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Design Design
ERROR DUE TO DISPENSING WITH LINE CHOKE the coils. If attenuation of
It is often tried at the beginning to only use X and Y capacitors to suppress interference in order to 40 dB/decade would be
dispense with a common-mode choke. However, this is contrary to the principle of the line filter required from each coil, the
of counteracting the interference current by adding a high-impedance filter element. The radio- calculation should be made
interference voltage using the same filter without common-mode choke has been measured as an with a corner frequency of one-
experiment. Figure 8 shows the measurement. tenth of the pulse frequency.
As expected, the interference The already-used X value of
emission in the lower-frequency the X capacitors can be used
range increases strongly without the Figure 9: Line filter with WE-CMB and WE-TI HV for the calculation of the coil:
WE-CMB line choke. At 200 kHz,
the quasi peak shows a value of
approximately 78 dBµV and the
average shows a value of 60 dBµV.
The permissible interference level is
exceeded up to 600 kHz both by the (Eq. 11)
quasi peak as well as by the average
measurement. A line filter without a As the coils for the differential-mode current are in series, the calculated value is divided by 2. The
line choke is inadequate. next larger inductance value of a WE-TI HV would be 470 µH. It should be ensured for the selection
Figure 8: Radio-interference voltage with line filter without WE-CMB
of a differential-mode coil that its rated current (I ) is far above the rated current of the SMPS input.
R
ADDITIONAL DIFFERENTIAL-MODE FILTER
If the differential-mode suppression using the WE-CMB and the X capacitors is not sufficient, an RESULT OF THE LINE FILTER
additional differential-mode filter consisting of two coils connected in series helps. Figure 9 shows In conclusion, a line filter for switch-mode power supplies without a common-mode choke is not
the design. 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-
The Würth Elektronik coils WE-TI HV and WE-PD2 HV or the WE-SD series are ideally suitable mode interference is required. Using a line filter, all interference levels are below the permissible
for the differential-mode suppression. The WE-UKW series is recommended in the case of high- limit value and the SMPS can pass an EMC test.
frequency interference. The Thomson oscillation equation can be used again here to calculate
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