NCP1653, NCP1653A
http://onsemi.com
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As a result, the on time t1 is reduced. Reduction of on time
makes the loss of the inductor and power MOSFET smaller.
Hence, it allows cheaper cost in the inductor and power
MOSFET or allows the circuit components to operate at a
lower stress condition in most of the time.
Output Feedback
The output voltage Vout of the PFC circuit is sensed as a
feedback current IFB flowing into the FB pin (Pin 1) of the
device. Since the FB pin voltage VFB1 is much smaller than
Vout, it is usually neglected.
(eq.14)
IFB +Vout *VFB1
RFB [Vout
RFB
where RFB is the feedback resistor across the FB pin
(Pin 1) and the output voltage referring to Figure 2.
Then, the feedback current IFB represents the output
voltage Vout and will be used in the output voltage
regulation, undervoltage protection (UVP), and
overvoltage protection (OVP).
Output Voltage Regulation
Feedback current IFB which represents the output voltage
Vout is processed in a function with a reference current
(Iref = 200 mA typical) as shown in regulation block
function in Figure 32. The output of the voltage regulation
block, low−pass filter on Vcontrol pin and the Icontrol =
Vcontrol / R1 block is in Figure 30 is control current Icontrol.
And the input is feedback current IFB. It means that Icontrol
is the output of IFB and it can be described as in Figure 32.
There are three linear regions including: (1) IFB < 96% ×
Iref, (2) 96% × Iref <IFB < Iref, and (3) IFB > Iref. They are
discussed separately as follows:
Figure 32. Regulation Block
Icontrol
Iref
Iref
96% IFB
Icontrol(max)
Region (1): IFB < 96% × Iref
When IFB is less than 96% of Iref (i.e., Vout < 96% RFB
× Iref), the NCP1653 operates in follower boost mode. The
regulation block output Vreg is at its maximum value.
Icontrol becomes its maximum value (i.e., Icontrol =
Icontrol(max) = Iref/2 = 100 mA) which is a constant. (eq.13)
becomes (eq.15).
Vout +h2R
SRȀvac Icontrol(max) Vref Vac
RMRCS Pout
(eq.15)
TVac
Pout
The output voltage Vout is regulated at a particular level
with a particular value of RMS input voltage Vac and output
power Pout. However, this output level is not constant and
depending on different values of Vac and Pout. The follower
boost operating area is illustrated in Figure 33.
Figure 33. Follower Boost Region
V
VV
Vout
ac(max)
ac(min) ac
Pout(min) Pout(max)
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Vin
96% Iref RFB
1. Pout increases, Vout decreases
2. Vac decreases, Vout decreases
Region (2): 96% × Iref < IFB < Iref
When IFB is between 96% and 100% of Iref (i.e., 96% RFB
× Iref < Vout < RFB × Iref), the NCP1653 operates in constant
output voltage mode which is similar to the follower boost
mode characteristic but with narrow output voltage range.
The regulation block output Vreg decreases linearly with
IFB in the range from 96% of Iref to Iref. It gives a linear
function of Icontrol in (eq.16).
(eq.16)
Icontrol +Icontrol(max)
0.04 ǒ1*Vout
RFB IrefǓ
Resolving (eq.16) and (eq.13),
Vout +Vac
ǒRMRCS
2R
SRȀvac Vref
0.04
Icontrol(max)
Pout
h)Vac
RFB IrefǓ(eq.17)
According to (eq.17), output voltage Vout becomes RFB
× Iref when power is low (Pout ≈ 0). It is the maximum value
of Vout in this operating region. Hence, it can be concluded
that output voltage increases when power decreases. It is
similar to the follower boost characteristic in (eq.15). On
the other hand in (eq.17), output voltage Vout becomes RFB
× Iref when RMS input voltage Vac is very high. It is the
maximum value of Vout in this operating region. Hence, it
can also be concluded that output voltage increases when
RMS input voltage increases. It is similar to another
follower boost characteristic in (eq.15). This characteristic
is illustrated in Figure 34.
Figure 34. Constant Output Voltage Region
V
VV
Vout
ac(max)
ac(min) ac
P
out(min) Pout(max)
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96% Iref RFB
1. Pout increases, Vout decreases
2. Vac decreases, Vout decreases
Iref RFB
Region (3): IFB > Iref
When IFB is greater than Iref (i.e., Vout > RFB × Iref), the
NCP1653 provides no output or zero duty ratio. The
regulation block output Vreg becomes 0 V. Icontrol also
becomes zero. The multiplier voltage VM in (eq.8)