Ferrite beads are passive devices that can filter high-frequency noise in a wide frequency range. It has resistance characteristics in the target frequency range and dissipates noise energy in the form of heat. The ferrite bead is connected in series with the power rail, and both sides of the bead are often grounded together with the capacitor. In this way, a low-pass filter network is formed to further reduce high-frequency power supply noise.

However, if the ferrite beads are not used properly in the system design, it will have adverse effects. There are some examples to illustrate: interference resonance is generated due to the combination of magnetic beads and decoupling capacitors used for low-pass filtering; The dependence of DC bias current leads to the decrease of EMI suppression ability of magnetic beads. After correctly understanding and fully considering the characteristics of ferrite beads, these problems can be avoided.

This paper discusses the precautions for system designers when using ferrite beads in power supply systems, such as impedance and frequency characteristics when DC bias current changes, and interference LC resonance effect. Finally, in order to solve the problem of interference resonance, damping techniques are introduced and the effectiveness of various damping methods is compared.

The device used to demonstrate the effect of ferrite beads as an output filter is a 2 A/1.2 A DC-DC switch regulator with independent positive and negative outputs (ADP5071). The ferrite magnetic beads used in this paper are mainly packaged by chip type surface mounting.

Simplified Model and Simulation of Ferrite Beads

Ferrite beads can be modeled as a simplified circuit consisting of resistors, inductors, and capacitors, as shown in Figure 1a. RDC corresponds to the DC resistance of the magnetic bead. CPAR, LBEAD and RAC represent parasitic capacitance, magnetic bead inductance and AC resistance related to magnetic bead (AC core loss) respectively.