Inductor is a component that can convert electrical energy into magnetic energy and store it. The structure of an inductor is similar to that of a transformer, but it has only one winding. The inductor has a certain inductance and it only hinders the change of current. If there is no current passing through the inductor, when the circuit is connected, it will try to prevent current from flowing through it; if there is current passing through the inductor, when the circuit is disconnected, it will try to maintain the current unchanged. Inductor is also called choke, reactor, and dynamic reactor.
Composition of inductor
An inductor is generally composed of a skeleton, winding, shielding cover, packaging material, magnetic core or iron core, etc.
Skeleton：The skeleton generally refers to the support for winding coils. Some large-sized fixed inductors or adjustable inductors (such as oscillation coils, choke coils, etc.) mostly wind enameled wire (or yarn-covered wire) around the skeleton, and then install the magnetic core or copper core, iron core, etc. into the inner cavity of the skeleton to increase its inductance.
The skeleton is usually made of plastic, bakelite, or ceramic. According to actual needs, it can be made into different shapes. Small inductors (such as color code inductors) generally do not use a skeleton, but directly wind enameled wire around the magnetic core. Hollow inductors (also known as air-core coils or hollow coils, mostly used in high-frequency circuits) do not use magnetic cores, skeletons, shielding covers, etc. Instead, they are wound on a mold first and then removed from the mold, and a certain distance is pulled between the coils of the coil.
Winding：The winding refers to a group of coils with specified functions and is the basic component of an inductor. Windings are divided into single-layer and multi-layer. Single-layer windings are in two forms: close winding (winding with the wire coil next to coil) and spaced winding (winding with a certain distance between each coil of wire). Multi-layer windings have various winding methods such as layered flat winding, random winding, and honeycomb winding.
Magnetic core and magnetic rod： Magnetic cores and magnetic rods generally use materials such as nickel-zinc ferrite (NX series) or manganese-zinc ferrite (MX series). They have various shapes such as "I"-shaped, columnar, cap-shaped, "E"-shaped, and tank-shaped.
Iron core： The iron core materials mainly include silicon steel sheets, permalloy, etc., and its shape is mostly "E"-shaped.
Shielding cover： To avoid the magnetic field generated by some inductors during operation affecting the normal operation of other circuits and components, a metal shielding cover is added to it (such as the oscillation coil of a semiconductor radio). Inductors with shielding covers will increase the loss of the coil and reduce the Q value.
Packaging materials： Some inductors (such as color code inductors, color ring inductors, etc.) are sealed with packaging materials after being wound. Packaging materials use plastic or epoxy resin, etc.
Classification of inductors
Self-inductor
When there is current passing through the coil, a magnetic field will be generated around the coil. When the current in the coil changes, the surrounding magnetic field also changes accordingly. This changing magnetic field can cause the coil itself to generate an induced electromotive force (induced electromotive force) (the electromotive force is used to represent the terminal voltage of the ideal power supply of an active element). This is self-induction.
An electronic component wound with a wire and having a certain number of turns and capable of generating a certain self-inductance or mutual inductance is often called an inductance coil. To increase the inductance value, improve the quality factor, and reduce the volume, a core or magnetic core made of ferromagnetic material is often added. The basic parameters of an inductor include inductance, quality factor, inherent capacitance, stability, current passing through, and operating frequency. An inductor composed of a single coil is called a self-inductor, and its self-inductance is also called the self-inductance coefficient.
Transformer
When two inductance coils are close to each other, the magnetic field change of one inductance coil will affect the other inductance coil. This influence is mutual induction. The magnitude of mutual induction depends on the self-inductance of the inductance coil and the degree of coupling between the two inductance coils. Components made using this principle are called transformers.
Common types of inductors
Inductance can be made by winding a conductive material around a magnetic core. Typical ones are copper wires. The magnetic core can also be removed or replaced with ferromagnetic materials. Core materials with a higher magnetic permeability than air can confine the magnetic field more tightly around the inductive element, thus increasing the inductance. There are many kinds of inductors. Most of them are made by winding enamel-coated wire around a ferrite bobbin. Some protective inductors place the coil completely inside the ferrite. The core of some inductive elements can be adjusted. Thus, the inductance can be changed. Small inductors can be directly etched on a PCB board using a method of laying a spiral trajectory. Small-value inductors can also be manufactured in integrated circuits using the same process as manufacturing transistors. In these applications, aluminum interconnects are often used as conductive materials. In any case, based on practical constraints, a circuit called a "gyrator" is still the most widely used. It uses a capacitor and an active element to exhibit the same characteristics as an inductive element. Inductive elements used to block high frequencies are often made of a metal wire passing through a magnetic core or magnetic bead.
Small inductors
Small fixed inductors are usually directly wound on a magnetic core with enameled wire. They are mainly used in circuits such as filtering, oscillation, notch, and delay. They have sealed and unsealed packaging forms, and both forms have vertical and horizontal shapes.
Vertical sealed fixed inductor：Vertical sealed fixed inductors use co-directional pins. The domestic inductance range is 0.1-2200μH (directly marked on the casing). The rated operating current is 0.05-1.6A. The error range is ±5% - ±10%. Imported inductance and current range is larger, and the error is smaller. Imported TDK series color code inductors have their inductance marked on the surface of the inductor with color dots.
Horizontal sealed fixed inductor： Horizontal sealed fixed inductors use axial pins. Domestic ones include LG1, LGA, LGX and other series. The inductance range of the LG1 series inductor is 0.1-22000μH (directly marked on the casing). The LGA series inductor adopts a ultra-small structure. Its appearance is similar to a 1/2W color ring resistor. Its inductance range is 0.22-100μH (marked on the casing with a color ring), and the rated current is 0.09-0.4A. The LGX series color code inductor also has a small packaging structure. Its inductance range is 0.1-10000μH. The rated current is divided into four specifications: 50mA, 150mA, 300mA, and 1.6A.
Adjustable inductor
Commonly used adjustable inductors include oscillation coils for semiconductor radios, line oscillation coils for televisions,Line linearity coils, intermediate frequency notch coils, frequency compensation coils for audio, and choke coils.
Oscillation coil for semiconductor radio: This oscillation coil forms the local oscillator circuit with a variable capacitor in a semiconductor radio to generate a local oscillator signal that is 465kHz higher than the radio signal received by the input tuning circuit. Its exterior is a metal shielding cover. Inside, it is composed of a nylon lining frame, an I-shaped magnetic core, a magnetic cap, and a pin base. On the I-shaped magnetic core, there is a winding wound with high-strength enameled wire. The magnetic cap is installed on the nylon frame inside the shielding cover and can be rotated up and down. By changing its distance from the coil, the inductance of the coil can be changed. The internal structure of the intermediate frequency notch coil of a television is similar to that of an oscillation coil, except that the magnetic cap is an adjustable magnetic core.
Line oscillation coil for television: The line oscillation coil is used in early black-and-white televisions. It forms a self-excited oscillation circuit (a three-point oscillator or an intermittent oscillator, a multivibrator) with peripheral resistance-capacitance elements and line oscillation transistors to generate a rectangular pulse voltage signal with a frequency of 15625HZ.
The magnetic core center of this coil has a square hole. The line synchronization adjustment knob is directly inserted into the square hole. By rotating the line synchronization adjustment knob, the relative distance between the magnetic core and the coil can be changed, thereby changing the inductance of the coil and keeping the line oscillation frequency at 15625HZ to generate synchronous oscillation with the line synchronization pulse sent by the automatic frequency control circuit (AFC).
Linearity coil: The line linearity coil is a nonlinear magnetic saturation inductance coil (its inductance decreases as the current increases). It is generally connected in series in the line deflection coil circuit and uses its magnetic saturation characteristic to compensate for the linear distortion of the image.
The line linearity coil is wound with enameled wire on an "I"-shaped ferrite high-frequency magnetic core or ferrite magnetic rod. There is an adjustable permanent magnet beside the coil. By changing the relative position of the permanent magnet and the coil, the inductance of the coil can be changed to achieve the purpose of linear compensation.
Choke inductor
A choke inductor refers to an inductance coil used in a circuit to block the AC current path. It is divided into high-frequency choke coils and low-frequency choke coils.
High-frequency choke coil: The high-frequency choke coil is also called a high-frequency choke. It is used to prevent high-frequency AC current from passing through.
The high-frequency choke coil works in a high-frequency circuit. It mostly uses a hollow or ferrite high-frequency magnetic core. The skeleton is made of ceramic material or plastic. The coil is wound in a honeycomb-type segmented winding or multi-layer flat winding segmented winding.
Low-frequency choke coil: The low-frequency choke coil is also called a low-frequency choke. It is applied in current circuits, audio circuits, or field output circuits. Its function is to prevent low-frequency AC current from passing through.
Generally, the low-frequency choke coil used in audio circuits is called an audio choke coil. The low-frequency choke coil used in field output circuits is called a field choke coil. The low-frequency choke coil used in current filtering circuits is called a filter choke coil.
Low-frequency choke coils generally use "E"-shaped silicon steel sheet cores (commonly known as silicon steel sheet cores), permalloy cores, or ferrite cores. To prevent magnetic saturation caused by passing a large DC current, an appropriate gap should be left in the iron core during installation.
Characteristics of inductor
The characteristics of an inductor are exactly opposite to those of a capacitor. It has the characteristic of preventing AC current from passing through and allowing DC current to pass through smoothly. When a DC signal passes through the coil, the resistance is just the voltage drop of the wire itself, which is very small; when an AC signal passes through the coil, an induced electromotive force will be generated at both ends of the coil. The direction of the induced electromotive force is opposite to the direction of the applied voltage, hindering the passage of AC. Therefore, the characteristics of an inductor are to pass DC and block AC. The higher the frequency, the greater the impedance of the coil. Inductors often work together with capacitors in circuits to form LC filters, LC oscillators, etc. In addition, people also use the characteristics of inductance to manufacture choke coils, transformers, relays, etc.
Functional uses of inductors
Inductors mainly play roles such as filtering, oscillation, delay, notch, etc. in circuits. They also have functions such as screening signals, filtering noise, stabilizing current, and suppressing electromagnetic interference. The most common role of inductors in circuits is to form an LC filter circuit together with capacitors. Capacitors have the characteristics of "blocking DC and passing AC", while inductors have the functions of "passing DC and blocking AC". If the DC current with many interference signals passes through the LC filter circuit, then the AC interference signals will be converted into heat energy by the inductor and consumed; when relatively pure DC current passes through the inductor, the AC interference signals in it will also be converted into magnetic induction and heat energy. The higher the frequency, the easier it is to be impeded by the inductor, which can suppress higher frequency interference signals.
Inductors have the characteristic of preventing AC current from passing through and allowing DC current to pass through smoothly. The higher the frequency, the greater the impedance of the coil. Therefore, the main function of inductors is to isolate, filter AC signals or form resonant circuits with capacitors, resistors, etc.
Applications of inductors in the automotive field
Main parameters of inductor
The main parameters of an inductor include inductance, allowable deviation, quality factor, distributed capacitance, and rated current.
Inductance, also known as self-inductance coefficient, is a physical quantity that represents the self-induction ability generated by an inductor.
The magnitude of the inductanceof an inductor mainly depends on the number of turns (number of windings) of the coil, the winding method, whether there is a magnetic core and the material of the magnetic core, etc. Generally, the more turns of the coil and the denser the wound coil, the greater the inductance. A coil with a magnetic core has a greater inductance than a coil without a magnetic core; the greater the magnetic permeability of the magnetic core, the greater the inductance of the coil.
The basic unit of inductance is the henry (referred to as henry for short), denoted by the letter "H". Commonly used units also include millihenry (mH) and microhenry (μH). The relationship between them is: 1H = 1000mH；1mH = 1000μH。
Allowable deviation refers to the allowable error value between the nominal inductance on the inductor and the actual inductance. Inductors generally used in oscillation or filtering circuits require higher precision, and the allowable deviation is ±0.2% - ±0.5%; while inductors used for coupling, high-frequency choke and other coils do not require high precision; the allowable deviation is ±10% - 15%.
The quality factor, also known as the Q value or merit value, is the main parameter for measuring the quality of an inductor. It refers to the ratio of the inductive reactance presented by the inductor when it works under an AC voltage of a certain frequency to its equivalent loss resistance. The higher the Q value of an inductor, the smaller its loss and the higher its efficiency. The quality factor of an inductor is related to the DC resistance of the coil wire, the dielectric loss of the coil skeleton, and the losses caused by the iron core, shielding cover, etc.
Distributed capacitance refers to the capacitance that exists between turns of the coil, between the coil and the magnetic core, between the coil and the ground, and between the coil and the metal. The smaller the distributed capacitance of an inductor, the better its stability. Distributed capacitance can increase the equivalent energy-consuming resistance and increase the quality factor. To reduce distributed capacitance, silk-covered wire or multi-strand enameled wire is often used, and sometimes honeycomb winding method is also used.
The rated current refers to the maximum current value that an inductor can withstand in an allowed working environment. If the working current exceeds the rated current, the inductor will change its performance parameters due to heating, and even be burned due to overcurrent.
Connection and difference between inductor and magnetic bead
Inductor is an energy storage element, while magnetic bead is an energy conversion (consumption) device;
Inductors are mostly used in power supply filtering circuits, while magnetic beads are mostly used in signal circuits for EMC countermeasures;
Magnetic beads are mainly used to suppress electromagnetic radiation interference. When inductors are used in this regard, they focus on suppressing conductive interference. Both can be used to deal with EMC and EMI problems; there are two ways of EMI, namely radiation and conduction. Different suppression methods are adopted for different ways. Magnetic beads are used for the former, and inductors are used for the latter;
Magnetic beads are used to absorb ultra-high frequency signals. For some RF circuits, PLLs, oscillation circuits, and circuits containing ultra-high frequency memory (DDR SDRAM, RAMBUS, etc.), magnetic beads need to be added to the power input part. Inductors are an energy storage element and are used in LC oscillation circuits, low-frequency and medium-frequency filtering circuits, etc. Their application frequency range rarely exceeds 50MHZ;
Inductors are generally used for circuit matching and signal quality control, and for general connections and power connections. Magnetic beads are used at the junction of analog ground and digital ground. Magnetic beads are also used for signal lines.
The size of the magnetic bead (more precisely, the characteristic curve of the magnetic bead) depends on the frequency of the interference wave that needs to be absorbed by the magnetic bead. Magnetic beads block high frequencies, have low DC resistance, and high high-frequency resistance. Because the unit of magnetic beads is nominal according to the impedance it generates at a certain frequency, and the unit of impedance is also ohm. The datasheet of magnetic beads generally comes with a characteristic curve of frequency and impedance. Generally, 100MHz is the standard. For example, 2012B601 means that the Impedance of the magnetic bead is 600 ohms at 100MHz.
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