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    Inductance knowledge
    Date:2013-06-17 11:29
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    Inductance [of an ideal inductor]
    The quotient of the voltage divided by the derivative of current versus time
              Inductance of an ideal inductor is an attribute of a closed loop. When the coil passes current, a magnetic field induction is formed in the coil, which in turn generates an induced current to resist current flow through the coil. The interaction between this current and the coil is called the electrical inductive reactance, which is the inductance, and the unit is "Henry (H)".
    Introduction to inductance
    Inductance is a property of a closed loop, that is, when the current through the closed loop changes, an electromotive force occurs to resist the change in current. This type of inductance is called self-inductance and is a property of the closed loop itself. Assuming that the current in a closed loop changes, the electromotive force is generated by another inductive action in another closed loop. This inductance is called a mutual inductance.
    Anchor point anchor
    When a current flows through the coil, a magnetic field is generated around the coil. When the current in the coil changes, the surrounding magnetic field also changes accordingly. The changed magnetic field can cause the coil itself to generate an induced electromotive force (induced electromotive force) (the electromotive force is used to indicate the terminal voltage of the ideal power source of the active component). It is self-consciousness.
    Anchor anchor point mutual inductance
        When the two inductor coils are close to each other, the change in the magnetic field of one inductor coil will affect the other inductor coil. This effect is mutual inductance. The magnitude of the mutual inductance depends on the degree to which the inductance of the inductor is coupled to the two inductors. The component made by this principle is called a transformer.
    basic structure
         The inductor can be made of a coiled core of electrically conductive material, typically a copper wire, or the core can be removed or replaced with a ferromagnetic material. A core material having a higher magnetic permeability than air can tightly constrain the magnetic field around the inductance element, thereby increasing the inductance. There are many types of inductors, most of which are made of enamel coated wire around a ferrite bobbin, while some protective inductors place the coil completely inside the ferrite. The core of some inductive components can be adjusted. This can change the size of the inductor. Small inductors can be etched directly onto the PCB, using a method of laying spiral traces. Small value inductors can also be fabricated in integrated circuits using the same process of fabricating transistors. In these applications, aluminum interconnects are often used as conductive materials. Regardless of the method used, the most practical application of constraints is a circuit called a "rotator" that uses a capacitor and active components to exhibit the same characteristics as an inductive component. Inductive components for high frequency isolation often consist of a wire that passes through a magnetic column or magnetic bead.
    Inductance characteristics
        Inductance is a physical quantity that measures the ability of a coil to generate electromagnetic induction. When the coil is supplied with an unsteady current, a varying magnetic field is generated around it. The greater the power supplied to the coil, the higher the strength of the excited magnetic field, and vice versa (before the magnetic induction reaches saturation).
           Inductors are generally divided into air core inductors and core inductors. The inductance of the hollow core inductor is a constant value and is simple to apply. Large magnetic core inductors are used more in the industry, and the accuracy of the inductance value is a key issue, which is of great significance both in theory and in practical applications.
           The analysis was carried out by the formula L = μ × Ae * N2 / l·. L represents the inductance, μ represents the magnetic permeability of the core, Ae represents the cross-sectional area of ??the core, N represents the number of turns of the coil, and lm represents the magnetic path length of the core. It can be seen that when a certain inductor is produced and formed, Ae, N, and lm are all fixed values, then only the magnetic permeability μ is affected by the magnitude of the inductance of the inductor.
    Inductance extreme value
    Minimum and maximum values:
          The minimum value of the inductance (L) is determined by the requirement of the minimum load current to be maintained. The current flowing through the inductor L is divided into two types of continuous and discontinuous operation. In either case, as long as the input and output voltages remain the same, the slope of the current waveform does not change due to the decrease in load current.
    If the load current I gradually decreases, when the minimum value of the fluctuating current in the inductor L is just zero, it is defined as the critical current Ioc, and Ioc should be equal to - half of the peak value of the current peak, that is,
    When Io < Ioc, iL will enter the discontinuous state Io ≥ Ioc when iL is continuous.
    The closed-loop control circuit of the single-ended forward converter is shown in the figure. In the figure, Cc is the distributed capacitance of the demagnetization reset winding Δ. The transfer function of the continuous state has two poles; the transfer function of the discontinuous state has only one pole. If you want to work stably during the state transition, you must carefully design carefully.
    Closed-loop control circuit for single-ended forward converter
          Another limiting factor for L values ??will appear when applied to multiple sets of output voltages. Because the control loop is only closed to the associated output, when the output current is below the threshold, the duty cycle is reduced to keep the voltage at this output constant. For the other auxiliary outputs, it is assumed that it carries a constant load, and in the case where the duty ratio is lowered, the voltage also drops. Obviously this is not desirable, so in the case of multiple sets of output voltages, in order to keep the auxiliary output voltage constant, the value of the inductance L should be greater than the minimum required. That is to say, if the auxiliary voltage is to be kept within a certain fluctuation range, the inductance of the main output must always exceed the critical value, that is, it is always in a continuous state.
    The maximum value of the inductor is generally limited by efficiency, volume, and cost. The cost of a large inductor with DC current is expensive. From the point of view of J, the inductance L is too large, which will slow down the reaction speed of the regulation system. Because the excessive L limits the maximum rate of change of the output current when there is a large transient change in the load.
    Inductive action
    The role of the inductor in the circuit:
         Electromagnetism, magnetoelectricity, and the two complement each other, always accompanied by the display. When a constant current flows through a wire, a constant magnetic field is always excited around the wire. When the wire is bent into a spiral coil, the law of electromagnetic induction can be used to conclude that a magnetic field has occurred in the spiral coil. Put this spiral coil in a current loop. When the DC current in this loop changes (such as from small to large or vice versa), the magnetic field in the inductor should also change. The changing magnetic field will bring a change of "new current." ", by the law of electromagnetic induction, this "new current" must be opposite to the original direct current direction, thus forming a certain resistance to changes in direct current in a short time. However, once the change is completed, the current is stabilized and the magnetic field is no longer changed, so that no obstacles will occur.
          From the above process, the core function of the inductor is to prevent the change of current. For example, in the process of small to large current, the inductor has a "lag" effect, which can resist this change for a certain period of time. On the other hand, just because the inductor has the function of storing a certain amount of energy, it can temporarily try to maintain its original state when the change is made, but it should be noted that when the energy is exhausted, it can only flow with the wave.
          The "straight blocking" characteristic of the inductor allows it to play a huge role in the circuit. In the board, the inductor is used in energy storage, filtering, delay and oscillation. It is an important component to ensure stable and safe operation of the board.
    Common species
    Anchor anchor small fixed inductor
         Small fixed inductors are usually wound directly on the core with an enameled wire. They are mainly used in circuits such as filtering, oscillation, notching, and delay. They are available in both sealed and unsealed packages. Both vertical and horizontal shapes are available.
    1. Vertical sealed fixed inductor The vertical sealed fixed inductor adopts the same-direction pin. The domestic inductance range is 0.1~2200μH (straight mark on the outer casing), the rated working current is 0.05~1.6A, and the error range is ± 5%~±10%, the inductance of the imported, the current range is larger, and the error is smaller. Imported TDK series color code inductors, the inductance of which is marked on the surface of the inductor with color points.
    2. Horizontal sealed fixed inductor The horizontal sealed fixed inductor uses an axial type pin.
       LC series inductors have inductances ranging from 0.1 to 22000μH (straight on the housing)
       The LC series inductors are ultra-small and similar in appearance to 1/2W color ring resistors. The inductance ranges from 0.22 to 100μH (marked on the housing with a color ring) and the rated current is 0.09~0.4A.
        The LC series color code inductors are also small package structures with inductance ranging from 0.1 to 10000μH and rated currents of 50mA, 150mA, 300mA and 1.6A.
    Anchor anchor adjustable inductor
    Commonly used adjustable inductors include an oscillating coil for a semiconductor radio, a line oscillating coil for a television, a linear coil, an intermediate frequency trap coil, a frequency compensation coil for sound, and a wave blocking coil.
    1. Oscillation coil for semiconductor radio: This oscillation coil is composed of a local oscillator circuit in a semiconductor radio and a variable capacitor, and is used to generate a local signal of 465 kHz higher than a radio signal received by the input tuning circuit. The outside is a metal shield, and the inside is composed of a nylon lining, an I-shaped core, a magnetic cap and a lead seat. The winding of the high-strength enameled wire is used on the I-shaped core. The magnetic cap is mounted on a nylon frame in the shield, and can be rotated up and down to change the inductance of the coil by changing its distance from the coil. The internal structure of the TV IF notch coil is similar to that of the oscillating coil, except that the magnetic cap has a tunable core.
    2. The oscillating coil for TV sets: The oscillating coil is used in the early black-and-white TV. It consists of a self-excited oscillating circuit with a peripheral RC and a oscillating transistor (three-point oscillator or intermittent oscillator, multi-resonant ,) used to generate a rectangular pulse voltage signal with a frequency of 15625HZ. The center of the core of the coil has a square hole, and the line synchronization adjustment knob is directly inserted into the square hole, and the synchronous adjustment knob is rotated to change the relative distance between the core and the coil, thereby changing the inductance of the coil and keeping the line oscillation frequency For the 15625HZ, the line sync pulse sent by the automatic frequency control circuit (AFC) generates synchronous oscillation.
    3. Linear coil: The linear coil is a kind of nonlinear magnetic saturation inductor (the inductance decreases with the increase of current). It is generally connected in series in the line deflection coil loop, and compensates by its magnetic saturation characteristics. Linear distortion of the image.
    The linear coil is wound with an enameled wire on a "work" type ferrite high-frequency core or a ferrite magnet, and an adjustable permanent magnet is placed beside the coil. The linear inductance is achieved by changing the relative position of the permanent magnet and the coil to change the inductance of the coil.
    Anchor anchor point blocking inductor
         A choke inductor is an inductive coil used in a circuit to block an AC current path. It is divided into a high frequency choke coil and a low frequency choke coil.
    1. High-frequency choke coil: The high-frequency choke coil is also called high-frequency choke coil, which is used to prevent high-frequency AC current from passing.
    The high-frequency choke coil works in the high-frequency circuit, and the hollow core or ferrite high-frequency core is mostly used. The skeleton is made of ceramic material or plastic, and the coil is wound by a honeycomb type or a multi-layer flat winding.
    2, low-frequency choke coil: low-frequency choke coil is also called low-frequency choke coil, it is applied to current circuit, audio circuit or field output and other circuits, its role is to prevent low-frequency AC current through.
    Generally, the low frequency choke coil used in the audio circuit is called an audio choke, and the low frequency choke coil used in the field output circuit is called a field choke, and the low frequency choke coil used in the current filter circuit. It is called a filter choke.
    The low-frequency choke coil generally adopts an "E"-shaped silicon steel sheet core (commonly known as a silicon steel sheet core), a permalloy core or a ferrite core. In order to prevent magnetic saturation caused by a large DC current, an appropriate gap should be left in the core during installation.
    Main classification
    Anchor anchor points are classified by structure
         Inductors can be divided into wirewound inductors and non-wire wound inductors (multilayer chip, printed inductor, etc.) according to their structure. They can also be divided into fixed inductors and adjustable inductors.
    According to the placement method: there are chip inductors, plug-in inductors. At the same time, the external shielding of the inductor becomes a shielded inductor, and the exposed coil is generally called an unshielded inductor. Fixed inductors are also divided into hollow electronic sensors, core inductors, core inductors, etc. According to their structural shape and pin way, they can also be divided into vertical co-directional pin inductors and horizontal axial pin inductors. , large and medium inductors, small and exquisite inductors and chip inductors.
    The adjustable inductor is further divided into a magnetic core adjustable inductor, a copper core adjustable inductor, a sliding contact adjustable inductor, a series mutual inductance adjustable inductor and a multi-tap adjustable inductor.
    Anchor anchor points are classified by working frequency
        Inductors can be divided into high frequency inductors, intermediate frequency inductors and low frequency inductors according to the operating frequency.
    Hollow inductors, core inductors, and copper inductors are typically intermediate frequency or high frequency inductors, while core inductors are mostly low frequency inductors.
    Anchor anchors are classified by purpose
         Inductors can be classified into oscillating inductors, correcting inductors, kinescope deflection inductors, choke inductors, filter inductors, isolated inductors, compensated inductors, etc., depending on the application.
    The oscillating inductor is further divided into a television line oscillating coil, an east and a pincushion correction coil, and the like.
    The kinescope deflection inductor is divided into a row deflection coil and a field deflection coil.
    The choke inductor (also known as the choke) is divided into a high-frequency choke coil, a low-frequency choke coil, a choke coil for an electronic ballast, a TV line frequency choke, and a TV airport frequency choke.
    The filter inductor is divided into a power supply (power frequency) filter inductor and a high frequency filter inductor.
    The main parameters
    The main parameters of the inductor are inductance, allowable deviation, quality factor, distributed capacitance and rated current.
    Anchor point anchor inductance
          The inductance is also called the self-inductance coefficient, which is a physical quantity indicating that the inductor generates self-inductance. The amount of inductance of the inductor depends mainly on the number of turns of the coil (number of turns), the winding method, the presence or absence of the core and the material of the core, and the like. Generally, the more coil turns, the denser the wound coil, and the greater the inductance. A coil with a core has a larger inductance than a coil without a core; a coil with a larger magnetic permeability has a larger inductance.
    The basic unit of inductance is Henry (referred to as Henry), which is indicated by the letter "H". Commonly used units are millihenry (mH) and microhenry (μH). The relationship between them is:
    Anchor anchor point tolerance
          The allowable deviation is the allowable error between the nominal inductance and the actual inductance on the inductor.
    Generally, inductors used in circuits such as oscillation or filtering require high precision, and the allowable deviation is ±0.2%~±0.5%. The accuracy of coils for coupling and high-frequency choke is not high; the allowable deviation is ±10. %~15%.
    Anchor anchor quality factor
          The quality factor, also known as the Q value or the figure of merit, is the main parameter for measuring the quality of the inductor. It refers to the ratio of the inductive reactance exhibited by the inductor to its equivalent loss resistance when operating at an AC voltage of a certain frequency.
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