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# "Recommended reading" inductance pure dry goods

2023-03-24【Archive】

Choke - also known as choke, reactor, dynamic reactor. Together with capacitors and resistors, they are called three main passive components. After capacitors and resistors, they quickly evolved into microcircuit components.

**Rationale**

**Self-Induction**: Electromagnetic induction occurs when current flowing through conductor itself changes. The coil is made of metal wires. When current flowing through coil changes, it causes obvious phenomenon of electromagnetic induction. The self-induced reverse electromotive force of coil prevents current from changing and plays role of current stabilization. In particular, if inductor is in a state where no current is flowing, it will attempt to block flow of current through it when circuit is on; if inductor is in a state where current is flowing, it will try to maintain current when circuit is off.

From an energetic point of view, an inductor can dump electrical energy into magnetic energy and release magnetic energy into electrical energy. The same inductor will block current differently at different frequencies. The general rule is to pass low frequency and block high frequency.

**Basic operating parameters of inductors**

Inductance, also known as self-inductance ratio, is a physical quantity that expresses ability of self-inductance to change current flowing through an inductor. The size of inductance reflects amount of energy stored and released by component. Inductance is an inherent characteristic of inductor itself, which depends on number of coil turns, winding method, core material, etc.

Formula: Ls=(k*µ*N²*S)/l

Where: μ is relative magnetic permeability of magnetic core

N is square of number of turns of coilS is cross-sectional area of the coil, in square metersL is length of coil, in meters k empirical factor

From formula:

The more coils and denser coils, greater inductance. A coil with a magnetic circuit has a higher inductance than a coil without a magnetic circuit, a coil with a higher permeability of magnetic circuit has a greater inductance. The basic unit of inductance is Henry (Henry), denoted by letter "H". Millihenry (mH), microhenry (µH), and nanohenry (nH) are commonly used. Conversion ratio: 1H=10^mH=10^6µH=10^9nH

**Inductance tolerance**

Tolerance refers to allowable error value between nominal inductance of an inductor and actual inductance. Inductors used in circuits such as oscillation or filtering require high accuracy, and tolerance is ±0.2%~±0.5%, ±20%.

**Inductive XL**

The resistance of an inductor to AC current is called inductive reactance XL, and unit is ohm. Its connection with inductance L and frequency of alternating current f is XL=2πfL

**Quality Factor Q**

Quality factor Q is main parameter characterizing quality of inductors.

Q is ratio of inductive reactance XL to its equivalent resistance when inductor is operated at a certain frequency of alternating voltage:

**Formula: Q = XL / R**

Since XL is related to frequency, Q value is related to frequency. Regular Q-F curves are bell-shaped. The Q value of an inductor is related to DC resistance of coil wire, dielectric loss of magnetic core, loss caused by shield or iron core, and influence of high frequency skin effect. The Q value reflects proportional relationship between useful work done by component and energy consumed by it. The higher Q value of inductor, lower loop losses and higher efficiency. The value of quality factor of inductor is usually from several tens to several hundreds. The communication and tuning circuits in receiving and transmitting modules require highlow Q-value requirements, and filter circuit has low Q-value requirements

**SRF Natural Resonant Frequency**

The frequency point at which parasitic capacitance and inductance of an inductor resonate is referred to as FSR. At FSR, inductive reactance and parasitic capacitance are equal and cancel each other out, and total reactance is 0, and inductance in FSR loses its energy storage capacitance, exhibiting a high-resistance pure resistance characteristic. That is, when FSR Q=0.

Formula: FSR=[2l(LC)1/2]-1

Stray capacitance refers to capacitance that exists between turns of a coil, between coil and magnetic core, between coil and ground, and between coil and metal. The smaller parasitic capacitance of inductor, better its stability. The presence of parasitic capacitance reduces quality factor of coil and worsens stability, so smaller parasitic capacitance of coil, better.

**DC resistance Rdc**

DC resistance - value of resistance of measuring element in DC state, unit of measure is Ohm. Characterize quality state of internal coil of component according to Ohm's law. The design of inductors requires that DC resistance be as low as possible. Usually nominal maximum.

**Rated current Ir**

Rated current refers to maximum current that inductor can withstand under acceptable operating conditions. The passage of current will cause element to heat up, and inductance of element will decrease as temperature rises. The rated current is value of current at which inductance of element drops by 30% or temperature of device heats up by 40°C. If operating current exceeds rated current, operating parameters of inductor will change due to heating and even burn out due to overcurrent. The rated current is maximum allowable operating current. For products of same series, rated current decreases as inductance increases. For inductors with a non-magnetic core, rated current depends on DC resistance: lower DC resistance, lower temperature rise and greater allowable current.

**The larger value of inductance, better?**

Before answering this question, let's look at formula:

The above formula is formula for calculating inductance, L is value of inductance, μ is magnetic permeability, N is number of turns of coil, A is cross-sectional area of the magnetic circuit, and ι is length of coil. The value of inductance value is associated with design parameters of inductor, which depend on cross-sectional area A of magnetic circuit in coil, length ι of coil, magnetic permeability μ of material of magnetic circuit and number of turns N of coil. Among them, N is a quadratic term, indicating that number of turns is main factor affecting inductance. If, in order to wind more turns on a magnetic circuit of same size and from same material, it is necessary to use a thinner wire, and rated current of inductor will decrease accordingly, that is, rated current of inductor is sacrificed with an increase in value of inductance. (Assuming same core).

So larger inductance value, better.

**How to choose right inductor?**

Basically determine appropriate inductor required based on package size of inductor, and minimum inductance value and rated operating current required by circuit design. In addition, it is necessary to comprehensively consider working environment of inductor and refer to parameters such as operating frequency and operating voltage.

**What are consequences of choosing wrong inductor?**

If you choose an unsuitable inductor, main functions of storing and filtering energy of inductor will not be performed, or it will cause a short circuit in circuit, leakage, more serious heating of inductor, and even cause PCB to self-ignite, which will affect use of circuit.

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