Behind The Mystery of Electromagnetic Compatibility Design 10
The result of the calculation is that high interference emissions can
be expected. For example, the product family standard EN 55022
can be used here for assessment of the interference emission. In
the frequency range of 0.15MHz to 0.5MHz, it defines a permissible
Quasi-Peak weighted interference level of 66dBµV to 56dBµV. Figure
2 shows the result of the measurement of the conducted radio
interference voltage of this switch-mode power supply without line filter.
Design of a Line Filter
Figure 3 shows the schematic design of a simple single-phase line
filter. Würth Elektronik provides various models of line chokes, such
as the WE-CMB series, for the construction of line filters. A line choke
basically consists of a MnZn ring core on which two geometrically
separated windings are wound in opposite directions. Figure 4
shows the design of the WE-CMB. In this case, the WE-CMB acts as
a filter coil that counteracts the current and reduces its amplitude.
A common mode choke with as-low-as-possible SRF in the lowest
frequency range should be selected because the switch-mode power
supply used here switches with very low pulse frequency. Low SRF
causes high attenuation in the lower frequency range.
The measurement shows that a
line filter is absolutely essential.
Figure 2: Radio interference voltage of a switch mode power supply without line filter (Source: Würth Elektronik)
The measurement
shows that a line
filter is absolutely
essential.
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