) and the flat winding method are the two methods for winding wire (Flat Wound). The greater the permeability of the magnetic core, the higher the inductance value; the magnetic core can be made of non-magnetic materials (such as air or ceramic; in this case, the inductance value is low, but there is no saturation current), or ferromagnetic materials (such as iron) (such as ferrite, Pomerol, etc.; the magnetic permeability of alloys is larger than that of ferrites; ferromagnetic materials have magnetic saturation and saturation current).

The inductance of the winding can offer a high current and a high inductance value; the larger the magnetic permeability of the magnetic core, the same inductance value, the less winding, the less winding can lower DC resistance; the same size, the less winding can be thicker, increase current. In addition, we frequently confront the problem of inductive whistling in power supply design. The essence is that a change in the magnetic field causes the conductor, i.e. the coil, to vibrate. The vibration's frequency falls barely within the audio frequency range, making it audible to the human ear. Firmer and less vibration in the alloy integrally constructed inductor.

2. Multilayer chip inductors

Multilayer chip inductors are made by drying and molding ferrite or ceramic paste, printing conductive paste alternately, and ultimately laminating and sintering into an integrated structure (Monolithic).

Figure. 3 Multilayer Chip Inductors 

Figure. 4 Thin Film Inductors 

3. Thin-film inductors

Thin-film inductors are made in a similar way as integrated circuits. A conductor layer is plated on the substrate, and then a coil is formed using a photolithography method. Finally, the packaging is completed by adding a dielectric layer, an insulating layer, and an electrode layer.

The inductor has the function of blocking current, which can be divided into two types: high-frequency blocking coil and low-frequency blocking coil; the inductor has the function of tuning and frequency selection; the color ring inductor's basic function is charging and discharging, but this basic charging and discharging function is limited. Color ring inductors offer a wide range of applications because of the numerous circuit phenomena that have been expanded. Filtering signals, filtering noise, stabilizing current, and minimizing electromagnetic wave interference are all common uses for inductors. Different inductor types perform differently in different circuits, as shown.

Figure. 5

In circuit design, there are three main types of applications for inductors:

• Power inductor: mainly used for voltage conversion, commonly used DCDC circuits use power inductors;

• Decoupling inductor: mainly used to filter noise on power lines or signal lines,  EMC  engineers should be familiar with;

• High-frequency inductors: Mainly used in radio frequency circuits to realize biasing, matching, filtering and other circuits.

A. Power inductor

In DCDC circuits, power inductors are widely utilized to maintain a constant current flow by storing and releasing energy. The majority of power inductors are wound inductors, which have a higher current and inductance.

Inductance value: Typically, the inductance value recommended by the DCDC chip specification should be used; the larger the inductance value, the smaller the ripple, but the size will become larger; typically, a small inductance can be used if the switching frequency is increased, but the increase in the switching frequency will increase the system loss. Reduce productivity;

Rated current: Temperature rise current and saturation current are the two rated currents for power inductors; as current passes through the inductor, the inductor heats up due to losses and causes a temperature rise. The bigger the current, the higher the temperature rise; the temperature rise current is the highest permissible current within the authorized temperature range.

The inductance value can be increased by increasing the magnetic permeability of the magnetic core, which is commonly made of ferromagnetic materials. Magnetic saturation occurs in ferromagnetic materials, which means that when the magnetic field strength exceeds a specific threshold, the magnetic induction intensity does not rise, and the magnetic permeability, or inductance, drops. The saturation current is the maximum allowed current within the rated inductance value range.

The calculation formula is commonly supplied in the specification book for DCDC circuit design to determine the peak (PEAK) current and root mean square (RMS) current. The temperature increase current is a measurement of the inductor's thermal effect. The thermal effect, according to  Joule's law, must take into account the integral of the current over time over a period of time; while choosing an inductor, the design RMS current must not exceed the inductor temperature increase current. The design peak current cannot exceed the inductor's saturation current in order to keep the inductance value steady within the design range.

DC resistance: The inductor's DC resistance will cause heat loss, causing a temperature rise and lowering DCDC efficiency; consequently, if efficiency is important, use an inductor with a low DC resistance, such as 15 milliohms.

It's also determined by the product's application temperature requirements, whether it needs to comply with  RoHS, automotive-grade Q200, or other standards, and PCB structure constraints.

The magnetic leakage of the inductor will be significant when high current is applied, affecting the surrounding circuits, such as the CPU. As a result, inductors with superior shielding performance should be chosen for high-current applications, and important signals should be avoided during layout.

B. Decoupling inductor

Chokes, which are commonly translated as choke coils in textbooks, are another name for decoupling inductors. The decoupling inductor's job is to filter out interfering signals on the line. It is a part of the  EMC device. EMC experts are mostly utilized to solve the product's radiated emission (RE) and conducted emission (CE) testing issues. The majority of decoupling inductors are copper wires directly wound around the ferrite ring, and they have a relatively simple structure. Differential mode inductors and common mode inductors, in my opinion, are the two types. The terms "common mode" and "differential mode" are not used again.

C-1. Differential mode inductor

Figure. 6

Differential mode inductance is a common winding inductance that is used to filter out some differential mode interference, primarily in conjunction with a capacitor to reduce power supply noise.

Differential mode interference is the interference between L-phase and N-phase for 220V mains; it is the interference between POE+ and POE- for POE; and it is actually power supply noise for the low-voltage DC power supply on the motherboard for the low-voltage DC power supply. The coil inductance is the principal purpose of the decoupling inductance, which can be wound into a greater inductance value to filter out low-frequency noise.

C-2. Common mode inductance

Figure. 7

Winding two coils with the same number of turns and opposing directions on the same ferrite ring produces common mode inductance. As demonstrated, the common-mode inductance is:

• When a common-mode component flows through the common-mode inductor, a magnetic field in the same direction will be formed in the two coils.  which will strengthen each other, which is equivalent to a high inductance to the common-mode signal; • When a differential mode component flows through the common-mode inductor, opposite magnetic fields will be formed in the two coils, which will cancel each other, which is equivalent to a high inductance to the differential-mode signal;

Another way to think about it is that when a frequency's common-mode interference runs on V+, the resulting alternating magnetic field induces a current on the other coil. The direction of the induced current is the same as that of V- The direction of the higher common-mode interference is the opposite, canceling out a part and reducing the common-mode interference, according to Lenz's law and the right-hand rule.

In two-wire or differential systems, such as 220V mains, CAN bus, USB signals, HDMI signals, and so on, common-mode inductors are commonly employed. Used to reduce the attenuation of important differential signals while filtering out common-mode interference.

The following considerations should be made while choosing common-mode inductors:

• If it's used for a power cord, the rated voltage and current should be considered to meet the working requirements; • Determine the frequency band of common-mode interference by testing, and the common-mode impedance should be high in this frequency band; • The differential mode impedance should be small, and it shouldn't have a big impact on the differential signal quality.

D. High-frequency inductance

From 100MHz to 6GHz, high-frequency inductors are mostly employed in radio frequency circuits of mobile phones, wireless routers, and other devices.

In  RF  circuits, high-frequency inductors serve the following purposes:

• Matching: use capacitors to create a matching network that eliminates impedance mismatches between the device and the transmission line, lowering reflection and loss.

• Filter: use an LC filter and a capacitor to filter out undesirable frequency components that could interfere with the device's operation.

• Isolation AC (Choke): The  RF signal is separated from DC bias and DC power in active RF circuits such as PA.

• Resonance: form an LC oscillation circuit with a capacitor as the oscillation source of the  VCO ;

• Balun: Balanced and unbalanced conversion, with capacitors to build LC baluns, to accomplish the conversion between single-ended RF signals and differential signals;

This time, various types of inductors have been introduced. The application possibilities of these inductors were briefly covered in the classification. The functioning principle and selection criteria of inductors in various application circuits will next be discussed in detail.