NCP1028 ON Semiconductor, NCP1028 Datasheet - Page 23

• Manufacturer Part Number: NCP1028
• Description: High?voltage Switcher For Medium Power Offline Smps
• Manufacturer: ON Semiconductor

Jittering
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
Skip-Cycle
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
Frequency jittering is a method used to soften the EMI
Skip cycle offers an efficient way to reduce the standby
Figure 41. Low Peak Current Skip Cycle Guarantees Noise-Free Operation
0
Figure 40. Modulation Effects on the Clock Signal by the
61.1kHz
Jitter ramp
Nominal peak
current
65kHz
adjustable
Jittering Sawtooth
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NCP1028
23
68.9kHz
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.
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.
Internal
sawtooth
Skip cycle
current limit