NCP1603
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23
a 9.0 V ESD zener diode. The 3.9 V maximum limit of this
V
ton
indirectly limits the maximum on time.
The V
control
processing circuit generates V
ton
from
control voltage V
control
and time information of zero
inductor   current. The circuit in Figure 61   makes
Equations 9 and 10 where the value of resistor R
1
is much
higher than the value of resistor R
2
(R
1
>> R
2
).
Figure 61. V
control
Processing Circuit
+
closed when zero current
10
C
control
V
control
R
1
R
2
R
3
C
1
C
3
V
ton
V
ton
+
T V
control
t
1
) t
2
for DCM
(eq. 9)
V
ton
+ V
control
for CRM
(eq. 10)
It is noted that V
ton
is always greater than or equal to
V
control
(V
ton
q V
control
).
In summary, the input impedance Z
in
in Equation 11 is
obtained from Equations 3 through 10.
Z
in
+
V
in
I
in
+
2LI
ch
C
ramp
V
control
(eq. 11)
Control voltage V
control
comes from the PFC boost
circuit output voltage (i.e., bulk voltage V
bulk
) that is a
slowly varying signal. The bandwidth of V
control
can be
additionally limited by inserting an external capacitor
C
control
to the V
control
pin (Pin 10) in Figure 62. The
internal 300 kW resistor and the capacitor C
control
create a
lowpass filter that has a bandwidth f
control
in Equation 12.
It is generally recommended to limit the bandwidth below
20 Hz to achieve power factor correction. Typical value of
C
control
is 0.1 mF.
C
control
u
1
2p300kW f
control
(eq. 12)
Figure 62. V
control
LowPass Filtering
300k
Regulation Block
10
C
control
V
control
V
reg
I
ref
I
ref
96%
I
FB1
V
control
Processing
Circuit
If the bandwidth of V
control
is much less than the 50 or
60 Hz line frequency, the input impedance Z
in
is slowly
varying or roughly constant. Then, the power factor
correction is achieved in DCM and CRM.
Maximum Power in PFC Section
Input and output power (P
in
and P
out
) are derived in
Equations 13 and 14 when the circuit efficiency ? is
obtained or assumed. The variable V
ac
stands for the RMS
input voltage.
P
in
+
V
ac
2
Z
in
+
V
ac
2
C
ramp
V
control
2LI
ch
(eq. 13)
P
out
+ h P
in
+
hV
ac
2
C
ramp
V
control
2LI
ch
(eq. 14)
From Equations 13 and 14, control voltage V
control
controls the amount of output power, input power, or input
impedance. The maximum value of the control voltage
V
control
is 1.05 V (i.e., V
control(max)
= 1.05 V). A parameter
called maximum power resistor R
power
(10.5 kW typical) is
defined in Equation 18 and restricted to have a maximum
?0% variation (i.e., 9.5 kW p R
power
p 11.5 kW) for
defining the maximum power in an application.
R
power
+
V
control(max)
I
ch
+
1.05 V
100 mA
+ 10.5 kW
(eq. 15)
It means that the maximum input and output power
(P
in(max)
and P
out(max)
) are limited to ?0% variation.
P
in(max)
+
V
ac
2
C
ramp
R
power
2 L
(eq. 16)
P
out(max)
+
hV
ac
2
C
ramp
R
power
2 L
(eq. 17)
The maximum input current I
ac(max)
to deliver the
maximum input power P
in(max)
is also derived in (eq.14).
The suffix ac stands for RMS value.
I
ac(max)
+
P
in(max)
V
ac
+
V
acC
ramp
R
power
2 L
(eq. 18)