Why Use Ferrite Cores?

This page explains the benefits of ferrite cores and the reasons to use them.

[Reason 1] You need simple suppression that does not require grounding

There are many noise suppression components that use capacitors, but because they work by discharging the noise to the ground, they are ineffective without a stable ground. Ferrite cores, on the other hand, can suppress noise without grounding.

When a cable is passed through a ferrite core, the magnetic fields generated by the cable concentrate inside the core due to the ferrite's ability to attract magnetic flux (μ”). The magnetic energy concentrated in the ferrite is converted to heat and dissipated by the magnetic loss of the ferrite, preventing emission as noise. In this way, noise suppression can be achieved without grounding. 

The magnetic flux of the noise current is reduced under the influence of high impedance.

 図


Supplemental information

 

The impedance of the ferrite core can be expressed by the following equation.

|Z|=R+jX


  • The X component (reactance) attenuates high-frequency signals by reflection
  • → X=ωLx : Lx∝μ’
  • The R component (resistance) dissipates the noise as heat
  • → R=ωLr : Lr∝μ”
             
When ferrite cores are used for prevention of EMI (electromagnetic interference), this R component is adjusted to be as large as possible in the noise frequencies.

* There are also applications (such as transformers) that take advantage of a small R (loss) component.
 

[Reason 2] Ferrite cores work against both normal-mode and common-mode noise

Normal-mode noise

In some cases, the noise current is superimposed on the signal current.

The signal current is generally low frequency, while the noise current in most cases is high frequency.

To reduce noise current, it is desirable to give much greater resistance to the noise current than the signal current. The noise current magnetic flux (the noise current itself) can therefore be reduced by passing the signal line though a ferrite core, which has a high degree of resistance, and where magnetic flux is generated by the noise current.

It works the same as the BLM!

BLMと同等のはたらき!   BLMと同等のはたらき!

Common-mode noise

Differential currents, or currents with different directions and different phases, exist in some cases.

They exist in cases in which the directions of the currents are reversed, and the noise affects both cables the same way (common noise).

In such cases, by passing the two cables through the ferrite core, the magnetic fluxes generated by the signal currents cancel each other out, and no resistance is imparted. On the other hand, since the magnetic fluxes generated by the noise currents do not cancel each other out, they are reduced in response to the ferrite's resistance (magnetic resistance), thereby reducing the noise current. Since the impact on the signal current is small, this approach is useful when you want to avoid dulling the waveform.

It works the same as the DLP/DLW!

DLP/DLWと同等のはたらき!   DLP/DLWと同等のはたらき!

[Reason 3] It can be retrofitted

No changes to the circuit board are required. This makes it possible to take countermeasures against noise just before the start of mass production.

In terms of functionality, ferrite cores are equivalent to chip ferrite beads (the Chip EMIFIL® BLM series) and chip common-mode choke coils (DLW/DLP), which are SMD components. The major difference is that no mounting on the circuit board is required. In other words, the advantage of ferrite is that it is easily retrofitted as a final countermeasure against noise.

Basically, noise countermeasures are implemented first using SMD components.

However, add-on parts such as ferrite cores are used when SMD noise countermeasures are impossible in the final product stages.