NCP1654
www.onsemi.com
16
Input and output power (P
in
and P
out
) are derived in (Equation 15) when the circuit efficiency η is obtained or assumed.
The variable V
ac
stands for the rms input voltage.
P
in
+
V
ac
2
Z
in
+
2pR
CS
@ (V
control
* V
CONTROL(min)
) @ V
REF
@ V
ac
2
Ǹ
R
M
R
SENSE
V
out
K
BO
(eq. 15)
T
(V
control
* V
CONTROL(min)
)V
ac
V
out
P
out
+ h P
in
+ h
2pR
CS
@ (V
control
* V
CONTROL(min)
) @ V
REF
@ V
ac
2
Ǹ
R
M
R
SENSE
V
out
K
BO
(eq. 16)
T
(V
control
* V
CONTROL(min)
)V
ac
V
out
Follower Boost
The “Follower Boost” is an operation mode where the
pre−converter output voltage stabilizes at a level that varies
linearly versus the ac line amplitude. This technique aims
at reducing the gap between the output and input voltages
to optimize the boost efficiency and minimize the cost of
the PFC stage (refer to MC33260 data sheet for more
details at http://www.onsemi.com ).
The NCP1654 operates in follower boost mode when
V
control
is constant, i.e. V
control
raises to its maximum value
V
CONTROL(max)
. Re−formulate (Equation 16) to become
(Equation 17) and (Equation 18) by replace V
control
by
V
CONTROL(max)
. If V
control
is constant based on
(Equation 15), for a constant load or power demand the
output voltage V
out
of the converter is proportional to the
rms input voltage V
ac
. It means the output voltage V
out
becomes lower when the rms input voltage V
ac
becomes
lower. On the other hand, the output voltage V
out
becomes
lower when the load or power demand becomes higher.
P
out
+ h
2pR
CS
@ (V
CONTROL(max)
* V
CONTROL(min)
) @ V
REF
@ V
ac
2
Ǹ
R
M
R
SENSE
V
out
K
BO
(eq. 17)
+ h
2pR
CS
@ DV
CONTROL
@ V
REF
@ V
ac
2
Ǹ
R
M
R
SENSE
V
out
K
BO
V
out
+ h
2pR
CS
@ DV
CONTROL
@ V
REF
2
Ǹ
R
M
R
SENSE
K
BO
@
V
ac
P
out
(eq. 18)
where
V
CONTROL(max)
is the maximum control voltage.
DV
CONTROL
is the gap between V
CONTROL(max)
and
V
CONTROL(min)
.
It is illustrated in Figure 39.
Figure 39. Follower Boost Characteristics
V
out
(Traditional Boost)
V
out
(Follower Boost)
V
in
P
out
Time
Time
Follower Boost Benefits
The follower boost circuit offers and opportunity to
reduce the output voltage V
out
whenever the rms input
voltage V
ac
is lower or the power demand P
out
is higher.
Because of the step−up characteristics of boost converter,
the output voltage V
out
will always be higher than the input
voltage V
in
even though V
out
is reduced in follower boost
operation. 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 MOSFET or allows the circuit
components to operate at a lower stress condition in most
of the time.
Reference Section
The internal reference voltage (V
REF
) is trimmed to be
±2% accurate over the temperature range (the typical value
is 2.5 V). V
REF
is the reference used for the regulation.
V
REF
also serves to build the thresholds of the fast transient
response, Overvoltage (OVP), brown out (BO), and
Undervoltage protections (UVP).
Output Feedback
The output voltage V
out
of the PFC circuits is sensed at
V
fb
pin via the resistor divider (R
fbL
and R
fbU
) as shown in
Figure 38. V
out
is regulated as described in (Equation 19).
V
out
+ V
REF
R
fbU
) R
fbL
R
fbL
(eq. 19)
