SG6901 Fairchild Semiconductor, SG6901 Datasheet - Page 16

• Manufacturer Part Number: SG6901
• Description: The highly integrated SG6901A is designed for power supplies with boost PFC and flyback PWM
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6901A • Rev. 1.0.2
To achieve good power factor, the voltage for V
V
to Equation 5. 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 ZO and a capacitor C
should be connected to ground. This establishes a
dominant pole f1 for the voltage loop:
Multi-Vector Error Amplifier
Although the PFC stage has a low bandwidth voltage
loop for better input power factor, the innovative multi-
vector error amplifier provides a fast transient response
to clamp the overshoot and undershoot of the PFC
output voltage.
0 shows the block diagram of the multi-vector error
amplifier. When the variation of the feedback voltage
exceeds
transconductance error amplifier adjusts its output
impedance to increase the loop response.
f
EA
1
=
should be kept as constant as possible, according
2
π
Figure 27. Multi-Vector Error Amplifier
×
Z
± 5%
1
O
×
C
EA
of
the
reference
Figure 26. Average-Current-Mode Control Loop
EA
RMS
(1µF ~ 10µF)
voltage,
and narrow
RMS
and
(5)
the
16
The average total input power can be expressed as:
From Equation 6, V
amplifier, controls the total input power and the power
delivered to the load.
PFC Over-Voltage Protection
Using a voltage divider from the output of PFC to the
OVP pin, the PFC output voltage can be safely
protected. Once the voltage on the OVP pin is over
OVPPFC, the OPFC is disabled. THE OPFC is not
enabled again until the OVP voltage falls below
OVPPFC.
Cycle-by-Cycle Current Limiting
SG6901A provides cycle-by-cycle current limiting for
both PFC and PWM stages. Figure 28 shows the peak
current limit for the PFC stage. The PFC gate drive is
terminated once the voltage on the ISENSE pin goes
below V
The voltage of V
relationship between V
The amplitude of the constant current, I
by the internal current reference according to:
I
Pin
∝
∝
∝
=
P
V
V
V
=
2
RMS
RMS
RMS
=
2
×
V
×
IN
R
V
×
×
×
PK
1.2V
(rms)
EA
AC
I
I
MO
R
R
AC
Vin
.
V
AC
V
I
RMS
×
×
RMS
I
V
×
IN
EA
2
V
(rms)
2
EA
RMS
EA
determines the voltage of V
PK
, the output of the voltage error
and V
RMS
is shown in Figure 28.
p
, is determined
www.fairchildsemi.com
PK
. The
(6)
(8)