What Is a Common Mode Choke?
A common mode choke is a filter designed to reduce a specific type of noise known as common mode noise.
Common mode noise is noise that travels over power and signal lines, characterized by its flow from the reference ground, through the wiring substrate, and back to the reference ground.
The core component of a common mode choke is an inductor, which effectively interrupts the flow of this noise.
Common mode chokes are essential in noise suppression and electromagnetic compatibility (EMC) measures for electronic equipment.
Uses of Common Mode Chokes
Common mode chokes are implemented in many electronic devices as part of EMC countermeasures, ensuring normal operation even under electromagnetic interference. EMC, or electromagnetic compatibility, refers to a device’s ability to neither emit nor be affected by electromagnetic interference. Such measures are critical in a wide range of electronic equipment, including home appliances, office automation equipment, and vehicle machinery.
Specifically, common mode chokes are utilized to eliminate common mode noise, as described below.
Principles of Common Mode Chokes
Conducted noise transmitted through power and signal lines is classified into two types: differential mode noise and common mode noise.
In an electrical circuit, the current generated in one part of the circuit passes through the load and returns through another path. This is known as differential mode, and noise that flows in this manner is called differential mode noise.
Conversely, in common mode, the current does not loop through the circuit but instead flows due to capacitance between the reference earth and the wiring on the board, returning to the reference earth from the opposite side.
High-frequency noise, in particular, can transmit as common mode noise due to lower impedance at these frequencies.
Common mode chokes are filters designed specifically to eliminate this type of noise. They consist of two conductors wound around a core. When currents flow in opposite directions (differential mode), their magnetic fluxes cancel each other out, and the choke does not filter. However, when currents flow in the same direction (common mode), the magnetic fluxes reinforce each other, acting as an inductor to filter out the noise.