Note that L2 could be a ferrite bead, depending on the current being drawn into this system. This example circuit shows the rectifier stage in a power system used to convert AC to DC. Note that L2 is typically a ferritic component (either a ferrite core inductor in high current systems, or a ferrite bead in low current systems). You would then follow this with a common-mode choke and further filtering on the output from your rectifier stage to produce DC power with low ripple.Īn example showing these filtering elements an AC input is shown below. In higher power systems, this same circuit design is used with inductor coils as these can generally handle several amps of current. These are sometimes used on the AC power stage of a system to provide differential-mode noise filtering to ground, i.e., as a Pi filter. Then you get what is essentially an LC filter that can provide low-pass filtering functions at sufficiently low frequencies. While a bead by itself can’t make a low pass or high pass filter, they can be used for more effective low-pass filtering at lower frequencies (e.g., 60 Hz AC or 120 Hz rectified DC ripple) when combined with shunt capacitors. frequency curve for the BLM18PG600SN1D ferrite bead from Murata. However, these components can be used with other ferrites to address specific types of noise on the input power section. In this way, they aren't the perfect filters. Above that band their inherent capacitance takes over and their impedance begins to drop again. They do block high frequencies, but only in a specific band their impedance tends to maximize and be totally resistive around 100 MHz to 1 GHz. Filtering With Ferrite Beads and Other Inductive Componentsįerrite beads are magnetic components, so it is tempting to think of ferrite beads as inductors that provide low pass filtering functions. As we'll see, the same logic that applies to ferrite cores on the input of a bridge rectifier stage in a power system does not apply to the power connection between a regulator and an integrated circuit. In this article, we'll go over some of the ways ferrites should not be used in a PCB, as well as how they actually operate in terms of their filtering behavior.

Ferrite beads are sometimes applied for EMI in two ways in attempts to minimize EMI, but the designer ends up creating a new EMI problem if these components are not used correctly. You're probably reading this article on a laptop that uses a ferrite to filter out conducted EMI from the supply line.Ī problem begins to arise when you try to apply the same logic to other areas of a PCB. These components are basically filters, and they do perform a useful function on power cords in many electronics. One of these areas relating to EMC in PCB design is the use of ferrite beads. EMI and EMC can be a tricky subjects, and it's often tempting to mis-apply design guidelines to try and reduce EMI.

Dual pi filter design