Introduction to inductor characteristic parameters and categories

1. What is an inductor?

Inductor is one of the three major passive components, and is listed as the most basic and most used electronic component in electronic circuits along with resistors and capacitors. Inductor is an electromagnetic induction component, usually formed by winding a certain number of turns of an insulated wire on a bracket or iron core to form a coil, which is called an inductor coil or inductor. The main uses of inductors include filtering and removing noise, suppressing instantaneous current, reducing electromagnetic interference (EMI) and power conversion.

Passive components refer to electronic components that cannot simulate power devices with equivalent circuits, so they play an important supporting role in electronic circuits.

2. Definition of inductance

Inductance is the ratio of the magnetic flux of the conductor to the current that produces the alternating magnetic flux inside and around the conductor when an alternating current passes through the conductor.

When a direct current passes through the inductor, only fixed magnetic lines of force appear around it, which do not change with time; however, when an alternating current passes through the coil, magnetic lines of force that change with time will appear around it. According to Faraday's law of electromagnetic induction - magnetoelectricity, the changing magnetic lines of force will generate an induced potential at both ends of the coil, and this induced potential is equivalent to a "new power source".

When a closed loop is formed, this induced potential will generate an induced current. Lenz's law shows that the total amount of magnetic lines of force generated by the induced current will try to prevent the change of the original magnetic lines of force. Since the change of magnetic lines of force comes from the change of external alternating current, objectively speaking, the inductor has the characteristic of preventing the change of current in the AC circuit. The inductor has a characteristic similar to inertia in mechanics, which is called "self-induction" in electricity. Usually, sparks will occur at the moment of opening or closing the gate switch. This is caused by the self-induction phenomenon that produces a high induced potential.

In short, when the inductor is connected to an AC power source, the magnetic lines of force inside the coil will change all the time with the alternating current, causing the coil to continuously generate electromagnetic induction. This electromotive force generated by the change of the current in the coil itself is called "self-induced electromotive force".

It can be seen that inductance is only a parameter related to the number of turns, size, shape and medium of the coil. It is a measure of the inertia of the inductor coil and has nothing to do with the external current.

3. Main characteristic parameters of inductance

3.1 Inductance (L)

Inductance L represents the inherent characteristics of the coil itself and has nothing to do with the current. Except for special inductor coils (color-coded inductors), inductance is generally not specifically marked on the coil, but marked with a specific name.

3.2 Inductive Reactance (XL)

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

3.3 Quality Factor (Q)

The quality factor Q is a physical quantity that represents the quality of the coil. Q is the ratio of the inductive reactance XL to its equivalent resistance, that is: Q = XL/R.

The higher the Q value of the coil, the smaller the loss of the circuit. The Q value of the coil is related to factors such as the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shield or iron core, and the influence of the high-frequency skin effect. The Q value of the coil is usually tens to hundreds. The use of magnetic core coils and multi-strand twisted coils can increase the Q value of the coil.

3.4 DC Resistance (DCR)

The resistance of an inductor coil measured under non-AC conditions. In inductor design, the smaller the DC resistance, the better. Its measurement unit is ohm, and it is usually marked with its maximum value.

3.5 Distributed Capacitance

The capacitance between turns of the coil, between the coil and the shield, and between the coil and the base is called distributed capacitance. The existence of distributed capacitance reduces the Q value of the coil and deteriorates its stability, so the smaller the distributed capacitance of the coil, the better. The use of segmented winding can reduce the distributed capacitance.

3.6 Impedance value

The impedance value of an inductor refers to the sum of all its impedances under current (complex number), including the AC and DC parts. The impedance value of the DC part is only the DC resistance of the winding (real part), and the impedance value of the AC part includes the reactance of the inductor (imaginary part). In this sense, the inductor can also be regarded as an "AC resistor".

3.7 Rated current

The continuous DC current intensity allowed to pass through an inductor. The intensity of this DC current is based on the maximum temperature rise of the inductor at the maximum rated ambient temperature. The rated current is related to the ability of an inductor to reduce winding losses by low DC resistance, and also to the ability of the inductor to drive away energy losses in the winding. Therefore, the rated current can be increased by reducing the DC resistance or increasing the size of the inductor. For low-frequency current waveforms, the RMS current value can be used to replace the DC rated current. The rated current has nothing to do with the magnetic properties of the inductor.

4. Inductor types

Classification by inductor form: fixed inductor, variable inductor.

Classification by magnetic properties: air core coil, ferrite coil, iron core coil, copper core coil.

Classification by working properties: antenna coil, oscillating coil, choke coil, trap coil, deflection coil.

Classification by winding structure: single-layer coil, multi-layer coil, honeycomb coil.

Classification by working frequency: high-frequency coil, low-frequency coil.

Classification by structural features: magnetic core coil, variable inductance coil, color-coded inductance coil, coreless coil, etc.