Active ripple filtering is the replacement of large passive components
in power filter circuits with smaller passive components and active
control circuitry. This letter focuses on common-mode filters, where a
large common-mode inductor (choke) is replaced by two smaller chokes and
active op-amp control. The technique is appropriate when improved
attenuation is required at relatively low frequencies and the
high-frequency filtering requirements are easily met. Smaller chokes
save significantly in material and winding costs. The technique is more
advantageous if wire-wound chokes can be replaced by planar printed
circuit board chokes. The use of the technique in an automotive
electromagnetic interference (EMI) filter application is explored in
detail.