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NCP1910
PFC Frequency Foldback
NCP1910 implements frequency foldback feature on PFC
section to improve the efficiency at light load. Thanks to
V in2 feed ? forward feature, the output power is proportional
to the (V CTRL ? V CTRL(min) ). The PFC frequency foldback
Vref
is hence done by comparing (V CTRL ? V CTRL(min) ) with
V fold , the voltage on Fold pin.
The simplified block diagram of PFC frequency foldback
feature is depicted in Figure 50.
PFCOSC
Vdd
Vfold
Vfold(max)
Ict(fold)
Ict
Ict(min)
“0” / ”1”
V PREF / 10%V PREF
Oscillator section
Vctrl
Grand Reset
Vctrl(min)
S
PFC BO
Q
Q
R
PFC OK
Figure 50. The PFC Frequency Foldback Block
Where:
? I Ct(min) limits the minimum operating frequency.
? I Ct and I Ct(min) provide the charging current for
oscillator and hence control the nominal operating
frequency.
? V fold determines the power level at which the
frequency foldback starts.
? I Ct(fold) steals the I Ct and hence reduces the
operating frequency according to the error
information between V fold and (V CTRL ?
V CTRL(min) ).
F sw(fold)
? The transient slope of frequency foldback vs. V CTRL
is fixed inside.
? V fold(max) is to limit the maximum power level of
frequency foldback, which is around 2 V typically.
The frequency foldback is disabled at startup, i.e. before
the PFCok signal in Figure 50 is asserted high.
The user can adjust the power level at which the frequency
foldback starts by adjust the resistor divider between V REF
pin and fold pin. Also, the frequency foldback can be
disabled by grounding fold pin.
The relationship between operating frequency and V CTRL
is depicted in Figure 51.
F sw
The slope is fixed internally.
The power level at which fre-
quency starts reducing is ad-
justable by modifying V fold .
V fold – 0.4
V fold
V CTRL ? V CTRL(min) T Power
Figure 51. The Relationship between Frequency and V CTRL
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