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CHAPTER 7: FILTER DESIGN AND REALIZATION
Gain [dB]
60
40
20
0
−20
−40
−60
Frequency [kHz] Attenuation required [dB] Resulting f0 [kHz]
200 20 63
300 25 71
700 35 93
1000 35 133
Table 7.8: Attenuation for crucial points in DM input emissions
Fundamental Switching frequency and harmonic clusters
Preliminary calculations considering only attenuation requirements
Filter response after adjustemt to avoid switching frequencies
0.1 1 10
Frequency [kHz]
100 1000
Figure 7.22: Bode plot of input DM filter
in Fig. 5.8 and 5.9. The same table, with these attenuations and resulting filter cut-off
frequencies calculated again with expression 7.2.1, has been filled in for specific rele-
vant frequencies, resulting in Table 7.8.
The filter resonance frequency has been chosen at 65kHz and once again, as it can be
seen in Fig. 7.22, it falls between the 5 th and 7 th switching frequency harmonic clusters.
This time, though, the fundamental frequency of 400Hz of the aircraft grid is much
closer to the switching harmonic clusters, so more care must be taken when position-
ing the filter cut-off frequency as more switching harmonics can be easily amplified
and could create problems this time too. Following the design guidelines used for the
output filter, the same filter values can be used: 9.4mH for the inductors and 68nF
for the capacitors. Fig. 7.23 emulates the insertion of the designed input DM filter by
adding the filter response to the measured DM emissions, as it has been done in the
previous chapter for the common mode output emissions, providing an indication on
how the filter should perform. Because in the output filter emissions measurements it
has been noted that the filter will not attenuate after roughly 1Mhz, this behaviour has
been included in the calculations, keeping constant the attenuation for frequencies af-
ter that point, to provide a more accurate emissions estimation. This filter has not been
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