A ferrite bead is a passive device that filters high frequency noise energy over a broad frequency range. However, improper use of ferrite beads in system design can lead to some detrimental issues. Some examples ripple noise measurement unwanted resonance due to combining the bead with a decoupling capacitor for low-pass filtering and the effect of dc bias current dependency that degrades the EMI suppression capability of the bead.
With proper understanding and consideration of the ferrite bead’s behavior, these issues can be avoided. This article discusses the important considerations that system designers need to be aware of when using ferrite beads in power supply systems such as impedance vs. Ultimately, to address the issue on the unwanted resonance, damping techniques will be introduced and a comparison of the effectiveness of each damping method will be presented. The ferrite beads used in the article are mainly chip type surface-mount packages. Ferrite Bead Simplified Model and Simulation A ferrite bead can be modeled as a simplified circuit consisting of resistors, an inductor, and a capacitor, as shown in Figure 1a. RDC corresponds to the dc resistance of the bead.
Tyco Electronics BMB2A1000LN2 measured ZRX plot. Ferrite beads are categorized by three response regions: inductive, resistive, and capacitive. In some cases, the simplified circuit model can be used to approximate the ferrite bead impedance characteristic up to the sub-GHz range. The Tyco Electronics BMB2A1000LN2 multilayer ferrite bead is used as an example. Figure 1b shows the measured ZRX response of the BMB2A1000LN2 for a zero dc bias current using an impedance analyzer. XL is the reactance at 30.
7 MHz, which is 233 Ω. 300 mΩ, is acquired from the manufacturer’s data sheet. Calculate RAC by subtracting RDC from Z. Because RDC is very small compared to the peak impedance, it can be neglected. Therefore, in this case RAC is 1. The ferrite bead model can be useful in noise filtering circuit design and analysis.
DC Bias Current Considerations 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 100 MHz and publish data sheets with frequency response curves at zero dc bias current. 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.
Würth Elektronik 742 792 510 bead. 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. LC Resonance Effect 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. 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. S21 response for a ferrite bead and capacitor low-pass filter. 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.
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. 5 MHz with as much as 10 dB gain. 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. As an example, Figure 5 shows an ADP5071 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. 4 MHz, the input voltage is 9 V, the output voltage is set at 16 V, and the load current of 5 mA. ADP5071 application circuit with a bead and capacitor low-pass filter implementation on positive output. Other factors that have an effect on the resonant peaks are the series and load impedances of the ferrite bead filter.