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NCP1653, NCP1653A
As a result, the on time t 1 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
depending on different values of V ac and P out . The follower
boost operating area is illustrated in Figure 33.
MOSFET or allows the circuit components to operate at a
lower stress condition in most of the time.
Vout
96% I ref R FB
Pout(min)
Pout(max)
Output Feedback
The output voltage V out of the PFC circuit is sensed as a
feedback current I FB flowing into the FB pin (Pin 1) of the
device. Since the FB pin voltage V FB1 is much smaller than
V out , it is usually neglected.
V in
1
V ac(min)
2
1. P out increases, V out decreases
2. V ac decreases, V out decreases
V ac(max) V ac
IFB + out
[ out
V * VFB1
RFB
V
RFB
(eq.14)
Figure 33. Follower Boost Region
Icontrol(max) Vout
Icontrol +
1 *
0.04
Vout +
RM RCS (eq.17)
) R
2 RS R vac Vref Icontrol(max)
FB Iref
where R FB is the feedback resistor across the FB pin
(Pin 1) and the output voltage referring to Figure 2.
Then, the feedback current I FB represents the output
voltage V out and will be used in the output voltage
regulation, undervoltage protection (UVP), and
overvoltage protection (OVP).
Output Voltage Regulation
Feedback current I FB which represents the output voltage
V out is processed in a function with a reference current
(I ref = 200 m A typical) as shown in regulation block
function in Figure 32. The output of the voltage regulation
block, low ? pass filter on V control pin and the I control =
V control / R 1 block is in Figure 30 is control current I control .
And the input is feedback current I FB . It means that I control
is the output of I FB and it can be described as in Figure 32.
There are three linear regions including: (1) I FB < 96% ×
I ref , (2) 96% × I ref <I FB < I ref , and (3) I FB > I ref . They are
discussed separately as follows:
Region (2): 96% × I ref < I FB < I ref
When I FB is between 96% and 100% of I ref (i.e., 96% R FB
× I ref < V out < R FB × I ref ), 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 V reg decreases linearly with
I FB in the range from 96% of I ref to I ref . It gives a linear
function of I control in (eq.16).
(eq.16)
RFB Iref
Resolving (eq.16) and (eq.13),
Vac
0.04 Pout Vac
h
According to (eq.17), output voltage V out becomes R FB
× I ref when power is low (P out ≈ 0). It is the maximum value
of V out in this operating region. Hence, it can be concluded
that output voltage increases when power decreases. It is
I control(max)
I control
similar to the follower boost characteristic in (eq.15). On
the other hand in (eq.17), output voltage V out becomes R FB
× I ref when RMS input voltage V ac is very high. It is the
maximum value of V out in this operating region. Hence, it
can also be concluded that output voltage increases when
96% I ref
I ref
I FB
RMS input voltage increases. It is similar to another
follower boost characteristic in (eq.15). This characteristic
Vout + h
Vac
Pout
Pout(max)
Figure 32. Regulation Block
Region (1): I FB < 96% × I ref
When I FB is less than 96% of I ref (i.e., V out < 96% R FB
× I ref ), the NCP1653 operates in follower boost mode. The
regulation block output V reg is at its maximum value.
I control becomes its maximum value (i.e., I control =
I control(max) = I ref /2 = 100 m A) which is a constant. (eq.13)
becomes (eq.15).
2 RS R vac Icontrol(max) Vref Vac
RM RCS Pout
(eq.15)
T
The output voltage V out is regulated at a particular level
with a particular value of RMS input voltage V ac and output
power P out . However, this output level is not constant and
is illustrated in Figure 34.
Vout Pout(min)
I ref R FB
1 2
96% I ref R FB
1. P out increases, V out decreases
2. V ac decreases, V out decreases
V ac(min) V ac(max) V ac
Figure 34. Constant Output Voltage Region
Region (3): I FB > I ref
When I FB is greater than I ref (i.e., V out > R FB × I ref ), the
NCP1653 provides no output or zero duty ratio. The
regulation block output V reg becomes 0 V. I control also
becomes zero. The multiplier voltage V M in (eq.8)
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