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Design                                                                                              Design



 corner point of the enclosing amplitude spectral density is needed first:  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
 (Eq. 3)
            of  the  interference  emission.  In
 The first corner frequency of the enclosing amplitude spectral density is analogous to this:  the frequency range of 0.15 MHz to
            0.5  MHz,  it  defines  a  permissible
            quasi-peak  weighted  interference
 (Eq. 4)    level of 66 dBµV to 56 dBµV. Figure 2
            shows the result of the measurement
 The amplitude of the first harmonic can be determined from this:  of the conducted radio-interference

            voltage  of  this  SMPS  without  line
            filter.
                                                    Figure 2: Radio-interference voltage of an SMPS without line filter

            The measurement shows that a line filter is absolutely essential.
 (Eq. 5)

 Based  on  the  assumption  that  the  parasitic  coupling  capacitance  (C )  between  the  SMPS  and   DESIGN OF A LINE FILTER
 P
 ground is 20 pF, the first harmonic common-mode current can now be determined:  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
 (Eq. 6)
            consists of a manganese-zinc ring core,
            on which there are two geometrically
 The radio-interference voltage is measured using a line-impedance–stabilization network (LISN)   separated windings wound in opposite
                                                    Figure 3: Single-phase line filter
 and  an  EMC test  receiver.  Due to the  parallel  connection  of the  50-Ω  input  impedance  of the   directions. Figure 4 shows the design
 EMC test receiver and the 50-Ω output impedance of the LISN, a total impedance (Z) of 25 Ω is   of  the  WE-CMB.  In  this  case,  the
 produced. The measured radio-interference voltage (V ) can now be calculated:  WE-CMB acts like a filter coil, which
 CM
            counteracts the current and reduces
            its  amplitude.  A  common-mode
            choke  with  an  as-low-as-possible
 (Eq. 7)
            self-resonance  frequency  (SRF)  in
            the  lowest-frequency  range  should

 Converted to decibel-microvolts, this gives:  be selected because the SMPS used
            here  switches  with  very  low  pulse
            frequency.  Low  SRF  causes  high
            attenuation  in  the  lower-frequency
            range.                                  Figure 4: Design of the WE-CMB
 (Eq. 8)



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