According to the editor, power electronic components are commonly referred to as "passive" components. These play a crucial role 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 topics starting this April to cater to readers' needs to deepen their understanding and effectively utilize these components. We warmly invite engineers from both manufacturers and users to contribute articles and look forward to receiving valuable feedback.
In our fifth installment of the Power Electronic Components lecture series, we 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 Inductors
#### 1.1 Wire Wound Chip Inductors
Wire wound chip inductors come in two main categories: power and high power.
(1) Low Power Wire Wound Chip Inductors
Low-power wire-wound chip inductors are manufactured by winding enameled wire around a ferrite or ceramic core, typically in smaller sizes. Their shape and structure vary. For instance, the structure shown in (a) features a skeleton with internal winding and is externally shielded by a magnetic material, formed by pressing a plastic film; the structure depicted in (b) involves winding on a rectangular skeleton, with the skeleton being made of ferrite or ceramic; (c) shows an I-shaped structure, with the skeleton made of ceramic, ferrite, or aluminum, and the size being relatively larger.
- (a): Externally molded plastic (with shielding);
- (b): Square ceramic or ferrite skeleton;
- (c): I-shaped ferrite skeleton chip inductor
The primary parameters include size, inductance, tolerance, Q value, DC resistance, maximum allowable current, and self-resonant frequency (SRF). The operating frequency of a chip inductor largely 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 used in high-frequency bands, including UHF or VHF sections. For the HF and UHF bands, the inductance of suitable inductors typically ranges from 0.1 to 1000 μH, while for the VHF band, it generally falls between 1.5 to 100 nH. The tolerance of inductance is usually at the +5% level, K level (±10%), or M level (±20%). The operating temperature range for chip inductors is generally -25°C to 85°C. One major drawback of small-power wire-wound chip inductors is their large size and significant magnetic leakage. Currently, most product specifications have already been replaced by laminated chip inductors.
(2) High-Power Wire-Wound Chip Inductors
In many circuits, there is a need for surface-mounted power inductors with inductances of mH and current levels of A. High-power wire-wound chip inductors are primarily used as energy storage elements and LC filter elements in DC/DC converters. These inductors are typically made using square or round I-shaped ferrite as the skeleton and enameled wires of varying wire diameters.
Common high-power wire-wound inductors have an inductance range of 1 to 330 μH, with an inductance base of 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 from ferrite materials using multi-layer production technology. Their structure closely resembles that of multilayer chip capacitors (MLCC), as illustrated. Since MLCIs have a closed magnetic circuit, they offer a shielding effect. The manufacturing methods for MLCIs are divided into dry and wet processes. The dry method primarily refers to the casting-through method, while the wet method includes overlapping printing, via forming, and printing hole forming.
MLCIs boast advantages such as small size, high reliability, and excellent magnetic shielding performance, making them ideal for high-density automated placement on STM equipment. Their inductance ranges from 1 nH to several tens of μH.
#### 1.3 Thin-Film Chip Inductors
Thin-film chip inductors are made by using thin-film technology to etch conductor coils onto ceramic or other substrates. They can maintain high Q values in the microwave range, offering high precision and stability, along with a smaller size. Thin-film chip inductors exhibit minimal parasitic capacitance, and their self-oscillation frequency is suitable for surface mounting. They can be assembled on PCBs using standard chip mounters and reflow soldering technology. Traditional inductive components, such as chokes and transformers, typically have non-standard lead forms and irregular shapes, requiring complex mounting equipment and manual assembly.
The wire frame is inserted into the slit, bent sideways, and then reshaped into a standard "gull-wing" shaped lead. In this way, each lead in the lead frame becomes a half-turn winding of the inductor, with the other half-turn formed by a lead on the PCB.
These components offer advantages in source systems. They use high-permeability and low-loss cores to enhance frequency characteristics and can function as signal processors (SI-IC) pulse transformers or broadband transformers. High-bandwidth, high-sensitivity cores are suitable for EMC applications, such as common-mode chokes or double-line chokes that suppress power and signal line interference (EMC).
Chip inductors are predominantly 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-level integration. The rapid trend toward lighter, thinner, shorter, and smaller electronics has created a vast market for chip inductors. Their applications in computers, communications, and audiovisual products are extensive. For instance, each mobile phone requires approximately 20 medium-sized chip inductors. In 2007, global mobile phone production reached 1.144 billion units (with China producing 549 million units), resulting in over 22 billion chip inductors being used. 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 demand large quantities 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 a critical parameter. It represents the ratio of the inductive reactance of the inductor when operating at a specific AC frequency to the DC resistance of the coil (Q = 2Ï€fL/R). The lower the inductor loss, the higher the Q value and efficiency. The Q value is closely linked to frequency; inductors with high Q values at high frequencies often have low Q values at low frequencies. Although different products may have the same inductance, the DC resistance varies due to the choice of winding diameter. In high-frequency loops, the DC resistance of the inductor significantly impacts the Q value, requiring careful consideration during design.
### 2. Inductors Based on Amorphous and Nanocrystalline Alloy Cores
Amorphous and nanocrystalline alloy cores can be manufactured in various shapes. The core used as an inductor is mainly ring-shaped. The conductor is wound around the ring-shaped iron core to create multiple turns, forming an inductor. Shown below is an amorphous nanocrystalline alloy ring inductor core and an inductor made from it.
Amorphous and nanocrystalline alloy inductors can be employed in various power supply EMI filters, switching power supply output filters, and power factor correction (PFC) circuits.
Switching power supplies and EMI filter circuits at the input of electronic ballasts are illustrated. In the diagram, L1 and L2 share a common core to form a common-mode inductor. L1 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, allowing very little interference.
Iron-based amorphous alloy cores are suitable for replacing silicon steel sheets and ferrites in the production of rectified power supply output filter inductors and reactors, as well as intermediate frequency and high-frequency (400Hz to 50kHz) switching power supply output filter inductors and audio equipment filter inductors. Shown below is a switching power supply output rectification filter circuit using a forward converter topology.
In the diagram, L is the filter inductor.
Filtering | Forward converter output rectification filter circuit employs switching power supplies, electronic ballasts, and frequency converter bridge rectification capacitor filter circuits. Due to the presence of large-capacity smoothing filter capacitors, the rectifier diodes are only turned on near the peak of the AC input line voltage, causing the AC input current to no longer be a sine wave but rather 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), polluting the power grid and affecting the safe and economical operation of other equipment in the system.
Using a power factor correction (PFC) circuit, a sinusoidal current maintaining 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, L is the boost inductor, and VD is the boost diode.
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