Small black and white electronic components on white surface.

EMC Mitigation: The Finer Points of Ferrite Beads

Mitigating electromagnetic compatibility (EMC) issues can be a complex and daunting task. So far on this blog, we’ve covered EMC mitigation techniques using two common components: Resistors and inductors. Now let us consider the role of ferrite beads and how they can be used to improve EMC performance so that your design satisfies EMC standards and regulations.

What Are Ferrite Beads and What Do They Do?

Ferrite beads are passive components that are widely used in EMC to suppress high-frequency noise by absorbing and reflecting it and dissipating some energy as heat.

Used properly, ferrite beads can selectively attenuate high-frequency aspects of the signal without affecting the more important lower-frequency elements. Used improperly, ferrite beads can invite some detrimental issues, such as increased reflections and ringing leading to data integrity and electromagnetic interference (EMI) issues.

Therefore, this little component can make a big difference in system design. Read on to learn how you can make ferrites work to your advantage for noise suppression and EMC mitigation.

Choosing the Right Ferrite Bead for EMC Mitigation

There are many considerations when selecting the right ferrite beads for specific EMC applications. Here we will help you demystify some of the properties of ferrite beads so you can ensure the best noise suppression possible for your design.

  1. Determine noise frequency (or frequency band).
  2. Determine the required noise attenuation.
  3. Select a bead that can handle the required current without saturating.
  4. Consider the DC resistance.

1. Determine Noise Frequency (or Frequency Band)

The frequency of the noise that needs to be suppressed is a critical factor in selecting a ferrite bead. Ferrite beads are most effective at suppressing high-frequency noise, typically in the range of tens of megahertz to several gigahertz. Lower-frequency noise may require other EMC components, such as inductors.

To choose the correct ferrite for a noise frequency, we must first identify the crossover frequency. The crossover frequency is the point where the resistance becomes greater than the reactance. Put another way, the crossover frequency of a ferrite bead is the frequency at which its impedance changes from being mainly reactive (inductive) at low frequencies to being mainly resistive at high frequencies. This is demonstrated in Figure 1. This frequency varies depending on the physical characteristics of the ferrite bead, such as its size, shape, and material composition.

If the frequency of the noise you wish to mitigate is less than the crossover frequency, the bead is inductive, which can cause reflections and ringing in the circuit and may lead to EMI issues. When the frequency of noise is greater than the crossover frequency, the bead is resistive. In this case, the RF energy is dissipated as heat and successfully removed from the circuit. 

 

 

Figure 1. Impedance curve of Murata BLM15HD182SN1 showing crossover frequency [1]

2. Determine the Required Noise Attenuation

When searching for ferrite beads you will typically see that the manufacturer only specifies the impedance at 100Mhz. However, this only tells part of the story. In Figure 2 both devices have a specified impedance at100 MHz = 1800 , but their impedance curves are drastically different, allowing different levels of attenuation at different frequencies. Therefore, to fully understand whether a ferrite bead will offer the required attenuation, it is important to review the impedance curves. This adds to the research time to find the right ferrite but is worth the effort!

Figure 2. Different impedance curves for devices with the same impedance @100 MHz [2] 

3. Select for DC Saturation

When a ferrite bead is exposed to a high DC voltage, it can overload the magnetic properties of the ferrite material so it cannot absorb any more energy, a failure known as DC saturation. At this point, the impedance of the ferrite bead drops to a very low value, and it is no longer effective at attenuating noise.

Manufacturers typically show impedance curves for 0 mA DC bias. As can be seen in Figure 3, the curves become less effective in suppressing noise as the DC bias increases.

 

Figure 3. Effect of DC bias current for a TDK MPZ1608S101A bead [3]

 

 

Ferrite beads are typically rated for a maximum current, but even below this max current there can be saturation of the ferrite bead. That’s why, as a rule of thumb, I typically use a ferrite bead where the specified max current is greater than twice the highest current in the trace, to avoid saturation.

4. Consider DC Resistance

The DC resistance of a ferrite bead is the resistance of the bead when a DC voltage is applied. It is typically very low, ranging from a few milliohms to a few ohms, but not insignificant!

Take power supply as an example. Filtering high currents through the ferrite may cause voltage drops that could be significant for low-voltage applications. Let’s look at one specific ferrite bead to understand this better. The typical resistance of BLM15HD182SN1 is 2.2 Ohms with a max DC current of 200mA. So, using the rule of thumb above, in an application of 100mA, this could result in a significant drop of up to 220mV.

Again, it’s rare for the DC resistance to be this significant, but best practice is always to check.

Further Reading

Need to know more about noise frequency, noise attenuation, DC saturation, or DC resistance? Here are some helpful resources to examine these characteristics.

Murata: SimSurfing

Wurth: Redexpert

References

[1] Murata, BLM15HD182SN1 - https://www.murata.com/en-sg/products/productdetail?partno=BLM15HD182SN1%23

[2] Murata, SimSurfing: https://ds.murata.co.jp/simsurfing/blm.html?lcid=en-us&md5=791a09fca963dce3d94b57316d67aed6

[3] Limjoco and Eco (2016), Ferrite Beads Demystified - https://www.analog.com/en/analog-dialogue/articles/ferrite-beads-demystified.html