At any given time, both common mode and differential mode currents may be flowing through a
circuit potentially leading to issues with radiated/conducted emissions, or overwhelming sensitive
circuitry, such as an op-amp or communication signals.
The most common mitigation strategy, perhaps behind careful board and cable placement, is the
use of a filter circuit that presents a high impedance to common mode currents, limiting the amount
the common mode signal that enters or leaves a PCB. X and Y capacitors are commonly used to
shunt the common mode noise away from the circuits of interest, while a common mode choke is
used to attenuate or dampen the noise.
When placing an inductive component such as a choke in the path of current, there are a few key
items one must consider. First and foremost, the downstream circuitry should not be negatively
impacted by the choke. In general, the winding polarity of a common mode choke can be set such
that the net flux in the core is largely canceled during normal operation and the choke appears
‘invisible’ aside from any leakage inductance and winding resistance. This cancellation can be understood with Faraday’s Law and use of the right hand rule – as current flows from the source and into the system through the choke, a voltage on the opposite winding will be developed to oppose the change in flux within the core. However, at the same time, current returning to the source will also pass through the choke and generates its own flux within the core. Assuming that the windings are ‘in phase’, the currents are balanced, and the number of turns are equal, these fluxes (and consequently generated voltages) will be equal in magnitude but opposite in direction, resulting in a negligible net increase in flux density. As an added benefit, because the flux contributions to the core from these differential currents cancel out, the CMC is highly unlikely to saturate during normal operation.


When selecting a common mode choke, winding inductance tells only part of the story. Each
parameter must be carefully considered with other non-ideal factors, such as DCR, and system level
considerations such as cooling, mechanical restraints, and thermal constraints to ensure customers
reach timely success during compliance testing and products are successfully released to the
market.