CHOKES 2~/3~ (EMI)

Common mode chokes are used for the suppression of common mode interference or common mode noise (CMN). They are in applications, like SMPS, which generate excessive high frequency noise. It is to avoid the conduction of the noise into the mains. 

Two or more windings are applied on the same core. They are connected the way that the load current passing through the separate windings causes opposing magnetic fields. Thus the magnetic flux in the core yields zero. This way the full inductance is available for the common mode noise signal. High permeability (µi) materials can be used which is mostly ferrite. Attention has to be paid to the fact that incomplete current compensation saturates the high µi core material. 

The advantage of using toroids is a minimized stray field and small, compact dimensions.
For winding separation we use special plastic separators or cable ties. 


Please note that the quality of a choke is not only determined by its rated inductance value:

  • The higher the inductance or turns on the same core the earlier the choke is driven into saturation by EMI Noise (especially pulses)
  • More layer chokes have a coupling capacitance which makes them less effective at high frequencies

Regarding this, two chokes of equal core dimension, where one which is rated at 2*1mH may be more effective than the other rated at 2*10mH. 

Common mode chokes may also be used as 1:1 transformers. See section transformers 



With the selection of certain core materials the type of application can be divided in different groups

Common mode chokes for EMI Noise Filter at frequencies ≤ 10MHz

These are our most sold common mode chokes. With that ferrite material (X) most of the common mode noise (CMN) is blocked by the choke and a small part is being absorbed. Single layer chokes of this type may be well effective at higher frequencies. 

Areas of application:

  • EMI suppression of SMPS, both, to the mains and to the load
  • EMI suppression of frequency converters, both, to the mains and to the load
  • EMI suppression of clock pulses of microprocessors as well as their harmonics on power, signal or transmission lines
  • Improving immunity against CMN, such as fast transients (burst) and radiated RF

Common mode chokes for EMI Noise Filter at frequencies ≥ 5MHz

These chokes (U-material) are used for suppression of common mode noise (CMN) above approx. 5MHz. The available core sizes are about the same as of the CMN chokes for lower frequencies (X-material). 

The ferrite material is high resistive and lower permeability (µi). Therefore the rated inductance will be lower than the ones with our X-material. Yet, a good attenuation will still be achieved. It is more important that there is a good attenuation over a wide frequency range. 

Because of the high frequency range the choke should be wound with only one layer in order to minimize capacitive coupling. Also it is better to use these chokes in open versions. Potting material acts as a dielectric and increases the capacitive coupling. 

Please note that at these frequencies the position of the choke i. e. on the PCB and the routing of the conductors are very important. The effectiveness of the choke can easily be reduced or eliminated by bypassing it. 

Areas of application:

  • EMI suppression of CMN up to approx. 300MHz on power, data and signal lines
  • Improving immunity against CMN, such as fast transients (burst), radiated RF, symmetrically operated data transmission and signal circuits like RS485, RS422, current loop, CAN bus etc.
  • In combination with CMN chokes for lower frequencies a good attenuation over a broad frequency band will be achieved
  • 1:1 transformer of high frequency signals such as fast pulses etc.

Common mode chokes for EMI Noise Filter for special applications

Amorphous or nanocristalline material (Z) in common mode noise (CMN) chokes enables an even broader use. Chokes with a certain inductance on a ferrite core can be realized on a smaller core with Z-material. 

However, according to the high permeability (µi) there will be a higher flux density which leads the core into saturation quicker. It is important that the CMN signal through each winding is the same. It is suggested to place an X-capacitor in close proximity to the choke. 

Areas of application:

  • inductive heating
  • appliances with very limited space
  • see also Common Mode Chokes for EMI Noise Filter at frequencies ≤ 10MHz


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