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CHAPTER 7: FILTER DESIGN AND REALIZATION
I[dBµA]
100
80
60
40
20
0
−20
J core
Limit Cat L
−40
0.1 1 10
Frequency [MHz]
Figure 7.5: Emission with DM filter using J material
7.4 DM output filter implementation
The DM filter with the aforementioned components has been built according to the
design methodology explained in section 7.3.1, however it presented immediately two
critical issues. First of all, even if a certain amount of attenuation has been introduced
by the filter, the DM output EMI, as shown in Fig. 7.5, still did not met the standards
limitation; furthermore the converter output current showed a clear indication of ex-
treme distortion and presence of high frequency interference. After detailed investi-
gation the problem was found in the ferrite toroid, that was not behaving as expected
because the filter capacitors caused large current transients, that caused saturation on
the cores with loss of their filtering capability. Therefore these inductors have been
replaced by another type of inductor, made of an amorphous alloy, with roughly the
same value (1.4mH) and the obtained output currents and DM emissions have been
compared.
Fig. 7.6 shows the HF emissions of the same output current when the DM filter has the
inductors made of the J ferrite and when they are made of the AMCC amorphous alloy.
There is a clear lack of attenuation for the ferrites at high frequency, but what is more
important, can be observed in Fig. 7.7 and 7.8. The output current waveform has peaks
that are extremely high when the ferrite solution is adopted, indicating that it is not
properly working, and this can be due only to saturation. Because of the advantages
of the new adopted material, later described, the ferrites have been ruled out for the
construction of the DM inductor.
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