"Bifilar winding of power leads is a two edged sword. While it reduces
core
flux from power line currents, it also reduces differential suppression in a
similar fashion. In a tightly coupled choke, the windings simply behave like
a transmission line. It takes a stand-alone choke in each conductor to add
significant
differential mode impedances, or a poorly coupled common core choke. You
will not obtain that with a bifilar or trifilar winding."
For a long time, Corcom has recognized this for use in their power-entry
EMI/RFI filters. Their high-performance line filters use a combination of
common-mode and differential-mode components. In the link below, note the
use of additional DM filtering in their FC10 product. The FC10B model uses
identical CM filtering but lesser differential mode filtering at a reduced
price. These filters will typically cure most any junk that appears on AC
lines of modern household appliances, including switched variable-speed
motor drives.
http://www.mouser.com/ds/2/418/NG_CS_1654001_CORCOM_PRODUCT_GUIDE_FC_SERIES_0611-370733.pdf
Paul, W9AC
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