What Are Ferrite Core Filters?

Ferrite core filters are a type of low-pass filter that is commonly used in switching power converters. They are effective at removing spikes and other high-frequency noise, improving the quality of the output voltage and overall converter performance.

They work by combining the magnetic properties of a ferrite core with the resistive and inductive characteristics of a coil over the ferrite core. The result is a ferrite bead that acts like a resistor, which impedes the flow of high-frequency signals and dissipates energy as heat.

These beads are typically placed around a power/ground pair on the incoming power cord, such as the one that comes out of your laptop or other electronic device. The ferrite core absorbs back EMF, attenuating the high frequency signals that travel on the power/ground lines.

EMI suppression is the most common reason why commercial designers use ferrite core filters. Government regulators want their products to not transmit radio-frequency radiation and tickle antennas, so they rely on ferrite beads as an easy way to meet these requirements.

If you're using a ferrite bead to suppress EMI on your circuit board, it's important to choose the right one. You can do this by carefully examining its impedance vs. frequency characteristics and considering dc bias current limitations.

The frequency range of a ferrite bead is called its "resistive band," and its impedance is called its "capacitive band." A ZRX plot (shown in Figure 1b) can help you determine the bead's response region. Ideally, the bead should be in the resistive band to reduce noise, and the capacitive band to suppress EMI.

Some manufacturers provide a frequency-impedance chart that shows the relative frequency range of a particular bead. These charts are helpful because they show you how to select a ferrite bead that will suppress the specific types of EMI you're trying to remove.

They're also a great source of general information about ferrite material and how they vary with frequency, which can be helpful when choosing the best one for your application. I particularly like their explanations of the two basic operating modes of soft ferrite materials.

Choosing the right bead depends on your frequency objectives and other design factors. Most importantly, it's crucial to choose a bead that is designed to suppress noise in its "resistive band." In the case of a ferrite bead with a high DC resistance, this may not be possible.

Another issue with ferrite beads is that their resonance points shift quickly as they are loaded by a direct current. This is different than inductors, which generally have almost no change in their resonance point under DC current conditions.

The resonant peaks are especially noticeable when the ferrite bead is loaded by light current, and they can degrade the system's dc bias regulation. The most efficient approach to reducing the peaking is to use a higher dc bias current, but this isn't always possible in practice.

Finally, if you're designing a high-speed digital device and you want to avoid the risk of a ferrite bead creating new EMI issues, make sure that you don't ignore any guidelines that are given on how they should be used. If you ignore these, your bead will do nothing to prevent EMI problems in your circuit.