What Do Ferrite Beads Do?

Introduction to Ferrite Beads

The full name of typical ferrite bead is the ferrite bead filter, which serves as an anti-interference electronic component with a significant effect in filtering out high-frequency noise.

So what do ferrite beads do？Generally, ferrite beads possess a high resistivity and magnetic permeability due to ferrite core, effectively acting as a series combination of resistor and inductor. However, both the resistance and inductance values vary with frequency. Compared to typical inductors, they exhibit superior high-frequency signals, presenting as resistive at high frequencies. This allows them to maintain a high impedance level over a relatively broad frequency range, thereby enhancing frequency modulation filtering performance.

In electronic circuit diagrams, the symbol for a ferrite bead is akin to that for an inductor; however, the model number indicates the use of a ferrite bead. Functionally within a electronic circuit, ferrite beads and inductors operate on the same principle, differing only in their frequency characteristics.

Unit of Ferrite Bead

It is important to note that the unit for ferrite beads is ohms, not henrys. This distinction is crucial because ferrite beads are rated based on the impedance they generate at a specific frequency, and impedance level is measured in ohms.

Datasheets for chip ferrite beads typically include characteristic curves showing frequency versus impedance. For example, 600R@100MHz indicates that at a frequency of 100MHz, the ferrite bead has an impedance of 600 ohms.

Main Parameters of Ferrite Beads

DC Resistance (mohm): This refers to the resistance value exhibited by this bead when a direct current flows through it.

Current rating (mA): This indicates the maximum allowable current for normal operation of the bead.

Impedance [Z]@100MHz (ohm): This pertains to the alternating current impedance.

Impedance-Frequency Characteristic: Describes the curve illustrating the variation of impedance with frequency.

Resistance-Frequency Characteristic: Describes the curve illustrating the variation of resistance with frequency.

Reactance-Frequency Characteristic: Describes the curve illustrating the variation of inductive reactance with frequency.

Classification and Selection of Ferrite Beads

Classification

Based on their applications, ferrite components can be roughly categorized into standard, high-current, and spike types.

1.Standard Type: Standard ferrite beads are used in situations where the current is not very high (generally less than 600mA) and no special requirements are needed. Their direct current resistance is usually a fraction of an ohm. They effectively suppress and absorb electromagnetic interference and radio frequency interference in electronic devices. The impedance range typically falls between several ohms to a few thousand ohms.

2.High-Current Type: This type of ferrite bead is suitable for situations requiring higher currents. Because they are used in high-current scenarios, their direct current resistance must be very low, about one order of magnitude lower than that of standard ferrite beads. Their impedance value is also generally smaller.

3.Spike Type: The characteristic of this type of ferrite bead is that its impedance rises sharply within a wide range of frequency, thereby achieving a high attenuation effect in that frequency region without affecting the signal.

Selection

When selecting specific types of ferrite beads, two primary aspects need to be considered: the level of noise interference in the electronic circuit and the magnitude of the current that needs to pass through. First, it's important to have a rough understanding of the frequency and intensity of the noise as different ferrite materials have distinct frequency impedance curves. Ideally, you should choose a bead with high impedance at the center frequency of the noise. For significant noise interference, opt for higher impedance beads; however, keep in mind that higher impedance isn't always better since it also increases the direct current resistance (DCR), leading to greater attenuation of useful signals. Generally, there are no precise standards for calculation and selection—it's mainly determined by practical application results, with impedances between 120Ω and 600Ω being commonly used.

Next, consider the magnitude of the current that will pass through. If the bead is used in the power cable section, select a model with a higher rated current. In contrast, when used on signal lines, the rated current requirement is generally lower. Note that typically, the greater the impedance of the bead, the smaller its rated current.

The choice of ferrite beads should be based on specific circumstances. For instance, for a 3.3V, 300mA power supply where the voltage drop can't below 3.0V, the DCR of the bead should be less than 1Ω. In such cases, selecting a 0.5Ω bead helps prevent parameter drift. Regarding noise suppression capabilities, if the requirement is to reduce a 100MHz, 300mVpp noise down to a 50mVpp level, assuming a load current of 45 ohms, you should select a bead with an impedance of 225Ω at 100 MHz range and a DCR of less than 1Ω.

It is important to note that the impedance of the ferrite bead may differ slightly from the specifications once voltage is applied. To choose the correct ferrite bead, consider the following key points:

1.The frequency range of unwanted signals;

2.The source of common-mode noise;

3.The required level of noise attenuation;

4.Environmental conditions such as temperature, DC voltage, and structural strength;

5.The sensitive circuit and load impedance;

6.Whether there is enough space on the PCB to place the ferrite bead.

Factors to Consider When Choosing Ferrite Beads

1.There is a wide variety of ferrite beads available, and manufacturers should provide technical specifications, particularly curves illustrating the relationship between impedance and frequency.

2.Some ferrite beads feature multiple holes, allowing wires to pass through them, which increases the component's impedance (the square of the number of passes through the bead). Using several beads in series can also enhance impedance.

3.Ferrite core is a magnetic material that can experience saturation when exposed to excessively high currents, causing a sharp decline in permeability. For high-current filtering, it is essential to use beads specifically designed for range of applications, and attention must be paid to their heat dissipation measures.

4.Ferrite beads are not only useful in power systems to pass filter from high-frequency noise (suitable for both DC and AC output voltage), but they also find extensive use in other circuits. They can be made very compact, making them particularly effective in digital circuit board where pulse signals contain high-frequency harmonics, which are a primary source of high-frequency radiation. In such scenarios, ferrite beads can be highly effective.

5.When selecting ferrite beads, it is crucial to consider their current-carrying capacity. Generally, derating is necessary, and in power connections, the effect of DC resistance on voltage drop must be taken into account.