Würth Elektronik 2022 13
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WE-SD Rod Core
Inductors+
WE-UKW EMI Suppression
6-Hole Ferrite Beads
Figure 7: Radio interference voltage with line filter without WE-CMB (Source: Würth Elektronik)
Error due to dispensing with Line Choke
It is often tried at the beginning to only use X and Y capacitors to
suppress interference in order to dispense with a common mode
choke. However, this is contrary to the principle of the line filter of
counteracting the interference current by adding a high-impedance
filter element. The radio interference voltage using the same filter
without common mode choke has been measured as an experiment.
Figure 7 shows the measurement.
As expected, the interference emission in the lower frequency range
increases strongly without the WE-CMB line choke. At 200kHz, the
quasi peak shows a value of approximately 78dBµV and the average
shows a value of 60dBµV. The permissible interference level is
exceeded up to 600kHz both by the quasi peak and by the average
measurement. A line filter without a line choke is inadequate.
Conclusion
A line filter for switch-mode power supplies without a common mode
choke is not sufficient. Individual capacitors are not sufficient to
completely suppress the interference emission. Additional longitudinal
coils before the line filter help if further suppression of the differential
mode interference is required. Using a line filter, all interference levels
are below the permissible limit value and the switch-mode power
supply can pass an EMC test. ■
A line filter for switch-mode power supplies
without a common mode choke is not sufficient.
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