PFC SPM
®
Design Guide
© 2005
Rev. 1.0.1-2012
Fairchild Semiconductor – System Engineering Group
7
The following is the block diagram of PFC controller ucc3818. The average current mode control is
composed of two types of loops- voltage loop and current loop. The voltage loop controls the output DC-Link
voltage. It regulates the output voltage. There is a 120[Hz] ripple voltage in the output DC-Link, which is
caused by the 60[Hz] ac input current. Hence, the voltage loop must be designed to be slow enough to reject
the 120[Hz] ripple voltage.
In general CCM (Continuous Current Mode) PFC IC, there is a multiplier. It multiplies V
EA
(the output of
voltage loop) by I
AC
(reference of input ac current shape) and then divides it by V
RMS
(reference of rms input
voltage). The output(
2
RMS
AC
EA
MO
V
I
V
K
I
) of a multiplier is the reference of the PFC’s input current(I
AC
).
The difference between the voltage of I
MO
pin(output of the multiplier) and the voltage of the current
sensing resister is amplified by the current loop. The V
EA
(output of the current loop) is the reference voltage
to the comparameter that generates the gating signal. Current loop should be fast enough to catch up with
the 120[Hz] input ac current. But too fast speed can distort the current shape due to the switching noise.
Therefore, the current loop must be designed to be fast enough to catch up with the 120[Hz] rectified input
current, but not too fast for switching noise immunity.
Current Loop Amplifier
Eq. 1 shows the open loop response of power stage. (Refer to UCC3818 datasheet)
sL
R
V
s
V
s
V
G
SH
DC
IAOUT
S
PST
4
)
(
)
(
(UCC3818)
(Eq.1)
where: V
S
: Voltage of shunt resistor
V
IAOUT
: Voltage of R
MO
V
DC
: DC link Voltage
R
SH
: Shunt resistance for current sensing (2mohm)
L : PFC inductance
s : j
(= j
2
f)
Voltage Loop Amplifier
Eq. 2 shows the open loop response of power stage. (Refer to UCC3818 datasheet)
EAOUT
OUTDC
OUT
IN
L
O
V
V
V
sC
P
G
.
.
.
(UCC3818)
(Eq.2)
where: P
IN
: Input power
C
OUT
: DC link capacitance
V
OUTDC
: DC link voltage
V
EAOUT
: Error amplifier output difference (
5V)
