Tips for Selecting Magnetic Beads in PCB design

In the design of electronic circuits, there is an endless variety of components with various functions. The use of magnetic beads to solve compatibility, noise, interference, etc. is the electronics industry and its products are unavoidable problems!
Magnetic beads are dedicated to suppressing signal lines, power lines on the high-frequency noise and spike interference, but can absorb electrostatic impulses, its DC impedance is not large, at high frequencies but has a high impedance, usually said 600R refers to the 100MHZ test frequency under the impedance value. Today we will give you some ways to choose the right magnetic beads in this article!

What is Box Build Assembly

A SMD Magnetic bead is a commonly used electronic component, usually made from materials with hysteresis properties, such as graphene magnetic materials or other magnetic materials. This component is mainly used in various electronic circuits, especially high-frequency circuits such as amplifiers, filters, modems, antennas, and transmitters. The principle is based on the phenomenon of magnetization, which uses magnets’ properties to transmit signals from one position to another.

The Difference Between Magnetic Beads and Inductors

  • Magnetic beads:
Magnetic beads are used to suppress high frequency noise and spikes on signal lines and power lines, and also have the ability to absorb electrostatic pulses. Magnetic beads have high resistivity and permeability, equivalent to resistance and inductance in series, but the value of resistance and inductance with frequency changes.
  • Inductors:
Inductors are energy storage components used in LC oscillator circuits, low and medium frequency filtering circuits, etc. The frequency range of their application rarely exceeds 50 MHZ.

Principle of Composition

Inductance is actually the wire so that a circle around the core, which constitutes the inductance, while the bead (plug-in), then the wire is wrapped around the periphery of a layer of ferrite magnetic material

Different Key Parameters

When choosing an inductor, we generally choose to look at the inductance value, and then look at the rated current. While in the selection of magnetic beads mainly look at xxx ohms at 100MHZ, we commonly used is 120 ohms @ 100MHZ, and the current.

Different Operating Principles

The inductor is an energy storage device that stores the electrical energy from the inductor into the magnetic field of the core and then releases it at the right time, while the magnetic bead consumes the high frequency signal that flows through him in the form of heat energy. At this point, the magnetic bead is equivalent to a resistor to the high frequency signal.

Circuit Symbols Differ

For inductors, there are two main circuit symbols on the left, and for magnetic beads, there are two circuit symbols on the right. Inductors are usually prefixed with L and beads are usually prefixed with FB or

Different Application Circuits

Inductors are generally used in power supply or signal storage filtering, for example, we commonly have the following three:

BucK Power Circuit

BOOST Power Circuit

LC Low Pass Filter Circuit

The Function of SMD Magnetic Beads

SMD Magnetic beads play a critical role in PCB manufacturing, mainly reflected in the following aspects:
  • Filtering:  SMD Magnetic beads exhibit high impedance characteristics at high frequencies, they can filter out high-frequency noise from circuits. This characteristic makes chip beads ideal for suppressing high-frequency noise and helps to improve the signal quality of circuits.
  • Suppression of Electromagnetic Interference (EMI): EMI is a common problem in electronic equipment and can cause signal distortion or equipment failure. SMD beads can absorb and dissipate electromagnetic energy in circuits, thus effectively reducing the effects of EMI and improving the stability and reliability of equipment.
  • Improve power supply stability: In power supply circuits, SMD beads can suppress power line noise and improve the purity of the power supply. This is especially important for electronic devices that require a stable power supply, such as digital circuits, RF circuits, etc.
  • Matching impedance: The impedance characteristics of SMD beads enable them to match the impedance of different circuits for lossless signal transmission. This is important to ensure the integrity and accuracy of signal transmission.
  • Prevent electrostatic interference: Electrostatic interference is one of the common sources of interference in electronic equipment. SMD beads can effectively absorb and dissipate static energy, thus preventing static electricity from causing damage to the circuit.

The Function of SMD Magnetic Beads

Noise interference and through-current factors need to be considered when selecting beads.
  1. Noise interference needs to consider the frequency and intensity of noise. Different models of magnetic beads have different frequency impedance curves. In the selection, the center frequency of the noise should be selected to correspond to the higher impedance beads, to suppress the noise better. The higher the noise interference, the higher the impedance bead selected. However, high-impedance beads will also produce greater attenuation of the useful signal. So higher impedance is not better, and the signal-to-noise ratio needs to be considered.
  2. For DC, you need to consider the size of the current rating. The higher the DC resistance of the bead, the lower the rated current. Selection needs to be based on the actual situation. 

How to Select Magnetic Beads Correctly ?

Therefore, the magnetic bead selection can be done according to the following steps.
First, you need to determine the frequency band of the noise that needs to be filtered out, and then select the appropriate AC impedance in this band (you can get the approximate impedance range through simulation, and the simulation model can be consulted with the manufacturer).
Second, determine the maximum current that the circuit will pass. The current flowing through the circuit determines the rated current of the bead. Once the rated current is determined, the DCR range of the bead can be calculated based on the voltage range needed for the back-end circuit.
Then: Select the package. The package can be selected according to the actual situation, such as the layout and structure of the single board. However, it should be noted that the impedance of the bead is different from the impedance on the specification after adding voltage.
To select the bead correctly, the following points must be noted:
1. What is the frequency range of unnecessary signals;
2. Where is the noise source;
3. Whether there is space to place magnetic beads on the PCB board;
4. How much noise attenuation is required;
5. What are the environmental conditions (temperature, DC voltage, structural strength);
6. What is the impedance of the circuit and the load?
Finally, magnetic bead selection also requires attention to derating. The derating criteria for magnetic devices are shown in the table below:
Hot Spot Temperature Derogation
DC Voltage Droop
Surge current derating
Surge Voltage Derating
Magnetic Beads

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