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NCP1910
? V ac,on is the rms ac voltage to starts PFC operating.
? V ac,off the rms ac voltage for line brown ? out
detection.
PFC Current Sense
I L
PFC Over ? Power Limitation (OPL)
This is a second OCP with a threshold that is line
dependent. Sense current I CS represents the inductor current
I L and hence represents the input current approximately.
Input voltage signal V LBO represents the rms input voltage.
The product (I CS x V LBO ) represents an approximated input
power (I L x V ac ). It is illustrated in Figure 48.
R CS
I CS CS
NCP1910
V in
I L
+
GND
R SENSE
I L
V CS
?
R SENSE
I CS + I L
I CS
R LBOU
R LBOL
Figure 47. PFC Current Sensing Configuration
The device senses the inductor current I L by the current
sense scheme in Figure 47. The device maintains the voltage
at CS pin to be zero voltage, i.e. V CS = 0 V, so that
R SENSE
(eq. 13)
R CS
Where:
? R SENSE is the sense resistor to sense I L .
? R CS is the offset resistor between CS pin and
R SENSE .
This scheme has the advantage of the minimum number
of components for current sensing. The sense current I CS
represents the inductor current I L and will be used in the PFC
duty modulation to generate the multiplier voltage V M ,
Over ? Power Limitation (OPL), and Over ? Current
Protection. Equation 13 would insist in the fact that it
provides the flexibility in the R SENSE choice and that it
allows to detect in ? rush currents.
R CS
CS
Current
mirror
LBO OPL
> 275 m VA?
C LBO
Figure 48. PFC Over ? Power Limitation Configuration
When the product (I CS x V LBO ) is greater than a
permissible level 275 m VA, the device turns off the PFC
driver so that the input power is limited. The OPL is
automatically deactivated when the product (I CS x V LBO ) is
lower than the 275 m VA level. This 275 m VA level
corresponds to the approximated input power (I L x Vac) to
be smaller than the particular expression in Equation 15.
I L
R CS
PFC Over ? Current Protection (OCP)
PFC Over ? Current Protection is reached when I CS is
larger than I S(OCP) (200 m A typical). The offset voltage of
the CS pin is typical 10 mV and it is neglected in the
calculation. Hence, the maximum OCP inductor current
I CS V LBO t 275 m VA
R SENSE 2 2 K LBO
p
@ V ac
t 275 m VA
(eq. 15)
R CS I S OCP
R CS
R LBOU ) R LBOL
threshold I L(OCP) is obtained in Equation 14.
I L OCP + + 200 m A (eq. 14)
R SENSE R SENSE
When over ? current protection threshold is reached, the
PFC drive goes low. The device automatically resumes
operation when the inductor current goes below the
I L @ V ac t
Where
R CS @ p
R SENSE @ K LBO
K LBO +
@ 97 m VA
R LBOL
threshold.
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