Common Mode Choke Query

[rahulksgift]

Hi All,

I have some doubts about ferrite bead and common mode choke,

1) In one of my design i used Common mode choke on power rail(+ve 3.8V and GND).
By mistake GND_IN(ground input of the choke) GND_OUT got shorted, effectively GND is bypassed.

Can Any one suggest me what will be the effect of this , Will Common Mode choke on the power Arm will work as single ferrite bead or inductor?

If it so, is there any change in the Saturation current specified in the datasheet?

Regards,
Rah

 

[WimRFP]

Hello,

Common mode chokes can only accept low common mode current. So if you run significant DC through the choke (without same DC in the other direction), it will saturate.

Manufactures may specify both differential mode current (that is the nominal load current) and the common mode current. Expect the common mode current to be 20...100 times below the differential mode current.

As you shorted one of the windings, the other winding will act as a short also. So by passing DC through the coil (without same DC return current) and short circuiting the other winding, it will result in a non-functioning component.

A normal choke (one winding only) is rated for a certain current. Above that current, inductance will drop rapidly with increasing current.

Common mode chokes have a closed magnetic circuit (for example ring core) and high permeability.

Normal chokes have an open magnetic circuit, are wound on low permeability material, or have at least an air gap to reduce the effective permeability so that the choke can have the DC current without saturation.

 

[rahulksgift]

Thanks sir, Its Really Helped me ,. Can you briefly explain how Common Mode noise generated!!!!!!
How can we measure it, and how we will know CM Choke required in particular path....

Thanks and regards,

RAh

 

[WimRFP]

The common mode current is the sum of all currents in a bundle of wires (for example a cable), don't forget to observe polarity. Differential current in two wires:

Id = (I1-I2)/2

Normally you want that the current that feeds a circuit, returns via the return wire (that is mostly close to the wire that carries the fied current). Example is mains wiring in a single-phase mains circuit. All current through the live conductor should return via the neutral.

As the live current equals the return current (but with opposite sign), the net magnetic flux induced in a core around both live and neutral wire will not induce any net magnetic field. So differential current will not induce net field in magnetic circuit.

When there is common mode current there will be a net induced magnetic flux in a magnetic core that encircles all wires. This is used in a residual current device. If some of the live current returns via safety ground (PE), or your body, the neutral current (N) will be somewhat less then the live (L) current. This results in a common mode current that will generate a magnetic flux in the core that is around the L and N wire. This flux will trip a switch that will cut the power.

For a signal you want the same mostly. The signal current should return via the intended return wire (for example a cable shield or the other conductor in a balanced system). As the return wire is close to the signal wire, there is little produced magnetic field. In addition, due to the small distance between the wire, magnetic interference will not induce differential voltage (between signal wire and return wire).

If there is a common mode component (current and or voltage) that will in the end produce both a magnetic and electric field. If something produces E of H field, it will be susceptible for it also.

By using common mode filters (that is a magnetic circuit around all wires) one can force the return current to go via the intended return path. Of course, the combination of frequency and common mode voltage (voltage*second product) should be such that the magnetic core material doesn't go into saturation.

How to measure common mode phenomena:
Common mode current can be measured directly with a current transformer. Common mode voltage can be measured between the star point and ground of a star resistor network (or capacitor network).

Common mode current exists when (some) part of the signal or supply current doesn't go back via the intended return conductors, but via other paths. If a circuit generates an interfering E-field outside the size of the circuit, this will very likely result in common mode current in wires/cables that enter the circuit.

Common Mode chokes/filters work best when they are at a position where the common mode impedance (that is Ucom/Icom) is small with respect to the common mode impedance of the CM filter.

I would recommend you to consult a good source on EMC.