A common mode choke is an essential component in most switch mode power supplies. Unlike a differential mode choke, a common mode choke is designed to pass differential mode current while blocking common mode noise. The windings of a common mode choke are connected out of phase so the impedance of the core is only seen by the common mode noise. A good common mode choke will deliver high attenuation of common mode noise in the frequency range of interest, typically 10 kHz to 10 MHz, and handle the full RMS power without overheating. Isolation is also an important design criteria for common mode chokes.


Most common mode chokes are designed using toroidal ferrite cores. Toroidal cores have the advantage of very low winding capacitance. Ferrites provide excellent noise attenuation over a broad frequency range and they are a low cost solution for most common mode choke designs. The winding is normally solid wire, but litz wire can be used to reduce high frequency winding losses. The design of a common mode choke relies on the successful choice of a core to provide attenuation. Most common mode chokes are PCB mount or surface mount.   West Coast Magnetics has designed a number of standard common mode chokes.


If you can’t find the common mode choke that is ideal for your application, West Coast Magnetics can design a custom common mode choke for you. Whether you need high attenuation in a small package, a specified creepage and clearance, or have a unique spec requirement we have a common mode choke design for you.   Here is an example of one of our custom common mode choke solutions.