Chokes, Noise - Common Mode & Differential

Background – Common Mode and Differential Mode Noise

We shall have a power supply input to the system we are working on. In this case, the power signal input to the system should return back the same without any additional noise. If any additional noise adds on to the ground plane, then there is a differential noise created. When we say additional noise is appended to ground plane, this could be because of improper ground plane on the board where different noise gets coupled. Differential noise need to be eliminated from the system. Otherwise, this might couple back to the power supply resulting in a radiation from the power supply lines. This is the main reason for the conducted emissions from the board. We have to remember that this is always at frequencies less than <30MHz. We have several compliance standards defined for this. The major sources of common mode noise are motors, unwanted radio signals and they enter the system causing interference. Y- capacitors on each line with other end connected to shield acts as suppressor for common mode noise.

The common mode noise scenario is different from differential mode noise scenario. In common mode scenario, the noise couples equally on both the line and ground paths from the input. This needs to be eliminated at the input without propagating further to the internal circuit. For this a common mode choke is required at the input. The functional requirement of common mode choke is to eliminate the noise coupled by providing high impedance and dissipate it as heat. X-capacitor works as differential mode suppressor.

Different types of Chokes:

Chokes are Common Mode and Differential Mode type. When we say choke, this is a simple inductor. These chokes are available in different constructions but the basic principle is an insulated wire wound on a magnetic core to maintain the coupling. Chokes that have both the properties of Common Mode and Differential Mode rejection are also available.

Notes on Common Mode Chokes

• Helps in EMI filtering of Power and signal lines

• For a Common mode choke, there is a winding run around a magnetic core. The magnetic core helps to couple the windings

• Common mode choke helps to eliminate noise by providing high impedance at those frequencies. This is same as bead functionality.

• Choke impedance increases with frequency

• Common mode chokes work as simple inductor for the common mode signals while they work as pass by for the differential mode signals

• The signal distortion is less when choke is used on high frequency signals, this is why they are preferred as filtering mechanism in high frequency circuits.

• You see scenarios where use common mode chokes on RS232, Ethernet, USB, CAN, power supply inputs. 

• Common mode chokes are available for very high currents also up to 200A

• Common mode chokes are available in various ranges from nH to H

• Common Mode Chokes are available in different mounting options like SMD/SMT, Radial, Through Hole

• Murata, EPCOS, TDK, Wurth Electronics, Coil Craft, Pulse, Panasonic, Nexperia, TDK-Lambda, Vishal are all different vendors of the Common Mode Chokes. Schurter, Schaffner are some of the vendors who has very high current rated chokes. 

Specifications of Common Mode Chokes:

• Inductance

• Impedance

• Maximum DC Current

• Maximum DC Resistance

• Maximum Rated Voltage/ Maximum Withstanding Voltage

• Rated Current

• Package/Mounting Type

• Operating Temperature/Storage Temperature

The withstanding voltage shall be more than 2 times of rated voltage for a specified time.

How does Common Mode Chokes work?

As common mode current increases, the magnetic flux produced inside the core and hence increasing the impedance. 

Why Common Mode Chokes does doesn’t work for suppression of differential mode signals?

The common mode chokes are constructed in such a way that when differential currents pass, the magnetic flux is cancelled out this doesn’t cause any increase in impedance.

Notes on Differential Mode Chokes:

• Inductors, 3-terminal capacitors, X-capacitors work as differential mode suppressors

Disadvantages of using chokes:

• Costly 

• Occupies board space

• Resistance causing drop (Minimal though) - Losses