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| NCP1236AD65R2G Datasheet, PDF (27/34 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller for Flyback Converters | |||
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NCP1236
LOW LOAD OPERATION
Frequency Foldback
In order to improve the efficiency in light load conditions,
the frequency of the internal oscillator is linearly reduced
from its nominal value down to fOSC(min). This frequency
foldback starts when the voltage on FB pin goes below
VFB(foldS), and is complete before VFB reaches Vskip(in),
whatever the nominal switching frequency option is. The
currentâmode control is still active while the oscillator
frequency decreases. Note that the frequency foldback is
disabled if the controller runs at its maximum duty cycle.
fOSC
Nominal fOSC
fOSC(min)
Skip
Vskip(in)
VFB(foldS)
FB
VFB(foldE)
Figure 47. Frequency Foldback when the FB Voltage Decreases
Skip Cycle Mode
â
+
+
Vskip
FB
KFB
S
Q
DRV stage
R
blanking
+
CS
tLEB
â
Figure 48. Skip Cycle Schematic
When the FB voltage reaches Vskip(in) while decreasing,
skip mode is activated: the driver stops, and the internal
consumption of the controller is decreased. While VFB is
below Vskip(out), the controller remains in this state; but as
soon as VFB crosses the skip out threshold, the DRV pin
starts to pulse again.
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