The basic principles and characteristics of inductance.
2026-05-16
1.The basic principle of inductance
In the electronic and electrical equipment system, resistance, capacitance and inductance are the three basic passive components. The function of inductance is to store electrical energy in the form of magnetic energy, and it becomes an indispensable core component in products such as power equipment, industrial control systems, new energy equipment, and communication electronics. From small to micro intelligent electronic devices, and from large to industrial power supplies and power control systems, inductance coils all play key roles in voltage stabilization, filtering, energy storage, and anti-interference, and are the "invisible cornerstone" that ensures the stable operation of circuits.
When a constant current flows through a coil, according to the right-hand screw rule, a directional static magnetic field will be formed. When an alternating current flows through the inductor, the generated magnetic field is an alternating magnetic field. The changing magnetic field produces an electric field, and there is an induced electromotive force on the coil, resulting in an induced current. The total magnetic force lines generated by the induced current strive to prevent the change of the original magnetic force lines. Since the change of the original magnetic force lines originates from the change of the external alternating power supply.
Therefore, from an objective perspective, the inductor coil has the characteristic of preventing changes in current in an alternating current circuit:
- When the current increases, the magnetic field becomes stronger, and the direction of the magnetic field change is the same as the original magnetic field direction. According to the left-hand spiral rule, the induced current generated is opposite to the original current direction, and the inductor current decreases;
- When the current decreases, the magnetic field becomes weaker, and the direction of the magnetic field change is opposite to the original magnetic field direction. According to the left-hand spiral rule, the induced current generated is the same as the original current direction, and the inductor current increases.
The inductor will impede the change of the flowing current and presents a high impedance to alternating current. For the same inductor, the greater the rate of current change, the greater the induced current generated, and thus the higher the impedance presented by the inductor; if the same rate of current change is maintained, when the inductance value is larger, the induced current generated is also larger, and thus the impedance presented by the inductor is also higher. Therefore, the impedance of the inductor is related to two factors: one is the frequency; the other is the inherent property of the inductor, which is also the value of the inductor.
The size of the inductor is related to the size of the coil and the material of the core. The actual characteristics of the inductor not only include the inductive effect, but also other factors, such as:
- The wire used to wind the coil is not an ideal conductor, and there is a certain resistance;
- The magnetic core of the inductor has certain heat loss;
- There is a distributed capacitance between the conductors inside the inductor.
The equivalent model of the actual inductor consists of a series connection of an inductor and a resistor, followed by a parallel connection with a capacitor. Here, C represents the equivalent capacitance between the electrodes, R is the equivalent resistance of the wire loss, and L is the true ideal inductor. The impedance of the inductor is derived as follows:
A: Calculate the impedance Z1 of the RL series branch. The total impedance of the series circuit is the sum of the impedances of each component directly:
A: Calculate the impedance Z1 of the RL series branch. The total impedance of the series circuit is the sum of the impedances of each component directly:
B: Calculate the total impedance Z of the RL series connection and the C parallel connection. The total impedance of the parallel connection follows the "reciprocal addition" rule: For two parallel branches, the calculation is as follows:
The actual frequency impedance curve of the inductor is in the shape of an inverted V. The highest point represents the self-resonance frequency of the inductor. At this resonant point, the inductive reactance and capacitive reactance cancel each other out, and the inductor becomes purely resistive at this time. When the operating frequency is lower than the self-resonance frequency, the inductor is inductive; when the operating frequency is higher than the self-resonance frequency, the inductor is capacitive. Therefore, in practical applications, it is necessary to ensure that the operating frequency of the inductor is much lower than its self-resonance frequency.
2. Characteristics of Inductance
The core characteristics of an inductor are that it allows direct current (DC) to pass through while blocking alternating current (AC), and it allows low-frequency signals to pass through while blocking high-frequency signals. It has the functions of hindering changes in current and storing and buffering energy.
【Allow DC, Block AC】
In a direct current (DC) circuit, the inductor exhibits a short-circuit characteristic (impedance nearly equal to 0), while in an alternating current (AC) circuit, it exhibits an obstructive effect. According to the formula for inductive reactance, the higher the frequency, the greater the inductive reactance of the inductor, and conversely, the smaller the inductive reactance. Therefore, the inductor has the function of allowing DC to pass through and blocking AC.
【Allow low-frequency, Block high-frequency】
For different frequency AC signals, the inductor shows a selective conduction characteristic: low-frequency signals are more likely to pass through (inductive reactance is small), while high-frequency signals are significantly suppressed (inductive reactance is large). This characteristic makes the inductor widely used in filter circuits.
【Hindering current changes】
According to Faraday's law of induction, when a current flows through a coil, a magnetic field is generated around the coil. When the current changes, the coil induces an electromotive force, generating an induced current in the opposite direction to the current, thereby hindering this current change. Therefore, the inductor can hinder the change of current and maintain the stability of the device's operating current.
【Energy storage and buffering】
The inductor stores energy through a magnetic field and realizes energy buffering in switching circuits. For example, in a DC/DC power supply, the inductor periodically stores and releases energy to stabilize the output voltage.
3. Classification and Applications of Inductors
- Power inductor: Usually used in DC/DC circuits, it stores and releases energy to maintain continuous current. Most power inductors are wound inductors. The selection of power inductors depends on the chosen DC/DC chip. Generally, the specification sheet of the DC/DC chip provides recommended inductance values and calculations of related parameters.
- Radio Frequency Inductor: Mainly used in the radio frequency circuits of mobile phones, wireless routers, etc., it is applied from 100 MHz to 6 GHz.. In radio frequency circuits, they mainly have the following functions:
Matching: Together with capacitors, they form a matching network to eliminate impedance mismatch between components and transmission lines, reducing reflection and loss;
Filtering: Together with capacitors, they form LC filters to filter out unwanted frequency components and prevent interference to the operation of the device;
Isolating AC: In active radio frequency circuits such as PA, they isolate the radio frequency signal from DC bias and DC power supply;
Resonance: Together with capacitors, they form LC oscillation circuits to serve as the oscillation source of VCO;
Barrel: Also known as balanced-unbalanced conversion, together with capacitors, they form LC barrels to achieve the conversion between single-ended radio frequency signals and differential signals.
Filtering: Together with capacitors, they form LC filters to filter out unwanted frequency components and prevent interference to the operation of the device;
Isolating AC: In active radio frequency circuits such as PA, they isolate the radio frequency signal from DC bias and DC power supply;
Resonance: Together with capacitors, they form LC oscillation circuits to serve as the oscillation source of VCO;
Barrel: Also known as balanced-unbalanced conversion, together with capacitors, they form LC barrels to achieve the conversion between single-ended radio frequency signals and differential signals.
- Decoupling inductor: The function of a decoupling inductor is to filter out interference on the circuit line. It is an EMC component. EMC engineers mainly use it to solve the testing problems of product radiation emission (RE) and conducted emission (CE). Decoupling inductors usually have a relatively simple structure and can be divided into differential-mode inductors and common-mode inductors. Differential-mode inductors are ordinary wound inductors used to filter out some differential-mode interference. They mainly form an LC filter together with capacitors to reduce power supply noise.
The common-mode inductor is a device for suppressing common-mode interference, with its magnetic core made of ferrite. It is a four-terminal device formed by symmetrically winding two coils of the same size and the same number of turns on the same ferrite ring-shaped magnetic core. The common-mode inductor impedes alternating current.
4. Selection of Inductors
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Clarify the application scenario: Determine the required application scenario of the inductor, such as: power conversion, EMI filtering, RF, energy storage, etc.
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Calculate the inductor value and accuracy: Based on the circuit requirements, calculate the inductor value and select the corresponding accuracy inductor. For example, for a BUCK switching inductor, estimate the inductor value based on switching frequency, power ripple, etc.
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Determine the current requirement: Based on the circuit current requirement, select an inductor with sufficient margin for saturation current and temperature rise. When selecting the inductor, the RMS current should not exceed the temperature rise current of the inductor. To ensure the stability of the inductor value within the design range, the peak current should not exceed the saturation current of the inductor. To improve reliability, derating design is necessary. Generally, it is recommended that the operating value should be derated to no more than 80% of the rated value.
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Select inductor type: Based on the above parameters, initially select one or more suitable inductors.
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Evaluate DC resistance (DCR): Based on the initially selected inductor, consult its DCR parameters, calculate the loss and temperature rise, and check if it meets the requirements.
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Check self-resonant frequency (SRF): Check whether the circuit operating frequency and the self-resonant frequency of the inductor meet the requirements according to the inductor application scenario.
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Verify the application of high-frequency quality factor Q: Verify whether the inductor Q value meets the requirements for high-frequency applications.
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Cost comparison: Finally, based on the comparison, select the model with better cost performance and stable supply.
5. What is inductance saturation?
When current flows through a coil, a magnetic field is generated. The magnetic core is magnetized under the influence of the magnetic field. The magnetic domains within the core gradually rotate. When the magnetic core is fully magnetized, the direction of the magnetic domains is all aligned with the magnetic field. Even if an external magnetic field is further applied, there are no magnetic domains in the core that can rotate anymore. At this point, the inductance enters the saturation state.
Methods for determining inductor saturation:
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Determine if the inductor is saturated by analyzing the inductor current waveform. This method is the most common and practical in engineering practice. From the waveform, it can be seen that when the inductor is not saturated, the inductor current is a triangular wave with a constant slope. When the inductor is saturated, the inductor current waveform will have a significant distortion, which is caused by the reduction in inductance after saturation.
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Measure if the inductor has abnormal temperature rise and listen for any abnormal buzzing.
In engineering practice, there are many situations where we may not be able to accurately know the core model, it is difficult to know the size of the inductor's saturated current, and sometimes it is not convenient to test the inductor current. At such times, we can also use methods such as measuring if the inductor has abnormal temperature rise or listening for any abnormal buzzing to preliminarily determine if saturation has occurred.
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