SG6903 Fairchild Semiconductor, SG6903 Datasheet - Page 14

• Manufacturer Part Number: SG6903
• Manufacturer: Fairchild Semiconductor

Green Mode PFC/Flyback-PWM Controller
PFC Output Voltage Control (RANGE)
applying active boost PFC and Flyback as a second stage,
the output voltage of PFC is usually designed around
250V at low line while it is 390V at high line. This can
improve the efficiency at low-line input. In SG6903, the
RANGE pin (open-drain structure) is used for the
two-level output voltage setting.
the PFC output voltage. The RANGE output is shorted to
ground when the V
while it is of high impedance (open) whenever the V
voltage drops below V
can be designed using below equations.
Interleave Switching
synchronize the PFC and Flyback stages. This reduces
switching noise and spreads the EMI emissions. Figure 4
shows that an off-time T
turn-off of the PFC gate drive and the turn-on of the
PWM.
© System General Corp.
Version 1.0.1 (IAO33.0061.B0)
Range
Range
For a universal input (90 ~ 264Vac) power supply
Figure 3 shows the RANGE output that programs
FIG.3 Range control two level output voltage
The SG6903 uses interleaved switching to
=
=
Open
Ground
⇒
⇒
V
RMS
O
V
RMS-L
=
O
voltage exceeds V
=
R
OFF
A
R
R
(1.6V). The output voltages
+
A
B
is inserted in between the
(
R
+
R
B
B
(
R
×
//
B
3
R
//
V
C
R
)
C
)
RMS-H
×
3
V
---- (3)
(1.95V)
RMS
- 14 -
PFC Operation
(PFC) is to shape the input current of a power supply. The
input current waveform and phase will follow that of the
input voltage. Using SG6903, 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 SG6903.
Charge® multiplier/divider can be expressed as:
I
identical fixed current sources. They are used to pull high
the operating point of the IMP and IPFC pins since the
voltage across R
The constant current sources I
60µA.
across Rs, better noise immunity is achieved. The output
of IEA will be compared with an internal sawtooth and
hence the pulse width for PFC is determined. Through the
average current-mode control loop, the input current Is
will be proportional to I
MP
, is the summation of I
The purpose of a boost active power factor corrector
The current source output from the Switching
Refer to Figure 5, the current output from IMP pin,
Through the differential amplification of the signal
I
I
MO
MO
FIG.4 Interleaved switching pattern
×
=
R
K
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2
×
S
=
goes negative with respect to ground.
I
I
AC
S
V
×
RMS
×
MO
R
V
S
.
MO
2
EA
-------------------- (5)
and I
(uA)
MR1
Product Specification
and I
MR1
------------ (4)
September 28, 2007
. I
MR2
MR1
are typically
and I
SG6903
MR2
are