Slide 1 Slide 2 Slide 3 Slide 4 Slide 5 Slide 6 Slide 7 Slide 8 Slide 9 Slide 10 Slide 11 Slide 12 Slide 13 Product List
common-mode-Slide6

As stated previously a common mode choke is really just a dual winding magnetic. As shown in the top left diagram the line current, shown in red, flows through the two windings, shown in yellow, and this creates a magnetic field within the magnetic core, shown in blue. However, because the magnetic field created by each winding oppose one another there is effectively no net field within the core. The common mode magnetic must provide the required inductance/impedance with a low enough winding resistance to ensure the part does not have excessive losses or overheat. However, because there is no net field there is no concern with saturation. This is further illustrated in the reluctance model in the top right corner. There are two opposing sources in the magnetic model, (Iline) times the number of turns (N) and Ineutral times the number of turns (N) essentially creating a no DC flux in the core. Because there is no need to handle a DC offset or store energy the common mode choke typically is designed using a high-perm, ungapped ferrite core. As a design example a 1.6mH, 2A common mode choke was created using a 0.6” OD, 5K perm, ferrite core with 25 turns of 26 GA wire per winding. This design would yield a resistance of 65mΩ (520mW of loss) and be approximately 2.89 cm³.

PTM Published on: 2014-10-17