How Ferrite Beads Work and How to Choose the Right One

July 1, 2017 Altium Designer

Snap-on ferrite

 

 

Sometimes I wish I could see electromagnetic waves. It would make detecting EMI much easier. Instead of having to mess around with complicated setups and signal analyzers, I could just look and see what all the fuss is about. While you may not be able to see EMI, you can sometimes hear it when it comes through audio circuits. One possible fix for that kind of interference is a ferrite bead. Unfortunately, ferrite beads can be a bit of a mystery. In order to use them properly, you’ll need to understand their electromagnetic characteristics and how they change during use. After you’ve got a handle on the theory behind ferrites you’ll need to carefully select one for your board. If you don’t you could end up causing more problems than you fix.

 

The Theory Behind Ferrite Beads

Ferrite beads are used to attenuate high-frequency signals. That might tempt you to think of them as an inductor, but they’re a little more complex than that. The simplified circuit model of a ferrite bead will help you understand its frequency characteristics. However, these attributes can change as a function of current and temperature.

 

A ferrite bead can be modeled as an inductor, capacitor, and resistor all in parallel, with a series resistor in front of them. The series resistor represents resistance to DC currents. The inductor is the primary component for attenuating high-frequency signals. The parallel resistor signifies AC current losses. The capacitor shows a parasitic capacitance. When looking at the impedance vs. frequency curve of a ferrite bead you can see that the impedance, which is mostly resistive, is extremely high in only a thin band. That section is where the inductance of the bead dominates. Above that band, the parasitic capacitance takes over and rapidly reduces high-frequency impedance.

 

Beads are usually rated for a certain DC current. Amperages above the specified value can damage the component. The troublesome thing is that this limit is drastically affected by heat. As temperature increases the rated current quickly decreases. Rated current also affects the ferrite’s impedance. As DC current increases, a ferrite bead will “saturate” and lose inductance. At relatively high currents, saturation can reduce impedance by up to 90%.

 

ESD in air
Load current can change the impedance of your ferrite.

 

How to Choose the Right Bead

Now that you’ve got the ferrite theory under your belt, it’s time to choose one for your circuit. This is not very difficult, you just have to pay attention the bead’s specifications.

 

Many people know that ferrite beads “attenuate high frequencies.” However, beads do not act like a wideband low-pass filter, since they can only help you stop a specific range of frequencies. You must choose a ferrite where your undesired frequencies are in its resistive band. If you go a little too low or a little too high the bead will not have the desired effect.

 

You should also see if the manufacturer can provide you with the impedance vs. load current curves for the bead. If your load currents are very high, you’ll need to select a bead that can handle them without saturating and losing its impedance.

 

Cautions

Ferrite beads are essentially resistive loads at high frequencies, which means they can cause a few problems in your circuit. When placing a bead you’ll need to think about voltage drop and heat dissipation.

 

In the days of higher voltage circuits, voltage drop wasn’t a big deal. Now we have lots of low power circuits that can use voltages down around 2 V. At those levels, you can’t afford to lose much. Ferrite beads necessarily cause a DC voltage drop in your circuit. It may not seem like much, but if your integrated circuits (ICs) have a short high-current draw state, the loss could become significant. Place your ferrite beads where they won’t cause voltage drop issues.

 

Since ferrite beads are resistive at high frequencies they primarily dissipate the absorbed energy as heat. This heat isn’t necessarily a problem, but if you forget about it when designing your active or passive cooling system, it can become one. If your system is especially noisy and the bead will be absorbing lots of high frequencies this heat could become more of an issue. Make sure to take the bead’s heat dissipation into account.

 

Thermometer at 45 degrees Celsius
Load current can change the impedance of your ferrite.

 

 

Ferrite beads can be quite useful, but only if you understand exactly how they work. Remember that they attenuate signals in a fairly small band, and their effectiveness depends on temperature and load current. In order to best use a ferrite bead, you should make sure it meets your exact specifications. Then, when placing the bead, be sure to take voltage drop and heat into account.

 

Dealing with things like ferrite beads can be difficult, but designing your circuit doesn’t have to be. Altium Designer is state of the art PCB design software with tools that can help you build the optimal circuit. It even has add-ons like the power delivery network analyzer, which can help you deal with problems like voltage drop and heat dissipation.


Have more questions about ferrite beads? Call an expert at Altium.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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