SG6901 Fairchild Semiconductor, SG6901 Datasheet - Page 15

• 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
Functional Description
SG6901A is a highly integrated PFC/PWM combination
controller. Many functions and protections are built in to
provide a compact design. The following sections
describe the operation and function.
Switching Frequency and Current Sources
The switching frequency can be programmed by the
resistor RI connected between RI pin and GND. The
relationship is:
For example, a 24KΩ resistor R
switching frequency. Accordingly, a constant current, I
flows through R
I
Line Voltage Detection (VRMS)
Figure 24 shows a resistive divider with low-pass
filtering for line-voltage detection on the VRMS pin. The
VRMS voltage is used for the PFC multiplier, brownout
protection, and range control.
For brownout protection, SG6901A is disabled with a
195ms delay if the voltage VRMS drops below 0.8V.
For PFC multiplier and range control, refer to the PFC
Operation section below for details.
Interleave Switching
The
synchronize the PFC and flyback stages, which
reduces
emissions. Figure 25 shows off-time, t
between the turn-off of the PFC gate drive and the turn-
on of the PWM.
For an universal input (90 ~ 264V
applying active boost PFC and flyback as a second
stage, the output voltage of PFC is usually designed
around 250V at low line and 390V at high line. This can
T
I
f
T
OSC
is used to generate internal current reference.
=
Figure 24. Line Voltage Detection Circuit
R
=
SG6901A
1.2V
I
R
(kΩ
switching noise and spreads the EMI
1560
I
(kΩ
)
I
:
(mA)
)
(kHz)
uses
interleaved
I
results in a 65KHz
AC
) power supply
switching
OFF
, inserted
(1)
(2)
to
T
,
15
improve the efficiency at low-line input. The RANGE pin
(open-drain structure) is used for the two-level output
voltage setting.
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 26 shows the total control loop for the
average-current-mode control circuit.
The current source output from the switching charge
multiplier-divider can be expressed as:
As shown in Figure 26, the current output from the IMP
pin is the summation of IMO and IMR1. IMR1 and IMR2
are identical fixed-current sources used to pull high the
operating point of the IMP and IPFC pins since the
voltage across RS goes negative with respect to
ground. Constant current sources IMR1 and IMR2 are
typically 60µA.
Through the differential amplification of the signal
across R
output of IEA is compared with an internal sawtooth
and the pulse width for PFC is determined. Through the
average current-mode control loop, the input current I
is proportional to IMO:
According to Equation 4, 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
transformer (CT) may be used to improve efficiency of
high-power converters.
I
I
MO
MO
Figure 25. Interleaved Switching Pattern
×
=
R
K
2
×
S
to
, better noise immunity is achieved. The
I
=
AC
V
I
RMS
S
maintain
×
S
×
V
can be determined since IMO should
R
2
EA
S
S
( )
. The value of R
µA
the
resolution.
S
should be small
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A
current
2
and
(3)
(4)
S