NCP1028_15 Datasheet, PDF (23/29 Pages) ON Semiconductor – High-Voltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

NCP1028
Jittering
Frequency jittering is a method used to soften the EMI
signature by spreading the energy in the vicinity of the main
switching component. The NCP1028 offers a "6%
deviation of the nominal switching frequency. The sweep
sawtooth is internally generated and modulates the clock
up and down with a fixed frequency of 300 Hz. Figure 40
shows the relationship between the jitter ramp and the
frequency deviation. It is not possible to externally disable
the jitter.
Jitter ramp
68.9kHz
65kHz
61.1kHz
adjustable
Internal
sawtooth
Figure 40. Modulation Effects on the Clock Signal by the
Jittering Sawtooth
Skip−Cycle
Skip cycle offers an efficient way to reduce the standby
power by skipping unwanted cycles at light loads.
However, the recurrent frequency in skip often enters the
audible range and a high peak current obviously generates
acoustic noise in the transformer. The noise takes its origins
in the resonance of the transformer mechanical structure
which is excited by the skipping pulses. A possible
solution, successfully implemented in the NCP1200 series,
also authorizes skip cycle but only when the power demand
as dropped below a given level. This is what Figure 41
shows, as implemented on the NCP1028.
Nominal peak
current
Skip cycle
current limit
0
Figure 41. Low Peak Current Skip Cycle Guarantees Noise−Free Operation
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