This model may change with respect to dc bias current, and in other cases, a more complex model is required. Selecting the right ferrite bead for power applications requires careful consideration not only of the filter bandwidth, but also of the impedance characteristics of the bead with respect to dc bias current. In most cases, manufacturers only specify the impedance of the bead at MHz and publish data sheets with frequency response curves at zero dc bias current.
However, when using ferrite beads for power supply filtering, the load current going through the ferrite bead is never zero, and as dc bias current increases from zero, all of these parameters change significantly. As the dc bias current increases, the core material begins to saturate, which significantly reduces the inductance of the ferrite bead. The degree of inductance saturation differs depending on the material used for the core of the component.
Figure 3a shows the typical dc bias dependency of the inductance for two ferrite beads. The current rating of ferrite beads is an indication of the maximum current the device can take for a specified temperature rise and it is not a real operating point for filtering purposes.
In addition, the effect of dc bias current can be observed in the reduction of impedance values over frequency, which in turn reduces the effectiveness of the ferrite bead and its ability to remove EMI.
Figure 3b and Figure 3c show how the impedance of the ferrite bead varies with dc bias current. System designers must be fully aware of the effect of dc bias current on bead inductance and effective impedance, as this can be critical in applications that demand high supply current.
Resonance peaking is possible when implementing a ferrite bead together with a decoupling capacitor. This commonly overlooked effect can be detrimental because it may amplify ripple and noise in a given system instead of attenuating it. In many cases, this peaking occurs around the popular switching frequencies of dc-to-dc converters. Peaking occurs when the resonant frequency of a low-pass filter network, formed by the ferrite bead inductance and the high Q decoupling capacitance, is below the crossover frequency of the bead.
The resulting filter is underdamped. Figure 4a shows the measured impedance vs. The resistive component, which is depended upon to dissipate unwanted energy, does not become significant until reaching about the 20 MHz to 30 MHz range. Below this frequency, the ferrite bead still has a very high Q and acts like an ideal inductor. LC resonant frequencies for typical bead filters are generally in the 0.
For typical switching frequencies in the kHz to 5 MHz range, additional damping is required to reduce the filter Q. As an example of this effect, Figure 4b shows the S21 frequency response of the bead and capacitor low-pass filter, which displays a peaking effect. Load current is in the microampere range. An undamped ferrite bead filter can exhibit peaks from approximately 10 dB to approximately 15 dB depending on the Q of the filter circuit.
In Figure 4b, peaking occurs at around 2. In addition, signal gain can be seen from 1 MHz to 3. This peaking is problematic if it occurs in the frequency band in which the switching regulator operates. This amplifies the unwanted switching artifacts, which can wreak havoc on the performance of sensitive loads such as the phase-lock loop PLL , voltage-controlled oscillators VCOs , and high resolution analog-to-digital converters ADCs.
The result shown in Figure 4b has been taken with a very light load in the microampere range , but this is a realistic application in sections of circuits that need just a few microamperes to 1 mA of load current or sections that are turned off to save power in some operating modes. This potential peaking creates additional noise in the system that can create unwanted crosstalk. As an example, Figure 5 shows an ADP application circuit with an implemented bead filter and Figure 6 shows the spectral plot at the positive output.
The switching frequency is set at 2. Resonant peaking occurs at around 2. Instead of attenuating the fundamental ripple frequency at 2. Other factors that have an effect on the resonant peaks are the series and load impedances of the ferrite bead filter. Peaking is significantly reduced and damped for higher source resistance. However, the load regulation degrades with this approach, making it unrealistic in practice.
The output voltage droops with load current due to the drop from the series resistance. Load impedance also affects the peaking response. Peaking is worse for light load conditions. This section describes three damping methods that a system engineer can use to reduce the level of resonant peaking significantly see Figure 7.
Method A consists of adding a series resistor to the decoupling capacitor path that dampens the resonance of the system but degrades the bypass effectiveness at high frequencies.
Method B consists of adding a small parallel resistor across the ferrite bead that also dampens the resonance of the system. However, the attenuation characteristic of the filter is reduced at high frequencies. This image shows why a ferrite bead is sometimes called a ferrite ring or ferrite choke. Ferrite beads are passive electronic components that can suppress high frequency signals on a power supply line. Standard ferrite beads can be acquired from specialized manufacturers such as Coilcraft, though certain projects may require customized beads.
In reality, a ferrite bead is a nonlinear component; the impedance it provides changes was the load current and voltage drop across the ferrite change. The simplified circuit model of a ferrite bead will help you understand its frequency characteristics. However, keep in mind that these attributes can change as a function of current and temperature. Load current can change the impedance of your ferrite.
Because ferrite bead impedance is inductive, ferrite bead inductors are used to attenuate high-frequency signals in electronic components. When a ferrite bead choke is placed on the power line connecting to an electronic device, it removes any spurious high frequency noise present on a power connection or that is output from a DC power supply. This ferrite clamp use is one of many approach to noise suppression, such as that from a switched-mode power supply.
This application of ferrite beads as a ferrite filter provides suppression and elimination of conducted EMI. Currents greater than the specified value can damage the component. The troublesome thing is that this limit is drastically affected by heat.
As temperature increases, the rated current quickly decreases. Rated current also affects the ferrite's impedance. As DC current increases, a ferrite bead will "saturate" and lose inductance. Ferrite Bead vs. Although a ferrite bead can be modeled as an inductor, ferrite bead inductors do not behave as a typical inductor. A simple yet accurate model of a ferrite bead connected to an AC power source.
A ferrite bead can be modeled as capacitors and inductors, and also a resistor in parallel with this RLC network wired with a series resistor. The inductor in this model represents a ferrite beads primary function of attenuating high-frequency signals, i. The parallel resistor in this model accounts for losses in eddy currents that are induced within the ferrite bead at high frequencies.
When looking at a ferrite bead impedance curve , the primarily resistive impedance is extremely high in only a thin band. The inductance of the bead dominates within this thin band. At higher frequencies, the ferrite bead impedance begins to appear capacitive over and the impedance rapidly decreases. Eventually, as frequency continues increasing, the capacitive impedance will drop to a very small value, and the ferrite bead impedance appears purely resistive.
The ferrite core in a ferrite bead provides a similar function as the ferrite core in a transformer. You may be wondering, are ferrite beads necessary for my design?
Like many engineering decisions, the answer is not so simple. If you know that your board will experience conducted EMI within a specific frequency range, and you need to attenuate these frequencies, then a ferrite bead may be the right choice for your design.
However, ferrite beads do not act like a wideband low-pass filter as they can only help attenuate a specific range of frequencies. The transformer itself is constructed by using a magnetic core in which coil inductor windings are made on a ferrite core component. Before designing a transformer, check your requirement and exact application including input voltage, output voltage, current and frequency of operation.
Sleeves and cores are often used on power and control cabling on electronic and electrical devices. Sleeves are usually installed after all cabling has been attached. For round cabling, a square plastic ferrite sleeve is often used. The core is contained in a hinged plastic casing that opens up to permit the insertion of the cable, then snaps together to secure the ferrite A5 core around the cable to suppress EMIs.
Another option is to use a ferrite sleeve suppression cored with a round plastic casing, sized to fit the cable used. Both the square and round cased ferrite cores attenuate any form of EMI emission and are often used either as a retrofit or for testing purposes when calculating ferrite core filter specifications and design requirements.
To begin choosing the correct type of ferrite core, bead, or sleeve you need for your design applications, use the suggested ferrite products in this handy chart. A single circuit protection on a PCB uses a single, two-lead ferrite bead. Protect multiple adjacent circuits on a PCP.
Use a multiline suppressor bead. For installation on outer cables for retrofit or testing purposes, use a square plastic casing ferrite sleeve. For installation on round power cables, use a round plastic casing ferrite sleeve. For flat cables in electronic devices, use a flat cable ferrite core.
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