Inductors – also known as chokes, reactors, and dynamic reactors. Together with capacitors and resistors, they are known as the three major passive components, and relay containers and resistors have rapidly developed into chip based components.
Basic Principles
Self induction phenomenon: electromagnetic induction phenomenon produced when the current flowing through the conductor itself changes. The coil is made of metal wires. When the current flowing through the coil changes, it will produce a very obvious electromagnetic induction phenomenon. The coil’s self induced reverse electromotive force blocks the change of current and plays a role in stabilizing the current. Specifically, if the inductor is in a state where there is no current flowing, it will attempt to block the flow of current when the circuit is connected; If the inductor is in a state with current flowing through it, it will attempt to maintain the current constant when the circuit is disconnected.
From an energy perspective, an inductor can convert electrical energy into magnetic energy and release magnetic energy into electrical energy. The same inductor has different blocking effects on currents with different changing frequencies, and its overall pattern is: low frequency on, high frequency on.
Main performance parameters of inductors
Inductance, also known as self inductance coefficient, is a physical quantity that represents the ability of an inductor to generate self inductance when the current flowing through it changes. The magnitude of inductance reflects the strength of the energy stored and released by the component. Inductance is an inherent characteristic of an inductor, which depends on the number of coil turns, winding method, magnetic core material, etc.
Formula: Ls=(k* μ* N ²* S)/L
Among them: μ Is the relative permeability of the magnetic core
N is the square of the number of coils
The cross-sectional area of the S-coil, in square meters
The length of the L coil, in meters
K empirical coefficient
From the formula, it can be seen that:
The more coils there are and the more densely wound the coils, the greater the inductance. A coil with a magnetic core has a greater inductance than a coil without a magnetic core; The higher the permeability of the magnetic core, the greater the inductance of the coil. The basic unit of inductance is Henry, denoted by the letter “H”. Commonly used units: milliHeng (mH), microHeng（ μ H) Naheng (nH). The conversion relationship is: 1H=10 ^ mH=10 ^ 6 μ H=10^9nH
Allowable error of inductance
The allowable deviation refers to the allowable error value between the nominal inductance on the inductor and the actual inductance. Inductors used in circuits such as oscillation or filtering require high accuracy, with an allowable deviation of ± 0.2% to ± 0.5%; The accuracy requirements for coils used for coupling, high-frequency resistance current, etc. are not high, and the allowable deviation is ± 10%~± 20%.
Inductive reactant XL
The magnitude of the inductance coil’s resistance to AC current is called inductance XL, measured in ohms. Its relationship with inductance L and AC frequency f is XL=2 π fL
Quality factor Q
The quality factor Q is a major parameter that characterizes the quality of an inductor.
Q is the ratio of inductance XL to its equivalent resistance when the inductor operates at a certain frequency of AC voltage:
Formula: Q=XL/R
Since XL is related to frequency, the Q value is related to frequency. The common Q-F curve is bell shaped. The Q value of the inductor is related to the DC resistance of the coil wire, the dielectric loss of the magnetic core, the loss caused by the shielding cover or the iron core, the influence of high-frequency skin effect and other factors. The Q value reflects the proportional relationship between the useful work done by the component during operation and the energy consumed by itself. The higher the Q value of the inductor, the smaller the loss of the circuit and the higher the efficiency. The Q value of an inductor usually ranges from tens to hundreds. The coupling and tuning circuits in the receiving and transmitting modules require high Q values, while the filtering circuit requires low Q values
Self resonant frequency SRF
The frequency point at which the parasitic capacitance and inductance of an inductor resonate is denoted as FSR. Under FSR, the inductance reactance and parasitic capacitance reactance are equal and cancel each other out, resulting in a reactance of 0. At FSR, the inductance loses its energy storage capacity and exhibits a high resistance pure resistance characteristic. At FSR, Q=0.
Formula: FSR=

[2 л (LC)1/2]-1
Parasitic capacitance refers to the capacitance that exists between turns of a coil, between coils and magnetic cores, between coils and ground, and between coils and metal. The smaller the parasitic capacitance of an inductor, the better its stability. The presence of parasitic capacitance reduces the Q value of the coil and deteriorates its stability. Therefore, the smaller the parasitic capacitance of the coil, the better.
DC resistance Rdc
DC resistance – The resistance value of a measuring element in DC state, measured in ohms. Characterize the quality status of the internal coil of the component, in accordance with Ohm’s law. In inductance design, it is required to keep the DC resistance as small as possible. Usually nominal as the maximum value.
Rated current Ir
Rated current refers to the maximum current that an inductor can withstand under the allowable working environment. The passage of current will cause the component to heat up, and the inductance of the component will decrease due to temperature rise. The rated current is taken as the current value when the inductance of the component decreases by 30% or the temperature rise of the component is 40 ℃. If the working current exceeds the rated current, the inductor will change its performance parameters due to heating, and even burn out due to overcurrent. The rated current is the maximum allowable working current, and for products of the same series, the inductance increases and the rated current decreases. For non magnetic core inductors, the rated current depends on the DC resistance. The smaller the DC resistance, the smaller the temperature rise, and the greater the allowable current.
Is the greater the inductance value, the better?
Before answering this question, let’s take a look at a formula: