NCP1010, NCP1011, NCP1012, NCP1013, NCP1014
100%
Peak current
at nominal power
Skip−cycle
current limit
25%
Figure 20. Low Peak Current Skip−Cycle Guarantees Noise−Free Operation
Full power operation involves the nominal switching
frequency and thus avoids any noise when running.
Experiments carried on a 5.0 W universal mains board
unveiled a standby power of 300 mW @ 230 Vac with the
DSS activated and dropped to less than 100 mW when an
auxiliary winding is connected.
Frequency Jittering for Improved EMI Signature
By sweeping the switching frequency around its nominal
value, it spreads the energy content on adjacent frequencies
rather than keeping it centered in one single ray. This offers
the benefit to artificially reduce the measurement noise on
a standard EMI receiver and pass the tests more easily. The
EMI sweep is implemented by routing the V
CC
ripple
(induced by the DSS activity) to the internal oscillator. As a
result, the switching frequency moves up and down to the
DSS rhythm. Typical deviation is
"3.3%
of the nominal
frequency. With a 1.0 V peak−to−peak ripple, the frequency
will equal 65 kHz in the middle of the ripple and will
increase as V
CC
rises or decrease as V
CC
ramps down.
Figure 21 portrays the behavior we have adopted.
V
CC
Ripple
VCC
OFF
67.15 kHz
65 kHz
62.85 kHz
VCC
ON
Internal Sawtooth
Figure 21. The V
CC
ripple is used to introduce a frequency jittering on the internal oscillator sawtooth.
Here, a 65 kHz version was selected.
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ONSEMI [ ON SEMICONDUCTOR ]
