Applications of Inductors and Ferrite Beads in High-Frequency Circuits
Applications of Inductors and Ferrite Beads in High-Frequency Circuits
A coil with more than one turn is conventionally called an inductor, while a coil with fewer than one turn (with the wire passing directly through the ferrite core) is conventionally called a ferrite bead. Inductors are energy storage components, while ferrite beads are energy conversion (dissipation) devices. Inductors are mostly used in power supply filtering circuits, while ferrite beads are mostly used in signal circuits for EMC countermeasures. Ferrite beads are mainly used to suppress electromagnetic interference, while inductors, in this respect, focus on suppressing conducted interference. Both can be used to address EMC and EMI issues. Inductors are generally used for circuit matching and signal quality control. Ferrite beads are used where analog and digital grounds are combined. Ferrite beads have high resistivity and permeability. They are equivalent to a resistor and an inductor in series, but both the resistance and inductance values change with frequency. They have better high-frequency filtering characteristics than ordinary inductors, exhibiting resistivity at high frequencies, thus maintaining high impedance over a relatively wide frequency range, thereby improving the FM filtering effect. Inductors can be used for power supply filtering. The circuit symbol for a ferrite bead is the same as that for an inductor, but the model number indicates that a ferrite bead is used. Functionally, ferrite beads and inductors operate on the same principle, differing only in their frequency characteristics. Ferrite beads are composed of ferrite metal, while inductors consist of a magnetic core and a coil. Ferrite beads convert AC signals into heat, while inductors store the AC signal and release it slowly. Ferrite beads offer significant impedance to high-frequency signals, typically rated at 100 ohms/100MHz. At low frequencies, their resistance is much lower than that of an inductor. Ferrite beads are a rapidly developing type of anti-interference component, inexpensive, easy to use, and highly effective at filtering high-frequency noise. In a circuit, simply passing a wire through it is sufficient (I usually use those that resemble ordinary resistors, with the wire already passed through and glued; surface-mount versions also exist, but are rarely seen for sale). When current flows through the wire, ferrite offers almost no impedance to low-frequency currents, but provides significant attenuation to higher-frequency currents. High-frequency current dissipates as heat within the bead. Its equivalent circuit consists of an inductor and a resistor in series, with the values of both components proportional to the bead's length. Many types of ferrite beads exist; manufacturers should provide technical specifications, particularly the impedance versus frequency curve. Some ferrite beads have multiple holes. Passing wires through these holes increases the component's impedance (by the square of the number of times the wire passes through the bead). However, at high frequencies, the increase in noise suppression is unlikely to be as significant as expected. Using multiple beads in series is generally better. Ferrite is a magnetic material and will experience magnetic saturation due to excessive current, causing a sharp decrease in permeability. High-current filtering should utilize specially designed ferrite beads, and careful heat dissipation is essential. Ferrite beads are not only used in power supply circuits to filter high-frequency noise (for both DC and AC outputs) but also widely applied in other circuits, and their size can be made very small. Especially in digital circuits, where pulse signals contain high-frequency harmonics, a major source of high-frequency radiation, ferrite beads can be particularly effective. Ferrite beads are also widely used for noise filtering in signal cables. Taking the HH-1H3216-500, commonly used for power supply filtering, as an example, the meanings of its model fields are as follows: HH is a series primarily used for power supply filtering; for signal lines, it's the HB series. 1 indicates one bead is packaged in one component; 4 indicates four beads packaged side-by-side. H indicates the constituent material; H, C, and M are for mid-frequency applications (50-200MHz), T for low-frequency applications (50MHz), and S for high-frequency applications (200MHz). 3216 is the package size: 3.2mm long and 1.6mm wide, i.e., a 1206 package. 500 is the impedance (typically at 100MHz), 50 ohms. Its product parameters mainly include three items: Impedance [Z]@100MHz (ohm): Typical 50, Minimum 37; DC Resistance (m ohm): Maximum 20; Rated Current (mA): 2500.
