6. Random Frequency Fluctuation (RFF):
Fluctuating switching frequency of an SMPS can
reduce EMI by spreading the energy over a wide
frequency range. The amount of EMI reduction is
directly related to the switching frequency variation,
which is limited internally. The switching frequency is
determined randomly by external feedback voltage
and an internal free-running oscillator at every
switching instant. This random frequency fluctuation
scatters the EMI noise around typical switching
frequency (67kHz) effectively and can reduce the cost
of the input filter included to meet the EMI
requirements (e.g. EN55022).
Figure 26. Random Frequency Fluctuation
© 2012 Fairchild Semiconductor Corporation
FSL117MRN • Rev.1.0.0
www.fairchildsemi.com
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