How to select and apply ferrites
Applications of Ferrite Suppression Devices: Ferrite suppression devices are widely used in PCBs, power lines, and data lines.
Applications of Ferrite Suppression Devices on PCBs:
The primary method of EMI design is source suppression, which is to suppress EMI at the source on the PCB. The idea is to confine the noise to a small area and avoid coupling high-frequency noise to other circuits that can generate stronger emissions by connecting cables.
The source of EMI on a PCB comes from the digital circuitry. Its high-frequency current creates a common-mode voltage drop between the power line and ground, causing common-mode disturbance. Decoupling capacitors on power or signal lines can short-circuit the high-frequency noise of IC switches, but decoupling capacitors often cause high-frequency resonances and create new disturbances. Adding a ferrite suppression bead to the power inlet of the board effectively attenuates high-frequency noise.
Applications of Ferrite Suppression Devices in Power Lines:
The power cord will transmit the disturbance of the external power grid and the noise of the switching power supply to the motherboard. Ferrite suppression devices are installed at the output port of the power supply and the inlet of the PCB power line, which can not only suppress the transmission of high-frequency disturbance between the power supply and the PCB, but also suppress the mutual disturbance of high-frequency noise between the PCB.
It is important to note that when ferrite devices are applied to power lines, bias currents are sometimes present. The impedance and insertion loss of the ferrite decrease as the bias current increases. When the bias current increases to a certain value, the ferrite suppression device becomes saturated. In EMC design, it is necessary to consider the problem of reduced insertion loss at saturation. The lower the permeability of the ferrite, the less the insertion loss is affected by the bias current, and the less saturated it is. Therefore, it is necessary to select materials with low permeability and components with large cross-sectional area for ferrite suppression devices used in power lines.
When the bias current is large, the outgoing wire (live wire of AC, positive wire of DC) and return wire (middle wire of AC, ground wire of DC) of the power supply can be threaded into a core/ring at the same time. This avoids saturation and suppresses common-mode noise.
Application of Ferrite Suppression Devices on Signal Lines:
The most commonly used area for ferrite suppression components is in signal lines. For example, in some products, the EMI signal will pass through the host-to-display cable to the host driver's circuit, and then coupled to the CPU, so that the circuit will not work properly. At the same time, data or noise from the motherboard can also be radiated through the connecting cables. Ferrite beads can be used between the drive circuit and the display circuit to suppress high-frequency noise. The data signal can be passed through the ferrite beads almost without loss.
Flat cables can also be treated with dedicated ferrite suppression devices to suppress noise before it is radiated.
Selection of ferrite suppression devices
Ferrite suppression devices are available in a variety of materials, shapes, and sizes.
In order to select the right suppression component that will be more effective at suppressing noise, the design engineer needs to know the frequency and intensity of the EMI signal to be suppressed, the effect of the required suppression, i.e., insertion loss, and the allowable footprint, including the core I.D., O.D., and length dimensions.
The size of the ferrite core/ring is determined: the greater the difference between the inner and outer diameters of the core/ring, the longer the axial direction, and the greater the impedance. But the inner diameter must be tightly wrapped around the wire. Therefore, to achieve large attenuation, a large magnetic ring should be used as much as possible.
Number of turns of a common-mode choke: Increasing the number of turns passing through the ring can increase the impedance of low frequencies, but the impedance of high frequencies decreases due to the increase in parasitic capacitance, so it is a common mistake to blindly increase the number of turns to increase the amount of attenuation. When the interference band to be suppressed is wide, different turns can be wound around the two magnetic rings. For example, a device may have two frequency points that exceed the standard radiation, one at 40 MHz and the other at 500 MHz. If the test determines that it is due to the common mode radiation of the cable. By putting a magnetic surround on the cable for 1 turn, the interference of 500MHz is significantly reduced and no longer exceeds the standard. However, the 40MHz frequency is still exceeded. Then use a magnetic ring to wrap the cable around the magnetic ring for more than 3 turns, and the 40MHz interference is reduced and no longer exceeds the standard. This is due to the fact that increasing the number of ferrite rings on the cable increases the impedance of the low frequency, and the impedance of the high frequency decreases as the number of winding turns increases. The reason for this phenomenon is the increase in parasitic capacitance. Since the effect of the ferrite ring depends on the impedance of the original common-mode loop, the lower the impedance of the original loop, the more obvious the effect of the magnetic ring. Therefore, when capacitive filters are installed at both ends of the original cable, the impedance is very low, and the effect of the magnetic ring will be more obvious.
Note: Ferrite beads are energy-intensive devices. It consumes high-frequency energy in the form of heat. This can only be explained by the characteristics of resistance and not inductance.
Different ferrite suppression materials have different optimal suppression frequency ranges, which are related to the initial permeability. In general, the higher the initial permeability of the material, the lower the frequency of applicable suppression.
The following table gives the relationship between the initial permeability and the optimal suppression frequency of commonly used high-frequency ferrite magnetic materials, in the case of DC or low-frequency AC bias current, it is necessary to consider the decline and saturation of the suppression performance, and try to choose materials with low permeability. On the other hand, for magnetic materials that use common-mode inductors applied at the AC input of power lines, it is necessary to suppress common-mode interference noise of 10 MHz or less, and it is necessary to select a magnetic material parameter with a higher initial permeability, and the range of 7000 to 10 K is recommended.
Initial permeability Optimal suppression frequency range/MHz
125 >200
850 30-200
2500 10-30
5000 <10
After selecting the ferrite material, it is necessary to select the shape and size of the suppression element. The shape and size of the suppression element affect the suppression effect on noise.
In general, the larger the volume of ferrite, the better the inhibition. When the volume is constant, the impedance of a long and thin shape is greater than that of a short and thick shape, and the suppression effect is better. However, in the case of DC or AC bias current, the problem of magnetic saturation should be taken into account. The larger the cross-sectional area (Ae parameter value) of the ferrite suppression element, the less saturated it is and the greater the bias current it can tolerate. In addition, the smaller the inner diameter of the ferrite, the better the inhibition effect.
In short, the principle of ferrite suppression device selection is to select a ferrite suppression device that is as long, thick as possible, and has as small an inner hole as possible under the condition that the space allows for use.
Mounting of ferrite magnetic materials
In most cases, the same ferrite suppression device should be installed as close as possible to the source of the nuisance. This prevents noise coupling to other places where noise can be more difficult to suppress. However, in I/O circuits, where wires or cables enter or exit the shield, the ferrite device should be installed as close to the inlet and outlet of the shield as possible to avoid noise coupling elsewhere before passing through the ferrite suppression device.
Note: If the ferrite magnetic material device is worn on the cable, it is recommended to seal it with a heat shrink tube.