SG6902 Fairchild Semiconductor, SG6902 Datasheet - Page 14

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

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Green-Mode PFC / Flyback-PWM Controller
OPERATION DESCRIPTION
The highly integrated SG6902 is designed for power
supplies with boost PFC and flyback PWM. It requires
very few external components to achieve green-mode
operation and versatile protections / compensation.
The patented interleave-switching feature synchronizes
the PFC and PWM stages and reduces switching noise. At
light loads, the switching frequency is continuously
decreased to reduce power consumption. If output loading
is reduced, the PFC stage is turned off to further reduce
power consumption.
The
average-current-mode control. The patented switching
charge multiplier-divider provides a high-degree noise
immunity for the PFC circuit. The proprietary
multi-vector output voltage control scheme provides a
fast transient response in a low-bandwidth PFC loop, in
which the overshoot and undershoot of the PFC voltage
are clamped. If the feedback loop is broken, the SG6902
shuts off PFC to prevent extra-high voltage on output.
Programmable two-level output voltage control reduces
the PFC output voltage at low line input to increase the
efficiency of the power supply.
For the flyback PWM, the synchronized slope
compensation ensures the stability of the current loop
under continuous-mode operation. Built-in line-voltage
compensation maintains constant output power limit.
Hiccup operation during output overloading is also
guaranteed. To prevent the power supply from drawing
large current during start-up, the start-up for PFC stage is
delayed 11.5ms after the operation of PWM stage.
In addition, SG6902 provides complete protection
functions, such as brownout, over-voltage, and RI pin
open/short protections.
Start-up
Figure 1 shows the start-up circuit of the SG6902. A
resistor R
The turn-on and turn-off thresholds are fixed internally at
16V/10V. During start-up, the hold-up capacitor must be
charged to 16V through the start-up resistor to enable
SG6902. The hold-up capacitor continues to supply V
before the energy can be delivered from auxiliary winding
of the main transformer. V
during this start-up process. This UVLO hysteresis
window ensures that hold-up capacitor is adequate to
© System General Corp.
Version 1.3.1 (IAO33.0022.B3)
PFC
AC
is utilized to charge V
function
DD
is
must not drop below 10V
DD
capacitor through S1.
implemented
by
DD
- 14 -
supply V
than 25µA start-up current, the value of R
to reduce power consumption. One 10µF capacitor should
hold enough energy for successful start-up. After start-up,
S1 switches so that the current I
multiplier. This helps reduce circuit complexity and
power consumption.
FIG.1 Start-up Circuit
Switching Frequency and Current
Sources
The switching frequency can be programmed by resistor
R
For example, a 24kΩ resistor for R
switching frequency. Accordingly, a constant current, I
flows through R
I
F
I
T
T
I
OSC
is used to generate internal current reference.
connected between RI and GND. The relationship is:
=
R
=
1.2V
I
R
DD
(kΩ
1560
I
during start-up. Since SG6902 consumes less
(kΩ
www.sg.com.tw • www.fairchildsemi.com
)
I
(mA)
:
)
(kHz)
----------------
-----------
Product Specification
AC
is the input for PFC
I
results in a 65kHz
September, 2007
AC
can be large
SG6902
(1)
(2)
T
,