Use of ferrite beads in power supply systems

An effective way to filter out high frequency power supply noise and cleanly share similar power supply rails (i.e., analog and digital rails for mixed-signal ICs) while maintaining high-frequency isolation between the shared supply rails is to use ferrite beads.

Ferrite beads are passive devices that filter out high-frequency noise over a wide frequency range. They are resistive in the target frequency range and dissipate noise energy in the form of heat. The ferrite bead is connected in series with the supply rail, and the sides of the bead are often grounded with a capacitor. This creates a network of low-pass filters that further reduce high-frequency power supply noise.

However, ferrite beads can have a detrimental effect if they are not used properly in the system design. Some examples include: interference resonance due to the use of beads and decoupling capacitors in conjunction with low-pass filtering; and the DC bias current dependence of beads resulting in reduced EMI suppression. These problems can be avoided with proper understanding and consideration of the characteristics of ferrite beads.

This paper discusses considerations for system designers using ferrite beads in power systems, such as impedance and frequency characteristics as DC bias current varies, and interference with LC resonance effects. Finally, damping techniques are presented to address the interference resonance problem and the effectiveness of each damping method 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 switching regulator with separate positive and negative outputs (ADP5071). The ferrite beads used in this paper are mainly in chip type surface mount packages.

Simplified Modeling and Simulation of Ferrite Beads

The ferrite bead can be modeled as a simplified circuit consisting of resistance, inductance, and capacitance as shown in Figure 1a. RDC corresponds to the DC resistance of the bead, and CPAR, LBEAD, and RAC denote the parasitic capacitance, bead inductance, and bead-dependent AC resistance (AC core loss), respectively.

 

 

Ferrite beads can be categorized according to three response regions: inductive, resistive, and capacitive. These regions can be determined by looking at the ZRX curve (shown in Figure 1b), where Z is the impedance, R is the resistance, and X is the reactance of the bead. To reduce high frequency noise, the beads must be in the resistive region; this is especially important for electromagnetic interference (EMI) filtering applications. This element acts as a resistor, blocking high-frequency noise and dissipating it as heat. The resistive region occurs after the bead crossover frequency (X = R) until the point at which the bead becomes capacitive. This capacitive point is located at the frequency where the absolute value of the capacitive reactance (-X) is equal to R.