Fig. 3 shows the circuit of the filter topology; common
mode (CM) and differential mode (DM) noises are dealt
with different portions of the filter. The equivalent circuits
of the CM and DM parts of the filter are depicted in Fig. 4
and Fig. 5, respectively. The noise voltage, measured from
the 50 Ω resistors contains both common-mode (CM) noise
and differential-mode (DM) noise. Each mode of noise is
dealt with by the respective section of an EMI filter. For Fig.
4 and Fig. 5, it is noticed that some elements of the filter
affect only common-mode (CM) and differential-mode
(DM) noises whereas some affect both common-mode
(CM) noise and differential-mode (DM) noise. The
capacitors CX1 and CX2 affect differential-mode (DM)
noise only. An ideal common-mode choke LC affects
common-mode (CM) noise only, but the leakage inductance
Lleakage between the two windings of LC affects
differential-mode (DM) noise. CY suppresses both
common-mode (CM) noise and differential-mode (DM)
noise, but its effect on differential-mode (DM) noise
suppression is practically very little because of the
relatively large value of CX2 .Similarly, LD suppresses
both differential-mode (DM) and common-mode (CM)
noises, but its effect on common-mode (CM) noise is