According to the editor, power electronic components are commonly referred to as "passive" components. These are essential elements in power electronic equipment and should be well-known to those involved in the design, development, production, marketing, and application of power electronic components and power supply technicians. This journal began offering lectures on "Power Electronics Component Knowledge" and related "Power Electronics Device Knowledge" starting in April this year to cater to readers' needs for increasing their knowledge and effectively utilizing these components. We warmly welcome engineers from both manufacturers and users to contribute articles and look forward to receiving valuable feedback.
In our fifth installment on Power Electronic Components, we will discuss new inductors. Mao Xingwu from the Shandong Linyi Electronics Research Institute contributed this article, which was adapted by Zhang Naiguo of this journal.
1. Chip Inductor
1.1 Wire Wound Chip Inductor
There are two main types: power and high power.
(1) Low Power Wire Wound Chip Inductors
Low-power wire-wound chip inductors are typically constructed with enameled wire wound around a ferrite or ceramic core (skeleton) of smaller dimensions. Their shape and structure vary; some examples include:
(a) A skeleton with internal windings, externally shielded by a magnetic material, often formed with a pressed plastic film;
(b) A rectangular skeleton with windings, where the skeleton is made of ferrite or ceramic materials;
(c) An I-shaped skeleton, made of ceramic, ferrite, or aluminum, with larger dimensions.
Key parameters include size, inductance, tolerance, Q value, DC resistance, maximum allowable current, and self-resonant frequency (SRF). The operating frequency primarily depends on the core material. Generally, inductors with ferrite skeletons are limited to middle and low-frequency operations, whereas those with aluminum, ceramic, or hollow skeletons can be utilized in high-frequency bands, including UHF (Ultra High Frequency) or VHF (Very High Frequency) sections. Inductors suitable for HF and UHF sections typically have an inductance range of 0.1 to 1000 μH, while those for the VHF section usually range from 1.5 to 100 nH. Tolerances are usually at the +5% level, K level (±10%), or M level (±20%). The operational temperature range for chip inductors is generally -25°C to 85°C. A drawback of small power wire-wound chip inductors is their relatively large size and significant magnetic leakage. Currently, most product specifications have been replaced by laminated chip inductors.
(2) High-Power Wire-Wound Chip Inductors
Many circuits require surface-mounted power inductors with inductances in the millihenry range and current levels in the ampere range. High-power wire-wound chip inductors are primarily used as energy storage elements and LC filter components in DC/DC converters. These high-power inductors are usually made with square or round I-shaped ferrite as the skeleton and enameled wires of varying wire diameters.
Commonly used high-power wire-wound inductors have an inductance range of 1 to 330 μH, with inductance bases at 1, 2.2, 3.3, 4.7, 5.6, 6.8, and 8.2.
1.2 Multilayer Chip Inductors
Multilayer chip inductors (MLCI) are wire-free inductors made of ferrite material and manufactured using multi-layer production techniques. Their structure is very similar to that of multilayer chip capacitors (MLCC), as illustrated. Since MLCIs have a closed magnetic circuit, they offer shielding effects. Manufacturing methods for MLCIs include the dry method (primarily casting through) and the wet method (including overlapping printing, via forming, and printing hole forming).
MLCIs boast small size, high reliability, excellent magnetic shielding performance, and suitability for high-density automated placement in STM equipment. Their inductance ranges from 1 nH to several tens of μH.
1.3 Thin-Film Chip Inductor
Thin-film chip inductors are inductors fabricated using thin-film technology to etch conductor coils onto ceramic and other substrates. They maintain high Q values in the microwave section, ensuring high precision and stability, and have a smaller footprint. Thin-film chip inductors exhibit minimal parasitic capacitance, and their self-oscillation frequency is suitable for surface assembly. They can be mounted on PCBs using standard chip mounters and reflow soldering technology. Traditional inductive components, such as chokes and transformers, generally have non-standard lead forms and irregular shapes, requiring complex mounting equipment and manual assembly.
Wire frames are inserted into slits, bent from side to side, and then shaped into a standard "gull-wing" lead form. In this way, each lead wire in the lead frame becomes a half-turn winding of the inductor coil, with the other half-turn formed by a lead on the PCB.
These components offer advantages in source systems. By using high permeability and low-loss cores, the frequency characteristics can be expanded, making them suitable for signal processing (SI-IC) pulse transformers or broadband transformers. High-bandwidth, high-sensitivity cores are ideal for EMC applications, such as common-mode chokes or double-line chokes that suppress power and signal line interference (EM-C). Different mode chokes can also be made using high permeability cores.
Chip inductors are primarily used in circuits involving filtering, resonance, oscillation, coupling, delay compensation, and impedance transformation.
Among the three passive components—resistors, capacitors, and inductors—inductors are the most technically challenging and the latest to achieve chip form. The rapid development of electronic devices toward being lighter, thinner, shorter, and smaller has created a vast market for chip inductors. The application of chip inductors in computer, communication, and audio-visual products is extensive. For instance, each mobile phone requires approximately 20 medium-sized chip inductors. With global mobile phone production reaching 1.144 billion units in 2007 (including 549 million units produced in China), the demand for chip inductors exceeded 22 billion. Beyond mobile phones, personal digital assistants (PDAs), digital cameras, portable CD players, VCDs, DVDs, video recorders, camcorders, color TVs, digital set-top boxes, fax machines, notebook computers, and program-controlled switches all require a significant number of chip inductors.
When selecting a chip inductor, one must consider parameters such as inductance, tolerance, frequency range, Q value, SRF, maximum allowable operating current, DC resistance, and others.
The quality factor Q value is an important parameter. It refers to the ratio of the inductive reactance of the inductor when operating at a certain AC frequency to the DC resistance of the coil (Q = 2Ï€fL/R). The smaller the inductor loss, the higher the Q value and efficiency. The Q value is closely related to frequency; inductors with high Q values at high frequencies often have low Q values at low frequencies. Although products may have the same inductance, the DC resistance varies depending on the winding diameter selected. In high-frequency loops, the DC resistance of the inductor significantly impacts the Q value, so care must be taken during design.
2. Inductors Based on Amorphous and Nanocrystalline Alloy Cores
Amorphous and nanocrystalline alloy cores can be shaped into various forms. The core used as an inductor is primarily ring-shaped. The conductor is wound around the ring-shaped iron core multiple times to create an inductor, as illustrated. Shown here is an amorphous nanocrystalline alloy ring inductor core and an inductor made from it.
Amorphous and nanocrystalline alloy inductors can be used in various power supply EMI filters, switching power supply output filters, and power factor correction (PFC) circuits.
Switching power supplies and electromagnetic interference (EMI) filter circuits at the input of electronic ballasts are depicted. In the diagram, Li and A share a common core to form a common-mode inductor. Li and L2 present high impedance to common-mode (asymmetric) interference signals and low impedance to differential-mode interference signals (symmetrical interference currents) and power supplies.
Iron-based amorphous alloy cores are suitable for replacing silicon steel sheets and ferrites in the production of rectified power supply output filters and reactors, intermediate frequency and high frequency (400 Hz to 50 kHz) switching power supply output filters, and audio equipment filters. Shown is a switching power supply output rectification filter circuit using a forward converter topology.
In the figure, L is the filter inductor.
Filtering | Forward converter output rectification filter circuit uses switching power supplies, electronic ballasts, and frequency converters of bridge rectification capacitor filter circuits. Due to the presence of large-capacity smoothing filter capacitors, rectifier diodes are only activated near the peak of the AC input line voltage, resulting in the AC input current becoming a spike with a large amplitude. This severely distorted current waveform has a very low fundamental component and a very high harmonic content, leading to a very low input power factor (only about 0.55 to 0.60), causing pollution to the power grid, and affecting the safe and economical operation of other equipment in the system.
Using a power factor correction (PFC) circuit as shown, a sinusoidal current that maintains the same phase as the AC line voltage can be generated at the input of the bridge rectifier. The power factor can reach 0.99, and the total current harmonic distortion (THD) can be less than 10% or even 3%, generating a boosted stable DC voltage (about 400V) at its output.
Among them, CiN and C. are PFC input and output capacitors respectively (CIN is used for high-frequency filtering, and large-capacity electrolytic capacitors are not allowed), VT is the PFC switch, IC is the power factor controller, and L is the boost inductor, VD is the boost diode.
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