Component selection is a fundamental task for hardware engineers. This article mainly introduces how to select inductors from the perspective of inductor technology and applications.
01 Basic Principles of Inductors
Inductors, along with capacitors and resistors, are the three basic passive components in electronics; the function of an inductor is to store electrical energy in the form of magnetic field energy.
Taking a cylindrical coil as an example, a brief introduction to the basic principles of inductors.
As shown in the figure above, when a constant current flows through the coil, according to the right-hand screw rule, a static magnetic field in the direction indicated in the diagram will be formed. When alternating current flows through the inductor, the magnetic field generated is an alternating magnetic field, and the changing magnetic field generates an electric field, resulting in induced electromotive force on the coil, producing induced current:
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When the current increases, the magnetic field strengthens, and the direction of the change in the magnetic field is the same as that of the original magnetic field. According to the left-hand screw rule, the induced current generated is opposite to the direction of the original current, causing the inductor current to decrease.
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When the current decreases, the magnetic field weakens, and the direction of the change in the magnetic field is opposite to that of the original magnetic field. According to the left-hand screw rule, the induced current generated is in the same direction as the original current, causing the inductor current to increase.
This is Lenz's law, and the final effect is that the inductor will hinder changes in the current flowing through it, meaning that the inductor presents high impedance to alternating current. For the same inductor, the higher the rate of change of current, the greater the induced current generated, and thus the higher the impedance presented by the inductor; if the rate of change of current is the same, for different inductors, the greater the induced current generated, the higher the impedance presented by the inductor.
Therefore, the impedance of an inductor is related to two factors: one is frequency; the other is the inherent property of the inductor, which is the inductance value, also known as inductance. According to theoretical derivation, the inductance formula for a cylindrical coil is as follows:
It can be seen that the size of the inductance is related to the size of the coil and the material of the core.
The actual characteristics of inductors not only include the function of inductance but also other factors, such as:
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The wire used to wind the coil is not an ideal conductor and has a certain resistance;
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The magnetic core of the inductor has certain thermal losses;
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There is distributed capacitance between the conductors inside the inductor.
Therefore, a more complex model is needed to represent the actual inductor, and the commonly used equivalent model is as follows:
The form of the equivalent model may vary, but it should reflect losses and distributed capacitance. Based on the equivalent model, two important parameters of the actual inductor can be defined:
❶ Self-Resonance Frequency
Due to the presence of Cp, it forms a resonant circuit with L, and its resonant frequency is the self-resonance frequency of the inductor. Before the self-resonance frequency, the impedance of the inductor increases with frequency; after the self-resonance frequency, the impedance of the inductor decreases with frequency, presenting capacitive characteristics.
❷ Quality Factor
This is the Q value of the inductor, the ratio of the power stored by the inductor to the power lost. The higher the Q value, the lower the losses of the inductor, which is a parameter directly related to the DC resistance of the inductor. The self-resonance frequency and Q value are key parameters for high-frequency inductors.
02 Inductor Process Structure
The inductor process can be roughly divided into three types:
2.1 Wire Wound Type
As the name suggests, it involves winding copper wire around a magnetic core to form a coil. There are two winding methods:
Round winding is very common and widely used, for example:
Flat winding is also very common; everyone must have seen a mosquito coil that breaks easily.
The advantage of flat winding is obvious, which is to reduce the height of the component.
From the formula mentioned earlier, it can be seen that the greater the permeability of the magnetic core, the larger the inductance value. The magnetic core can be:
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Non-magnetic materials: such as air core, ceramic core, which seemingly cannot be called magnetic cores; thus, the inductance value is small, but there is basically no saturation current.
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Ferromagnetic materials: such as ferrite, permalloy, etc.; the permeability of alloys is greater than that of ferrite; ferromagnetic materials exhibit magnetic saturation phenomena and have saturation currents.
Wire wound inductors can provide large currents and high inductance values; the greater the permeability of the magnetic core, for the same inductance value, the less winding is needed, which can reduce DC resistance; for the same size, less winding can allow for thicker wire, increasing current.
In addition, in power supply design, the problem of inductor whistling is often encountered, which is essentially caused by changes in the magnetic field leading to vibrations of the conductor, i.e., the coil, with the vibration frequency falling within the audible range, making it audible. Integrated alloy inductors are relatively sturdy and can reduce vibrations.
2.2 Multilayer Type
The manufacturing process of multilayer inductors: drying and forming ferrite or ceramic slurry, alternately printing conductive slurry, and finally stacking and sintering into a monolithic structure.
Multilayer inductors are smaller in size compared to wire wound inductors, standardized packaging, suitable for automated high-density mounting; the integrated structure has high reliability and good heat resistance.
2.3 Thin Film Type
Thin film inductors use a process similar to IC manufacturing, depositing a layer of conductive film on a substrate, then using photolithography to form coils, and finally adding dielectric layers, insulation layers, and electrode layers, and packaging.
The manufacturing process of thin film devices is shown in the figure below.
The precision of the photolithography process is very high, resulting in narrower lines and clearer edges. Therefore, thin film inductors have
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Smaller size, 008004 package
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Smaller Value Step, 0.1nH
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Smaller tolerance, 0.05nH
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Better frequency stability
03 Applications and Selection of Inductors
Inductors, from a technological perspective, are primarily led by three major Japanese manufacturers: TDK, Murata, and Taiyo Yuden. These three companies have complete product lines that can basically meet most needs.
All three have corresponding selection software, with products and related parameter curves for inductors, capacitors, and all series.
Personally, I feel that TDK and Murata are a bit more advanced, as seen from the quality of their official websites. Coilcraft's website seems a bit low-end, after all, websites also require investment.
In circuit design, inductors mainly have three major types of applications:
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Power inductors: mainly used for voltage conversion, commonly used in DCDC circuits that require power inductors;
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Decoupling inductors: mainly used to filter out noise on power lines or signal lines, which EMC engineers should be familiar with;
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High-frequency inductors: mainly used in RF circuits for biasing, matching, filtering, and other circuits.
3.1 Power Inductors
Power inductors are usually used in DCDC circuits, maintaining continuous current by accumulating and releasing energy.
Most power inductors are wire-wound inductors, which can handle large currents and high inductance;
Multilayer chip power inductors are also becoming more common, usually with lower inductance values and currents. Their advantages are low cost and ultra-small size, making them widely used in products with significant space constraints, such as Mobile phones.
Power inductors need to be selected based on the chosen DCDC chip. Typically, the specifications of DCDC chips include recommended inductance values and calculations for related parameters, which will not be elaborated here. This will explain how to select inductors from the perspective of the inductors themselves.
Generally, the inductance value recommended in the DCDC chip specifications should be used; the larger the inductance value, the smaller the ripple, but the size will increase; usually, increasing the switching frequency allows for smaller inductors, but increasing the switching frequency will increase system losses and reduce efficiency;
Power inductors generally have two rated currents, namely temperature rise current and saturation current;
When current passes through the inductor, due to losses, the inductor heats up and generates a temperature rise; the larger the current, the greater the temperature rise; within the rated temperature range, the maximum allowable current is the temperature rise current.
Increasing the permeability of the magnetic core can enhance the inductance value, typically using ferromagnetic materials for the core. Ferromagnetic materials exhibit magnetic saturation, meaning that when the magnetic field strength exceeds a certain value, the magnetic induction strength no longer increases, resulting in a decrease in permeability, which also means a decrease in inductance. Within the rated inductance value range, the maximum allowable current is the saturation current.
Hysteresis Loop:Magnetic materials-------ferrite, specific gravity meter, porous material density meter, liquid density meter, solid particle volume tester, magnetic material density meter.
For DCDC circuit design, it is necessary to calculate the peak (PEAK) current and root mean square (RMS) current, and the specification usually provides the calculation formulas.
The temperature rise current is an assessment of the thermal effect of the inductor. According to Joule's law, the thermal effect needs to consider the integral of current over time for a period; when selecting inductors, the designed RMS current should not exceed the inductor's temperature rise current.
To ensure that the inductance value remains stable within the design range, the designed peak current should not exceed the inductor's saturation current.
To improve reliability, derating design is necessary, and it is generally recommended that the working value should be derated to no more than 80% of the rated value. Of course, excessive derating will significantly increase costs and needs to be considered comprehensively.
The DC resistance of the inductor will generate thermal losses, leading to temperature rise and reducing DCDC efficiency; therefore, when efficiency is sensitive, inductors with low DC resistance should be selected, such as 15 milliohms.
Additionally, consider the product's application temperature requirements, whether it needs to meet RoHS, automotive grade Q200, and other standards, as well as PCB structural limitations.
In applications with large currents, the leakage magnetic field of the inductor can be considerable, affecting surrounding circuits, such as CPUs. I have previously encountered a situation where the leakage magnetic field of an X86 CORE inductor caused the CPU to fail to start. Therefore, for large current applications, it is advisable to choose inductors with good shielding performance and to avoid key signals during layout.
Decoupling inductors, also known as Chokes, are usually translated as choke coils in textbooks. The role of decoupling inductors is to filter out interference on the line, belonging to EMC devices, which EMC engineers mainly use to solve issues related to product radiation emission (RE) and conducted emission (CE) testing.
Decoupling inductors usually have a relatively simple structure, mostly consisting of copper wire directly wound on ferrite rings. Personally, I think they can be divided into differential mode inductors and common mode inductors. The concepts of common mode and differential mode will not be elaborated here.
Differential Mode Inductors
Differential mode inductors are ordinary wire-wound inductors used to filter out some differential mode interference, mainly forming an LC filter with capacitors to reduce power supply noise.
For 220V mains electricity, differential mode interference refers to the interference between the L phase and N phase; for POE, it refers to the interference between POE+ and POE-; for low-voltage DC power supplies on motherboards, it is essentially power supply noise.
When selecting differential mode inductors, attention should be paid to the following points:
Ferrite Bead is also commonly used to filter out noise from low-voltage DC power supplies on the motherboard, but there is a difference between ferrite beads and decoupling inductors.
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Ferrite beads are made of ferrite material, and at high frequencies, the magnetic loss (equivalent resistance) of ferrite becomes very large, converting high-frequency noise into thermal energy for dissipation.
❶ Equivalent circuit model of ferrite bead
A common mode inductor is formed by winding two coils with the same number of turns in opposite directions on the same ferrite core.
The common mode inductor shown in the above figure:
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When there is a common mode component flowing through the common mode inductor, according to the right-hand rule, a magnetic field with the same direction is formed in the two coils, reinforcing each other, which corresponds to a high inductive reactance to the common mode signal.
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When there is a differential mode component flowing through the common mode inductor, according to the right-hand rule, a magnetic field with opposite directions is formed in the two coils, canceling each other out, which corresponds to a low inductive reactance to the differential mode signal.
To understand it in another way: when a common mode interference of a certain frequency flows through V+, the alternating magnetic field formed will induce a current in the other coil. According to the left-hand rule, the direction of the induced current is opposite to the direction of the common mode interference on V-, which cancels out part of it, reducing the common mode interference.
Common mode inductors are mainly used in two-wire or differential systems, such as 220V mains, CAN bus, USB signals, HDMI signals, etc. They are used to filter out common mode interference while having minimal attenuation of useful differential signals.
When selecting common mode inductors, the following points need to be noted:
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The DC resistance should be low and should not have a significant impact on voltage or useful signals.
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For power lines, consider the rated voltage and current to meet operational requirements.
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Determine the frequency band of common mode interference through testing, and the common mode impedance should be relatively high within that frequency band.
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Differential mode impedance should be low and should not significantly affect the quality of the differential signal.
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Consider the package size and design for compatibility. For example, for common mode inductors used in USB signals, choose a package that can be compatible with two 0402 resistors. When common mode inductors are not needed, 0402 resistors can be directly soldered to reduce costs.
The following figure shows the common mode impedance and differential mode impedance of a certain common mode inductor.
If the frequency of common mode interference is around 10MHz, the filtering effect is very good, but if it is 100kHz, it may not be effective. If the differential signal rate is high, above 100M, it may affect signal quality.
3.3 High-frequency inductors
High-frequency inductors are mainly used in RF circuits of products such as mobile phones and wireless routers, with applications ranging from 100MHz to 6GHz.
High-frequency inductors mainly serve the following functions in RF circuits:
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Matching: Together with capacitors, they form matching networks to eliminate impedance mismatches between devices and transmission lines, reducing reflections and losses.
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Filtering: Together with capacitors, they form LC filters to filter out unwanted frequency components and prevent interference from affecting device operation.
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AC isolation: In active RF circuits such as PA, they isolate RF signals from DC bias and DC power.
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Resonance: Together with capacitors, they form LC oscillation circuits, serving as oscillation sources for VCO.
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Balun: This is a balanced-unbalanced converter, which, together with capacitors, forms an LC balun to achieve conversion between single-ended RF signals and differential signals.
The three structures introduced earlier can all be used to make high-frequency inductors, and their characteristics are as follows:
The multilayer type is formed through sintering, creating an integral structure, also known as monolithic.
Multilayer chip inductors have the lowest Q value compared to the other two types, but their biggest advantage is low cost and high cost-performance ratio, making them suitable for most applications without special requirements. TDK and Taiyo Yuden's high-frequency inductors are all multilayer types, with no wire-wound or thin-film types.
TDK's MLK series, Murata's LQG series, and Taiyo Yuden's HK series have basically the same products, being the cheapest and offering high cost-performance.
Of course, with the improvement of process technology, there are now high Q value series of multilayer chip inductors, such as TDK's MHQ series and Taiyo Yuden's HKQ series.
TDK's multilayer inductors are better and more comprehensive, and there is also an MLG series with 0402 packaging, inductance values can be made as low as 0.3nH, Value Step 0.1nH, tolerance 0.1nH, approaching the performance of thin-film inductors, and the price is still low.
Current process levels have improved, and wire-wound inductors can also be made in 0402 packaging.
The wire-wound type process allows the wire to be thicker than multilayer and thin-film structures, thus achieving extremely low DC resistance. This also means extremely high Q values, while supporting larger currents. Replacing the non-magnetic ceramic core with a ferrite core can achieve higher inductance values, suitable for medium frequencies.
Murata's LQW series can achieve 03015 packaging, with a minimum inductance value of 1.1nH; Coilcraft's 0201DS series can achieve 0201 packaging, claiming to be the smallest wire-wound inductor in the world.
Using photolithography technology, the process accuracy is extremely high, so the inductance value can be very small, the size can also be very small, with high precision and stable inductance value, and a higher Q value.
Murata's LQP series can achieve 01005 packaging, with a tolerance of 0.05nH for high-precision products, and a minimum inductance value of 0.1nH. These three parameter values can be said to be the current limits of inductance. Additionally, Abracon's ATFC-0201HQ series can also achieve a minimum of 0.1nH.
Murata has three types of high-frequency inductors, comparing inductors with the same inductance value (1.5nH), the same packaging, and the same tolerance.
It can be seen that the Q value of the wire-wound type is significantly higher than the other two types, while the inductance value of the thin film type has better frequency stability than the other two types. Of course, the cost of the multilayer type is significantly lower than the other two types.
When selecting high-frequency inductors, in addition to determining the inductance value, rated current, operating temperature, and packaging size, attention should also be paid to self-resonant frequency, Q value, inductance value tolerance, and inductance value frequency stability.
The inductance value usually needs to be determined based on simulation, actual debugging, or reference design. In most cases, multilayer chip high-frequency inductors can meet the requirements, but some special occasions may require attention:
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For inductance values that are larger and self-resonant frequencies that are lower, it is important to ensure that the operating frequency is far below the self-resonant frequency.
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For high-power RF equipment, with a larger PA bias current, it is necessary to choose wire-wound types to meet the current requirements; at the same time, high-power devices have higher temperature rises, so operating temperature must be considered.
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For some broadband devices, if a stable inductance value is required within the bandwidth, then thin film inductors should be chosen.
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In high-precision VCO circuits, as an LC resonant source, only thin film inductors can improve the tolerance to 0.05nH.
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For devices like Mobile phones and wearable devices, size may be the most critical factor, and thin film inductors may be a better choice.
Some high-frequency inductors have directionality, and the direction of the surface mount installation has a certain impact on the inductance value, as shown in the figure below:
It can be seen that when the marking point faces the side, the inductance value changes significantly, so when mounting the chip, care should be taken to ensure that the marking point on the inductor faces upwards.
Additionally, during layout, care should be taken to ensure that two inductors are not placed too close together, with at least a distance of more than 20 mils. The reason is that the magnetic fields will affect each other, thereby affecting the inductance value, as referenced in the previous common mode inductor schematic.
Conclusion:When selecting components, it is important to understand the principles and applications of the devices, considering various factors such as cost, derating, and compatibility.
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