Datasheet

RUW15 Series
Isolated Ultra-Wide Input DC/DC Converters
KDC_RUW15.B02 Page 8 of 13
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EMC FILTERING AND SPECTRA
FILTERING
The module includes a basic level of filtering, sufficient for many applications. Where lower noise levels are desired, filters can easily be added to achieve any required
noise performance. A DC/DC converter generates noise in two principle forms: that which is radiated from its body and that conducted on its external connections. There
are three separate modes of conducted noise: input differential, output differential and input-output.
This last appears as common mode at the input and the output, and cannot therefore be removed by filtering at the input or output alone. The first level of filtering is to
connect capacitors between input and output returns, to reduce this form of noise. It typically contains high harmonics of the switching frequency, which tend to appear
as spikes on surrounding circuits. The voltage rating of this capacitor must match the required isolation voltage. (Due to the great variety in isolation voltage and required
noise performance, this capacitor has not been included within the converter.)
Input ripple is a voltage developed across the internal Input decoupling capacitor. It is therefore measured with a defined supply source impedance. Although simple
series inductance will provide filtering, on its own it can degrade the converter stability. A shunt capacitor is therefore recommended across the converter input terminals,
so that it is fed from a low impedance.
If no filtering is required, the inductance of long supply wiring could also cause a problem, requiring an input decoupling capacitor for stability. An electrolytic type will
perform well in these situations. The input-output filtering is performed by the common-mode choke on the primary. This could be placed on the output, but would then
degrade the regulation and produce less benefit for a given size, cost, and power loss.
The metallic case of the product is connected internally to output 0V and as such also has reinforced isolation to the input. Tinned tabs on the casing may be soldered
to PCB slots to provide mechanical support and extra electrical connection to the casing to optimise EMC shielding. Rail standard EN50155 references EN50121-3-2 for
conducted EMI which is less severe than EN55011 so the RUW15 also meets these levels comfortably.
The RUW15 has been evaluated to meet the requirements of EN55011 curve B with an external filter as shown. The limit line shown is quasi peak curve B.
EMC FILTER AND VALUES TO OBTAIN SPECTRA AS SHOWN
C1
L1
C2 C3
DC
DC
C4
C5
C1, C3 1μF Polyester or ceramic capacitor
C2 100μF Electrolytic capacitor
C4 & C5 4.7nF 250 VAC Y Rated
L1 4.7µH Common-mode choke (Murata part number 52106C)
The following filter circuit shows the input filter typically required to meet CISPR22 Quasi-PeakCurve B.