Inductor Selection: Basic Principles

Basic knowledge and importance of inductance

Inductance is a component that can convert electrical energy into magnetic energy and store it. Its structure is similar to that of a transformer, but it has only one winding. It has a certain inductance, and its characteristic is that it allows direct current and blocks alternating current. When current flows through a conductor, an electromagnetic field is generated. Inductance is a physical quantity that measures the ability of a coil to generate electromagnetic induction. When current is passed through a coil, a magnetic field is generated around the coil, and magnetic flux passes through it. The greater the current passed, the stronger the magnetic field and the greater the magnetic flux. The magnetic flux passing through the coil is proportional to the current passed. Their ratio is called the self-inductance coefficient, which is the inductance.

The role of inductance

Pass direct current and block alternating current: isolate and filter alternating current signals, or form a resonant circuit with capacitors, resistors, etc., and have a limited current effect on alternating current. It can form a high-pass or low-pass filter, a phase shift circuit, and a resonant circuit with resistors or capacitors; tuning and frequency selection: an inductor coil and a capacitor in parallel can form an LC tuning circuit. When the inherent oscillation frequency of the circuit is equal to the frequency of the non-AC signal, the inductive reactance and the capacitive reactance of the circuit are also equal, and the electromagnetic energy oscillates back and forth between the inductor and the capacitor, which is the resonance phenomenon of the LC circuit. When resonating, the inductive reactance of the total loop current is the smallest and the current is the largest, so the LC resonant circuit has the function of selecting frequency and can select an AC signal of a certain frequency

Signal screening, noise filtering, current stabilization and electromagnetic wave interference suppression: For example, the magnetic ring inductor and the connecting cable form an inductor, which is a commonly used anti-interference component in electronic circuits and has a good shielding effect on high-frequency noise. Normal and useful signals can pass smoothly and can well suppress high-frequency interference signals

Application of inductors in circuits

In communication circuits, inductors are used for signal filtering and frequency selection to ensure stable signal transmission. For example, in radio frequency circuits, biasing, matching, filtering and other functions are implemented to ensure the quality of wireless communication

In power circuits, inductors play the role of energy storage and filtering. They are commonly found in DC-DC conversion circuits. They accumulate and release energy to maintain continuous current, stabilize power output, and reduce voltage fluctuations and noise

In various electronic devices, such as mobile phones, computers, and televisions, inductors play an indispensable role. From power management on the motherboard to signal processing, they are inseparable from the participation of inductors, which affects the performance and stability of the equipment.

Preparation before selection

Clear circuit requirements

It is crucial to determine the operating frequency range of the circuit, because the performance of inductors varies at different frequencies. For example, the operating frequency of inductors used for high-frequency signals is usually higher, generally above 1GHz, and the resonant frequency can be as high as 12GHz; while the operating frequency of inductors used for general signals is relatively low, and the resonant frequency point is generally within a few hundred megahertz

Understand the circuit's requirements for signal integrity. If the circuit has high requirements for signal accuracy and stability, it is necessary to select an inductor that can ensure high-quality signal transmission to avoid signal distortion and interference

Consider environmental factors

The ambient temperature has a significant impact on the performance of the inductor. Temperature changes may cause changes in the parameters of the inductor. For example, at high temperatures, the resistivity of the material may increase, resulting in a decrease in the Q value and an increase in the loss of the inductor. Therefore, it is necessary to understand the ambient temperature range in which the inductor works and select an inductor with stable performance within this temperature range

Humidity may also affect the performance of the inductor, especially for some inductors that are not well protected. A humid environment may cause rust and corrosion of its internal components, thus affecting the normal operation of the inductor.

Understand cost constraints

On the premise of meeting the circuit performance requirements, cost is an important consideration. The prices of inductors of different types, specifications and brands vary greatly, and it is necessary to find a balance between performance and cost. For example, some high-end inductors have superior performance but are expensive. If the circuit does not have particularly stringent performance requirements, you can choose an inductor with a higher cost performance; at the same time, you must also consider the long-term use cost of the inductor, including its stability, reliability, and possible maintenance costs.

Core selection principles

Selection of inductance value

Determine the appropriate inductance value according to the specific function and design requirements of the circuit. For example, in the LC oscillation circuit, the inductance value and the capacitance value jointly determine the oscillation frequency; in the filter circuit, the inductance value affects the filtering effect and frequency characteristics

Pay attention to the error range of the inductance value. Generally, the error range of the inductance is ±10% - 20%. In the circuit with high requirements for the accuracy of the inductance value, it is necessary to select an inductor with a smaller error to avoid unstable circuit performance due to inductance value deviation

Quality factor (Q value)

The Q value is also called the quality factor. It is the ratio of the inductor's ability to store energy to its energy loss in the form of heat energy. It reflects the efficiency of the inductor in the AC circuit. The higher the Q value, the better the performance of the inductor is usually; the Q value is affected by factors such as material, frequency, temperature and manufacturing process. Materials with high magnetic permeability can reduce the loss of inductors, thereby increasing the Q value; the Q value usually decreases with increasing frequency; as the temperature rises, the material resistivity increases, and the Q value may decrease; the manufacturing process, including the winding of the coil and the assembly of the magnetic core, will also affect the Q value; in high-frequency circuits, inductors with high Q values help reduce signal distortion, improve signal integrity, reduce losses, and improve circuit efficiency and stability

Importance of DC resistance (DCR)

DC resistance is the DC internal resistance of the inductor coil winding, and its size affects the DC loss and temperature rise of the circuit. The larger the DCR, the greater the power loss on the inductor at the same current, which will cause the inductor to heat up and affect the stability and efficiency of the circuit. When selecting an inductor, on the premise of meeting other performance requirements, you should try to choose an inductor with a small DC resistance to reduce energy loss and heating problems. For example, in a high-current power supply circuit, an inductor with a low DCR can effectively reduce the voltage drop and improve the efficiency of the power supply.

Self-resonant frequency (SRF)

Due to the existence of parasitic capacitance of the inductor, LC oscillation will occur, and its resonant frequency is the self-resonant frequency of the inductor. Before the self-resonant frequency, the impedance of the inductor increases with the increase of frequency; after the self-resonant frequency, the impedance of the inductor decreases with the increase of frequency, and it becomes capacitive.

In actual applications, an inductor with a resonant frequency point higher than the operating frequency should be selected to ensure that the inductor is inductive within the operating frequency range and plays its due role. If the operating frequency exceeds the resonant frequency, the inductor will lose its inductance characteristics and cannot work properly.

Determination of rated current

The rated current includes the inductor saturation current Isat and the inductor temperature rise current Irms. Generally, the smaller value of Isat and Irms is taken as the rated current of the inductor; the inductor saturation current refers to the DC current allowed when the inductance value drops by 30%, and the inductor temperature rise current is the DC current allowed when the inductor temperature rises by 40℃ at 20℃

The operating current of the inductor must be less than the rated current, otherwise the inductance value will change, affecting the normal operation of the circuit. When designing the circuit, the inductor with a rated current large enough should be selected according to the maximum current in the circuit, and a certain margin should be left. It is generally recommended that the rated current be 1.3 times the maximum output current in the circuit, and the rated current should be used at a reduced rate to improve the reliability of the circuit.

Selection Misunderstandings and Precautions

Only focusing on one parameter of the inductor and ignoring the influence of other parameters. For example, only pursuing a high Q value without considering whether the inductance value, rated current and other parameters meet the circuit requirements may cause the circuit to not work properly; not considering the working environment of the inductor, such as temperature, humidity and other factors, selecting an inductor with unstable performance in the actual working environment, thereby affecting the reliability and stability of the circuit

Precautions

When selecting an inductor, it is necessary to comprehensively consider multiple parameters to ensure that each parameter can meet the requirements of the circuit and cooperate with each other to achieve the best circuit performance

Refer to the inductor datasheet to understand the detailed parameters, performance curves and application precautions of the inductor, which will help to correctly select and use the inductor

For some special application scenarios, such as high temperature, high pressure, high frequency and other environments, it is necessary to select an inductor specially designed for such environments to ensure its reliability and stability

Summary

The core principles of inductor selection include determining the appropriate inductance value according to circuit requirements, paying attention to the quality factor (Q value) to improve inductor efficiency and signal quality, selecting inductors with small DC resistance (DCR) to reduce energy loss and heat generation, ensuring that the self-resonant frequency (SRF) is higher than the operating frequency to ensure the inductor characteristics, and determining the appropriate rated current with a certain margin for derating.

Correct inductor selection is crucial to the performance, stability and reliability of the circuit. Appropriate inductors can ensure the normal operation of the circuit, improve signal quality, reduce energy loss, and reduce the probability of failure, thereby improving the performance and service life of the entire electronic device.

With the continuous development of electronic technology, the performance requirements for inductors are getting higher and higher. In the future, inductors may develop in the direction of smaller size, higher performance, and lower loss to meet the needs of increasingly miniaturized and high-performance electronic devices. At the same time, the application of new materials and manufacturing processes will also bring new opportunities and breakthroughs to the development of inductors.