| In the PCB circuits of electronic devices, a large number of inductive components and EMI filter components are used. These components include chip inductors and chip beads. The following describes the characteristics of these two devices and analyzes their common and special applications. The advantage of surface mount components is that they are small in size and can meet the requirements of actual space. Except for the impedance value, current carrying capacity and other similar physical properties, the other performance characteristics of through-hole connectors and surface mount devices are basically the same.
In the occasions where chip inductors are needed, the inductors are required to achieve the following two basic functions: circuit resonance and choke reactance. Resonant circuits include resonance generating circuits, oscillation circuits, clock circuits, pulse circuits, waveform generating circuits, etc. Resonant circuits also include high-Q bandpass filter circuits. In order for the circuit to resonate, there must be capacitance and inductance in the circuit at the same time. There is parasitic capacitance at both ends of the inductor, which is caused by the ferrite body between the two electrodes of the device being equivalent to the capacitor medium. In the resonant circuit, the inductor must have high Q, narrow inductance deviation, and stable temperature coefficient to meet the requirements of narrow band and low frequency temperature drift of the resonant circuit. High Q circuits have sharp resonant peaks. Narrow inductor bias ensures that the resonant frequency deviation is as small as possible. Stable temperature coefficient ensures that the resonant frequency has stable temperature variation characteristics. The difference between standard radial lead inductors and axial lead inductors and chip inductors is only the packaging. The inductor structure includes a coil wound on a dielectric material (usually alumina ceramic material), or an air-core coil and a coil wound on a ferromagnetic material. In power applications, when used as a choke, the main parameters of the inductor are DC resistance (DCR), rated current, and low Q value. When used as a filter, wide bandwidth characteristics are desired, so the high Q characteristics of the inductor are not required. Low DCR can ensure minimum voltage drop. DCR is defined as the DC resistance of the component in the absence of AC signal.
Chip beads The function of chip beads is mainly to eliminate RF noise existing in the transmission line structure (PCB circuit). RF energy is an AC sine wave component superimposed on the DC transmission level. The DC component is the desired useful signal, while the RF energy is useless electromagnetic interference transmitted and radiated along the line (EMI). To eliminate these unwanted signal energies, chip beads are used to play the role of high-frequency resistors (attenuators). The device allows DC signals to pass through and filters out AC signals. Usually high-frequency signals are above 30MHz, however, low-frequency signals are also affected by chip beads.
Chip beads are composed of soft magnetic ferrite materials, forming a monolithic structure with high volume resistivity. Eddy current loss is inversely proportional to the resistivity of ferrite materials. Eddy current loss is proportional to the square of the signal frequency.
Benefits of using chip beads: Miniaturization and lightweight have high impedance in the RF noise frequency range, eliminating electromagnetic interference in transmission lines. Closed magnetic circuit structure, better eliminate signal crosstalk. Excellent magnetic shielding structure. Reduce DC resistance to avoid excessive attenuation of useful signals. Significant high-frequency characteristics and impedance characteristics (better elimination of RF energy). Eliminate parasitic oscillations in high-frequency amplifier circuits. Effectively work in the frequency range of several MHz to hundreds of MHz.
To correctly select magnetic beads, you must pay attention to the following points: What is the frequency range of the unwanted signal. Who is the noise source. How much noise attenuation is required. What are the environmental conditions (temperature, DC voltage, structural strength). What are the circuit and load impedances. Is there space to place magnetic beads on the PCB board? The first three can be determined by observing the impedance frequency curve provided by the manufacturer. In the impedance curve, all three curves are very important, namely resistance, inductance and total impedance. The total impedance is described by ZR22πfL()2+:=fL. The typical impedance curve is shown in the figure below:
Through this curve, select the magnetic bead model with the maximum impedance in the frequency range where you want to attenuate noise and the smallest signal attenuation at low frequency and DC. The impedance characteristics of chip magnetic beads will be affected under excessive DC voltage. In addition, if the operating temperature rise is too high or the external magnetic field is too large, the impedance of the magnetic beads will be adversely affected.
Reasons for using chip magnetic beads and chip inductors: Whether to use chip magnetic beads or chip inductors depends mainly on the application. Chip inductors are required in resonant circuits. When it is necessary to eliminate unwanted EMI noise, using chip magnetic beads is the best choice.
Applications of chip beads and chip inductors: Chip inductors: Radio frequency (RF) and wireless communications, information technology equipment, radar detectors, automotive electronics, cellular phones, pagers, audio equipment, PDAs (personal digital assistants), wireless remote control systems, and low-voltage power supply modules. Chip beads: Clock generation circuits, filtering between analog circuits and digital circuits, I/O input/output internal connectors (such as serial ports, parallel ports, keyboards, mice, long-distance telecommunications, local area networks), between radio frequency (RF) circuits and susceptible logic devices, filtering high-frequency conducted interference in power supply circuits, computers, printers, video recorders (VCRS), EMI noise suppression in television systems and mobile phones. |
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
