sg6901asz-f116 Fairchild Semiconductor, sg6901asz-f116 Datasheet - Page 14

• Manufacturer Part Number: sg6901asz-f116
• Description: Pfc/flyback-pwm Controller
• Manufacturer: Fairchild Semiconductor

PFC / Flyback PWM Controller
Interleave Switching
The SG6901A uses interleaved switching to synchronize
the PFC and flyback stages, which reduces switching
noise and spreads the EMI emissions. Figure 3 shows
off-time, T
gate drive and the turn-on of the PWM.
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 4
shows the total control loop for the average-current-mode
control circuit of SG6901A.
The current source output from the switching charge
multiplier-divider can be expressed as:
As shown in Figure 4, the current output from IMP pin,
I
identical fixed-current sources used to pull high the
operating point of the IMP and IPFC pins since the
voltage across R
Constant current sources I
© System General Corp.
Version 1.0.1 (IAO33.0062.B0)
I
MP
MO
, is the summation of I
=
K
×
I
FIG.3 Interleaved Switching Pattern
AC
OFF
V
RMS
×
, inserted between the turn-off of the PFC
V
S
2
EA
goes negative with respect to ground.
(µA)
MR1
------------
MO
and I
and I
MR2
MR1
are typically 60µA.
. I
MR1
and I
MR2
(4)
are
- 14 -
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
According to Equation 5, the minimum value of R
maximum of R
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) may
be used to improve 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
to ground. This establishes a dominant pole f1 for the
voltage loop:
The average total input power can be expressed as:
From Equation 7, V
amplifier, controls the total input power and the power
delivered to the load.
I
Pin
∝
∝
∝
=
f
MO
1
O
S
EA
V
V
V
, better noise immunity is achieved. The output of
=
and a capacitor C
2
=
RMS
RMS
RMS
×
should be kept as constant as possible, according to
×
2
V
R
π
IN
R
V
×
×
×
(rms)
EA
AC
2
×
I
I
MO
R
AC
Vin
V
Z
=
AC
V
RMS
1
×
×
O
RMS
I
I
S
×
IN
V
www.sg.com.tw • www.fairchildsemi.com
×
EA
2
V
×
(rms)
C
2
S
EA
R
EA
can be determined since I
----------------
S
--------------------
EA
S
. The value of R
EA
(1µF ~ 10µF) should be connected
----------------------
, the output of the voltage error
S
Product Specification
is proportional to I
September 26, 2007
S
RMS
should be small
MO
and narrow
should not
SG6901A
RMS
MO
(5)
(6)
(7)
2
IEA
and
and
:
is