Inductance refers to the current intensity that can be induced when the coil moves in a magnetic field, and the unit is "Henry" (H). It also refers to parts made of this property.
Inductor (inductance coil) and transformer are both electromagnetic induction elements wound with insulated wires (such as enameled wires and yarn-wrapped wires), and they are also one of the commonly used components in electronic circuits, and related products such as * * * mode filters.
Brief introduction of inductance
Diàn' g?n [inductor], plural: inductor and transformer are electromagnetic induction elements wound with insulated wires (such as enameled wires and yarn-wrapped wires), and are also one of the commonly used components in electronic circuits, as well as * * * mode filters and other related products.
Edit this self-feeling and mutual feeling
inductance
When current passes through the coil, a magnetic field will be generated around the coil. When the current in the coil changes, the magnetic field around it changes accordingly, and this changing magnetic field can cause the coil itself to generate induced electromotive force (electromotive force is used to represent the terminal voltage of the ideal power supply of active components), which is self-inductance.
mutual inductance
When two inductance coils are close to each other, the change of magnetic field of one inductance coil will affect the other inductance coil, which is mutual inductance. The magnitude of mutual inductance depends on the self-inductance of the inductance coil and the coupling degree of the two inductance coils, and the components made by using this principle are called transformers.
In this section, edit the functions of the inductor and circuit graphic symbols.
(I) Circuit graphic symbol of inductor An inductor is a series of coaxial turns made of enameled wire, yarn-wrapped wire or plastic-coated wire wound on an insulating skeleton or on a magnetic core or iron core. It is represented by the letter "L" in the circuit. The diagram on the left is its circuit graphic symbol, and the diagram on the right is its physical diagram.
(II) The function of inductance The main function of inductance is to isolate and filter AC signals or form graphic symbols of harmonic inductance with capacitors and resistors.
Vibration circuit.
Type of inductor
Classification by structure
Inductors can be divided into wire wound inductors and non-wire wound inductors (multilayer chip inductors, printed inductors, etc.). ) According to its different structures, it can be divided into fixed inductance and adjustable inductance. According to the installation method, there are patch inductors and plug-in inductors. At the same time, the inductance with outer shield becomes a shielded inductance, and the coil is a bare vertical and horizontal inductance.
Exposed inductance is usually called unshielded inductance. Fixed inductors are divided into hollow electronic surface inductors and magnetic patch inductors.
Magnetic core inductance, magnetic core inductance, etc. It can also be divided into vertical coaxial pin inductors, horizontal axial pin inductors, large and medium-sized inductors, small and exquisite inductors and chip inductors according to their structural shapes and pin modes. Adjustable inductors can be divided into core adjustable inductors, copper core adjustable inductors, sliding contact adjustable inductors, series mutual inductance adjustable inductors and multi-tap adjustable inductors.
Classification by working frequency
Inductors can be divided into high frequency inductors, medium frequency inductors and low frequency inductors according to working frequency. Air-core inductance, magnetic core inductance and copper core inductance are generally medium-frequency or high-frequency inductance, while magnetic core inductance is mostly low-frequency inductance.
Classification by purpose
Inductors can be divided into oscillation inductance, correction inductance, kinescope deflection inductance, choke inductance, filter inductance and isolation inductance according to their uses.
Inductor, compensation inductor, etc. Oscillating inductance is divided into TV line oscillating coil, east-west pincushion correction coil, etc. The deflection coils of CRT are divided into line deflection coils and field deflection coils. Choke inductance (also known as choke coil) is divided into high frequency choke, low frequency choke, electronic ballast choke, TV line frequency choke and TV field frequency choke. Filter inductance is divided into power supply (power frequency) filter inductance and high frequency filter inductance.
Edit the main parameters of the sensor in this section.
The main parameters of inductor include inductance, allowable deviation, quality factor, distributed capacitance and rated current.
inductance
Inductance, also known as self-inductance coefficient, is a physical quantity indicating the self-inductance ability of inductor. Ring inductor
The inductance of the inductor mainly depends on the number of turns of the coil, the winding mode, whether there is an iron core and the material of the iron core. Generally speaking, the more turns of the coil, the denser the wound coil and the greater the inductance. The inductance of the coil with magnetic core is greater than that of the coil without magnetic core; The greater the permeability of the magnetic core, the greater the inductance. The basic unit of inductance is Henry (abbreviated as Hen), which is represented by the letter "H". Commonly used units are milli-Heng (mH) and micro-Heng (μH), and the relationship between them is:1h =1000 MH1MH =1000 μ h.
allowable deviation
Allowable deviation refers to the allowable error between the nominal inductance and the actual inductance of the inductance. Inductors commonly used in oscillating or filtering circuits require high accuracy, and the allowable deviation is 0.2% ~ 0.5%. However, the accuracy of coils used for coupling and high frequency chokes is not high; The allowable deviation is 10% ~ 15%.
quality index
Quality factor, also known as Q value or quality factor, is the main parameter to measure the quality of inductance. It refers to the ratio of inductance to its equivalent loss resistance when the inductance works under a certain frequency AC voltage. The higher the Q value of the inductor, the smaller the loss and the higher the efficiency. The quality factor of the inductor is related to the DC resistance of the coil wire, the dielectric loss of the bobbin and the loss caused by the magnetic core and shield.
distributed capacitance
Distributed capacitance refers to the capacitance between coil turns and between coil and iron core. The smaller the distributed capacitance of the inductor, the better its stability.
rated current
Rated current refers to the maximum current that the inductor is allowed to pass under normal working conditions. If the working current exceeds the rated current, the performance parameters of the inductor will change due to heating, and even burn out due to overcurrent.
Related reading of inductance
* * * Mode inductance and iron-based nanocrystalline alloy 1. Introduction With the increasing application of switching power supply in industrial and household appliances, the mutual interference between electrical appliances has become an increasingly serious problem, and the electromagnetic environment has attracted more and more attention. There are many kinds of electromagnetic interference, among which * * * mode interference below 30MHz is a very important one, which is mainly transmitted by conduction, causing great harm to the safe and normal operation of the instrument and must be controlled. Usually, a * * * mode filter is attached to the input terminal to reduce the external * * mode interference from entering the instrument through the power line, and at the same time prevent the * * * mode interference generated by the instrument from entering the power grid. The core of * * mode filter is the * * mode inductance of soft magnetic core, and its performance determines the level of filter. 2.* * * mode noise and * * * mode inductance * * * mode noise are mainly generated from the turn-on and turn-off of various switching devices, which can be decomposed into different harmonic forms and have a wide spectrum range. For interference signals below 30MHz, they generally propagate through conduction. * * * mode inductance consists of soft magnetic core and two groups of coils wound in the same direction, as shown in figure 1. For the differential mode signal, because the magnetic fields generated by the two groups of coils are in opposite directions and cancel each other, the iron core is not magnetized, and there is no suppression effect on the signal. For the * * * mode signal, because the magnetic fields generated by the two groups of coils are not offset, they are superimposed on each other, and the iron core is magnetized. Due to the high permeability of the iron core material, the iron core will produce a great inductance, and the impedance of the coil will inhibit the passage of * * * mode signals. 3. Relationship between performance of * * * mode inductance device and material performance In order to filter out * * * mode interference more effectively, the * * mode inductance must first have a large enough inductance, so high permeability of iron core material is the most basic requirement of * * * mode inductance. On the other hand, the frequency characteristics of iron core materials are also the key factors to determine the performance of devices. Because the frequency spectrum of * * * mode interference is very wide, the impedance of iron core to * * * mode interference has a maximum only in a certain frequency band. Therefore, in order to filter out the * * * mode interference in a certain frequency band, the frequency characteristics of the iron core should make the impedance of the device have the greatest mismatch with the subsequent circuits in this frequency band, so as to generate sufficient loss (called insertion loss) for the * * * mode interference. For the * * * mode signal, the * * mode inductance can be equivalent to the series connection of a resistor and an inductor. At this time, the total impedance of the device is: where: is the inductance related to pure inductance caused by the real part of iron core permeability. It is the impedance related to the loss caused by the imaginary part of the magnetic permeability of the iron core. L0 is the inductance of the air-core inductor. In the actual * * * mode inductance, XL is the reflection of * * * mode interference, while XR is the part absorbed and consumed due to core loss. These two parts form the suppression of * * * mode interference. Therefore, the total impedance of * * * mode inductor core represents the device's ability to suppress * * * mode interference. Most core suppliers of * * mode inductors use the relationship between impedance (or insertion loss after making devices) and frequency to express the frequency characteristics of their products. The relationship between permeability and frequency of materials is complex. Generally speaking, the real part of magnetic permeability decreases with the increase of frequency; The imaginary part of permeability is very low at first, and there is a peak at a certain frequency (called cutoff frequency). How does it decrease with frequency? It should be noted that the variation law of device impedance with frequency is different from that of magnetic permeability, because impedance is not only determined by magnetic permeability, but also related to frequency. Generally speaking, the impedance and frequency characteristics of * * mode inductors are determined by factors such as core size, material characteristics and coil turns. 4. Advantages of Nanocrystalline Alloys In order to obtain the best suppression effect of * * mode interference, the * * mode inductance core must have high permeability and excellent frequency characteristics. In the past, ferrite was mostly used as the core material of * * mode inductors, which has excellent frequency characteristics and low cost advantages. However, ferrite also has some insurmountable weaknesses, such as poor temperature characteristics and low saturation magnetic inductance, which limit its application. In recent years, the appearance of Fe-based nanocrystalline alloy has added an excellent magnetic core material for * * * mode inductance. The manufacturing process of iron-based nanocrystalline alloy is as follows: firstly, the amorphous alloy thin strip with the thickness of about 20-30 microns is made by rapid solidification technology, and then it is rolled into iron core and further processed into nanocrystals. Compared with ferrite, nanocrystalline alloys have some unique advantages:? High saturation magnetic induction intensity: Bs of iron-based nanocrystalline alloy reaches 1.2T, which is more than twice that of ferrite. As a * * * mode inductance core, an important principle is that the core cannot be magnetized to saturation, otherwise the inductance will drop sharply. However, in practical application, there are many occasions with strong interference intensity (such as high-power variable frequency motors). If ordinary ferrite is used as the * * * mode inductor, the iron core may be saturated, and the noise suppression effect under high-intensity interference cannot be guaranteed. Because of the high saturation magnetic induction, the anti-saturation characteristics of nanocrystalline alloys are obviously better than those of ferrite, which makes nanocrystalline alloys very suitable for occasions with high current resistance and strong interference. ? High initial permeability: the initial permeability of nanocrystalline alloy can reach 654.38+ million, which is much higher than that of ferrite. Therefore, the * * mode inductor made of nanocrystalline alloy has large impedance and insertion loss at low magnetic field, and has excellent suppression effect on weak interference. This is especially suitable for weak anti-jamming * * * mode filters that require minimum leakage current. In some specific occasions (such as medical equipment), the equipment causes leakage current through the capacitance to the ground (such as human body), which is easy to form * * * mode interference, and the equipment itself has extremely strict requirements for this. At this time, it may be the best choice to make * * * mode inductance with nanocrystalline alloy with high permeability. In addition, the high permeability of nanocrystalline alloys can reduce the distribution parameters such as coil turns and parasitic capacitance, thus increasing the peak frequency of insertion loss spectrum caused by distribution parameters. At the same time, the high permeability of nanocrystalline iron core makes the * * mode inductance have higher inductance and impedance value, or reduces the volume of iron core on the premise of the same inductance. ? Excellent temperature stability: Curie temperature of iron-based nanocrystalline alloy is above 570℃. Under the condition of large temperature fluctuation, the performance change rate of nanocrystalline alloy is obviously lower than that of ferrite, and it has excellent stability and the performance change is close to linear. Generally, in the temperature range of -50oC- 130oC, the change rate of main magnetic properties of nanocrystalline alloys is within 10%. In contrast, the Curie temperature of ferrite is generally below 250oC, and the change rate of magnetic properties sometimes reaches more than 100%, which is nonlinear and difficult to compensate. This temperature stability of nanocrystalline alloy, combined with its unique low loss characteristics, provides relaxed temperature conditions for device designers. ? Flexible frequency characteristics: Nanocrystalline iron core can obtain different frequency characteristics through different manufacturing processes, and appropriate coil turns can obtain different impedance characteristics to meet the filtering requirements of different frequency bands, and its impedance value is much higher than that of ferrite. It should be pointed out that any filter can not expect to achieve noise suppression in the whole frequency range with one core material, but should choose different core materials, sizes and turns according to the filter frequency band required by the filter. Compared with ferrite, nanocrystalline alloys can obtain the required frequency characteristics more flexibly by adjusting the process. Iron-based nanocrystalline alloys have been widely used in switching power supply transformers, transformers and other fields since they were developed in the late 1980s. Nanocrystalline alloys have attracted more and more attention in the field of anti-interference filters because of their high permeability, high saturation magnetic inductance and flexible and adjustable frequency characteristics. Foreign countries have iron-based nanocrystalline alloy * * * mode inductor cores that can be supplied in large quantities. With the gradual deepening of people's understanding of nanocrystalline alloys, it can be predicted that the application prospect of * * * mode inductors made of nanocrystalline alloys will be more and more broad in China. It's the first time I saw * * * mode inductor * * mode choke, also called * * * mode choke, which is often used to filter * * * mode electromagnetic interference signals in computer switching power supply. In the board design, the * * mode inductance also plays the role of EMI filtering, which is used to suppress the electromagnetic wave generated by high-speed signal lines from radiating outward. Various CMC tips: EMI (electromagnetic interference) The motherboard inside the computer is mixed with various high-frequency circuits, digital circuits and analog circuits. When they work, a large number of high-frequency electromagnetic waves will interfere with each other, which is EMI. EMI will also be emitted through motherboard wiring or external cables, resulting in electromagnetic radiation pollution, which not only affects the normal work of other electronic equipment, but also does harm to human body. The chip on PC board is not only the object of electromagnetic interference, but also the source of electromagnetic interference. Generally speaking, these electromagnetic interferences can be divided into two categories: series mode interference (differential mode interference) and * * * mode interference (grounding interference). Take two PCB traces on the motherboard (wires connecting motherboard components) as an example. The so-called serial mode interference refers to the interference between two wires. * * * Mode interference is interference caused by the potential difference between two traces and PCB ground. The interference current in series mode acts between two signal lines, and its conduction direction is consistent with waveform and signal current; * * * mode interference current acts between the signal line and the ground line, and the interference current flows through half of the two signal lines in the same direction, and the ground line serves as a common * * * loop. Cross-mode interference and * * mode interference If the * * mode current generated by the board is not attenuated and filtered (especially the * * mode current on high-speed interface lines such as USB, IEEE 1394 interface), then the * * mode interference current can easily generate electromagnetic radiation through the interface data line-the * * mode radiation generated by the * * mode current in the cable. The FCC of the United States, CISPR22 of the Special Committee on International Radio Interference and GB9254 of China all have restrictions on the conducted interference and radiation emission of communication ports of information technology equipment. In order to eliminate the interference signal and all kinds of induced interference input on the signal line, the filter circuit must be arranged reasonably to filter out the interference of * * * mode and series mode, and the * * * mode inductance is an integral part of the filter circuit. * * mode inductance is essentially a bidirectional filter: on the one hand, it is necessary to filter out the * * mode electromagnetic interference on the signal line, on the other hand, it is necessary to suppress the electromagnetic interference emitted by itself to avoid affecting the normal work of other electronic devices in the same electromagnetic environment. Schematic diagram of internal circuit of * * * mode inductor The diagram above is our common schematic diagram of internal circuit of * * * mode inductor. In actual circuit design, multi-level * * * mode circuit can be used to better filter out electromagnetic interference. In addition, we can also see a patch * * * mode inductor on the motherboard, which has almost the same structure and function as the vertical * * * mode inductor.
Repair CMC
Second, from the working principle, why * * * mode inductance can prevent EMI? To understand this, we need to start with the structure of * * * mode inductor. * * * mode inductance filter circuit The above picture shows a filter circuit of * * * mode inductance, and La and Lb are * * * mode inductance coils. These two coils are wound on the same iron core with the same number of turns and phase (opposite winding directions). In this way, when the normal current in the circuit flows through the inductance of * * * mode, the current generates reverse magnetic fields in the inductance coils wound in the same phase, which cancel each other out. At this time, the normal signal current is mainly affected by the coil resistance (and a little damping caused by leakage inductance); When * * * mode current flows through the coil, due to the same direction of * * mode current, the same magnetic field will be generated in the coil, which will increase the inductance of the coil, make the coil present high impedance and produce strong damping effect, thus attenuating the * * * mode current and achieving the purpose of filtering.
In fact, if one end of this filter circuit is connected to the interference source and the other end is connected to the interfered equipment, then La and C 1, Lb and C2 form two sets of low-pass filters, which can control the EMI signal in * * * mode on the line at a very low level. The circuit can not only suppress the entry of external EMI signals, but also attenuate the EMI signals generated when the circuit itself works, and effectively reduce the EMI interference intensity.
Tips: Leakage inductance and differential mode inductance For an ideal inductance model, when the coil is wound, all the magnetic flux is concentrated in the center of the coil. However, in general, the annular coil will not be completely wound or wound loosely, which will cause magnetic leakage. * * * mode inductance has two windings, and there is a considerable gap between them, which will lead to magnetic leakage and form differential mode inductance. Therefore, the * * * mode inductance generally has a certain attenuation ability to the differential mode interference.
In the design of filter, we can also use leakage inductance. For example, in a common filter, only one * * * mode inductor is installed, and a proper amount of differential mode inductor is generated by using the leakage inductance of the * * * mode inductor, so as to suppress the differential mode current. Sometimes it is necessary to artificially increase the leakage inductance of the * * * mode choke and improve the differential mode inductance to achieve better filtering effect.
From the overall design of kanban cards, we can see the * * * mode inductors on some motherboards, but on most motherboards, we will find that this component is omitted, and some even have no reserved position. Is this motherboard qualified?
Undeniably, the * * * mode inductance has a good suppression effect on the * * * mode interference of the high-speed interface of the motherboard, which can effectively prevent EMI from forming electromagnetic radiation through cables to affect the normal work of other peripherals and our health. But at the same time, it should be pointed out that the anti-EMI design of board is a huge system engineering, and the design using * * * mode inductance is only a small part of it. The circuit board designed with * * * mode inductance at the high-speed interface is not necessarily excellent in the overall EMI prevention design. Therefore, from the * * * mode filter circuit, we can only see one aspect of the board design, which is easy to be ignored by everyone and makes the mistake of seeing only the trees but not the forest. Only by knowing the overall anti-EMI design of the board can we evaluate the quality of the board. So, what does an excellent circuit board design usually do to prevent EMI?
● Motherboard layout design For excellent motherboard layout design, most clock wiring will take shielding measures or be close to the ground wire to reduce EMI. For multi-layer PCB design, adjacent PCB wiring layers will adopt the open-loop principle to prevent conductors from forming loops from one layer to another. If the wiring forms a closed loop, it will act as an antenna and enhance the intensity of EMI radiation.
Unequal signal line lengths will also cause unbalanced impedance of the two lines, resulting in * * * mode interference. Therefore, in the design of the board, the signal line will be snake-shaped to make its impedance as consistent as possible and weaken the * * * mode interference. At the same time, the serpentine will also minimize the bending and swinging during wiring, so as to reduce the area of the annular area and thus reduce the radiation intensity.
Main board serpentine wiring
In the design of high-speed PCB, the length of the trace is generally not an integer multiple of the clock signal wavelength 1/4, otherwise it will produce resonance and serious EMI radiation. At the same time, wiring should ensure that the return path is minimal and unobstructed. For the design of decoupling capacitor, it should be set as close as possible to the power supply pin, and the area surrounded by the power supply line and ground line of the capacitor should be as small as possible to reduce the ripple, noise and EMI radiation of the power supply. Of course, the above are only a small part of the principles in the EMI prevention design of PCB. The layout design of the motherboard is a very complicated and profound knowledge, and even many DIYer have such knowledge: whether the layout design is excellent or not has a very significant impact on the overall performance of the motherboard.
● Cutting off the wiring of the motherboard If you want to completely isolate the electromagnetic interference between the motherboard circuits, it is absolutely impossible, because we can't "package" the electromagnetic interference one by one, so we should adopt other methods to reduce the degree of interference. The metal wire in the motherboard PCB is the chief culprit in transmitting interference current, which transmits and emits electromagnetic interference signals like antennas, so it is a useful method to "cut off" these "antennas" at appropriate places. If the "antenna" is broken, and then a circle of insulator is enclosed, its interference to the outside world will naturally be greatly reduced. The leakage of electromagnetic radiation can be further reduced if a filter capacitor is used at the disconnection point. This design can obviously increase the stability of high-frequency operation and prevent EMI radiation. Many large motherboard manufacturers have adopted this method in their design. Calculation formula of inductance: inductance is calculated according to the following formula: coil formula impedance (ohm) = 2 * 3. 14 159 * F (working frequency) * inductance (mH), and 360 ohm impedance is required for setting. Therefore: inductance (mH) = impedance (ohm) ÷ (2*3. 14 159) ÷ F (working frequency) = 360 ÷ (2 * 3.14/kloc-0. {( 18* circle diameter (inch)) +(40 * circle length (inch)}] circle diameter (inch) turns = [8.116 * {(18 * 2.047)+ Calculation formula of air-core coil inductance: l = (0.01* d * n * n)/(l/d+0.44) Coil inductance L unit: micro-winding diameter D unit: cm coil turns N unit: turn coil length L unit: cm frequency inductance and capacitance calculation formula: L = 25330.3/[(F0 *] 25KHZ=0. 125 resonant capacitance: ... 1000pf, which can be determined by itself or by q value: l unit: the calculation formula of micro-constant coil inductance is 1. For a circular magnetic core, the following formula can be used: (iron) l = N2. AL l = inductance (H) H-DC=0.4πNI/l N= coil turns (turns) AL= inductance H-DC= DC magnetizing force I= passing current (A) l= magnetic circuit length (cm) l and AL value can be used. For example, if T50-52 is used, and the coil has five and a half turns, its L value is T50-52 (indicating an outside diameter of 0.5 inch) and its AL value is about 33 NH L = 33. (5.5) 2 = 998.25 NH ≈ 1 μ h When a current of 10A flows, The value of l can be introduced into an empirical formula by l=3.74 (look-up table), h-DC = 0.4 π ni/l = 0.4× 3.14× 5.5×10/3.74 =18.47 (look-up table) and the degree. (negative seventh power of 10) μs is the relative permeability of the inner core of the coil, μs= 1 N2 is the cross-sectional area of the square s coil of the coil in the air-core coil, and the unit is the coil length, the unit is square meter L, and the unit is m k coefficient, which depends on the ratio of the radius (r) to the length (l) of the coil. The unit for calculating inductance is Henry (h).