SG6902 Fairchild Semiconductor, SG6902 Datasheet - Page 16

• Manufacturer Part Number: SG6902
• Description: Green-Mode PFC / Flyback-PWM Controller
• Manufacturer: Fairchild Semiconductor

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Green-Mode PFC / Flyback-PWM Controller
FIG.5 Average-Current-Mode Control Loop
PFC Operation
The purpose of a boost active power factor corrector (PFC)
is to shape the input current of a power supply. The input
current waveform and phase follow that of the input
voltage. Average-current-mode control is utilized for
continuous-current-mode operation for the PFC booster.
With the innovative multi-vector control for voltage loop
and switching charge multiplier-divider for current
reference, excellent input power factor is achieved with
good noise immunity and transient response. Figure 5
shows the total control loop for the average-current-mode
control circuit of SG6902.
The current source output from the switching charge
multiplier-divider can be expressed as:
I
summation of I
fixed-current sources. R
are used to pull high the operating point of the IMP and
IPFC pins since the voltage across R
respect to ground. The constant current sources I
I
Through the differential amplification of the signal across
R
compared with an internal sawtooth and the pulse width
for PFC is determined. Through the average current-mode
control loop, the input current, I
© System General Corp.
Version 1.3.1 (IAO33.0022.B3)
I
I
IMP
MR2
MO
MO
S
, better noise immunity is achieved. The output of I
, the current output from the IMP pin, is the
×
=
are typically 60µA.
K
R
×
2
=
I
AC
V
I
S
RMS
×
×
V
MO
R
2
EA
S
and I
(µA)
--------------------
2
MR1
and R
------------
. I
MR1
S,
3
are also identical. They
is proportional to I
and I
S
goes negative with
MR2
are identical
MR1
(4)
(5)
MO:
EA
and
- 16 -
is
According to Equation 5, the minimum value of R
maximum of R
not exceed the specified maximum value.
There are different concerns in determining the value of
the sense resistor R
enough to reduce power consumption, but large enough to
maintain the resolution. A current transformer (CT) can
improve the efficiency of high-power converters.
To achieve good power factor, the voltage for V
V
Equation 4. Good RC filtering for V
bandwidth (lower than the line frequency) for voltage
loop are suggested for better input current shaping. The
transconductance error amplifier has output impedance
Z
be connected to ground (as shown in Figure 5). This
establishes a dominant pole f1 for the voltage loop:
The average total input power can be expressed as:
V
total input power and the power delivered to the load.
Pin
f
∝
∝
∝
=
1
O
EA
EA
V
V
V
=
(>90kΩ) and a capacitor C
2
=
RMS
RMS
RMS
, the output of the voltage error amplifier, controls the
×
2
V
should be kept as DC as possible, according to
π
IN(rms)
R
V
×
×
×
EA
AC
×
I
I
MO
R
AC
Vin
R
V
AC
V
1
RMS
×
×
RMS
0
I
×
IN(rms)
V
×
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C
EA
2
V
2
S
EA
EA
can be determined, because I
----------------
S
----------------------
. The value of R
EA
Product Specification
(1µF ~ 10µF) that should
S
RMS
September, 2007
should be small
and narrow
MO
SG6902
RMS
should
(6)
(7)
2
and
and