SG6905SZ Fairchild Semiconductor, SG6905SZ Datasheet - Page 17

• Manufacturer Part Number: SG6905SZ
• Description: IC PWM CTLR PFC/FLYBACK 20-SOP
• Manufacturer: Fairchild Semiconductor

Specifications of SG6905SZ

• Mode: Average Current
• Frequency - Switching: 65kHz
• Current - Startup: 10µA
• Voltage - Supply: 16 V ~ 20 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

© 2007 Fairchild Semiconductor Corporation
SG6905 • Rev. 1.1.2
Interleave Switching
The SG6905 uses interleaved switching to synchronize
the PFC and flyback stages. This reduces switching
noise and spreads the EMI emissions. Figure 26 shows
that an off-time t
the PFC 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 follows that of
the input voltage. Using SG6905, 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 27 shows the
total control loop for the average-current-mode control
circuit of SG6905.
The current source output from the switching charge
multiplier/divider can be expressed as:
Refer to Figure 27, the current output from IMP pin, I
is the summation of 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
to ground. The constant current sources I
are typically 60µA.
Through the differential amplification of the signal
across R
output of IEA is compared with an internal sawtooth
and hence the pulse width for PFC is determined.
Through the average current-mode control loop, the
input current I
There are different concerns in determining the value of
the sense resistor R
to reduce power consumption, but it should be large
enough
transformer (CT) may be used to improve the efficiency
of high power converters.
I
I
MO
MO
=
•
Figure 26. Line-Voltage Detection Circuit
R
K
2
•
=
S
I
to
AC
, better noise immunity is achieved. The
V
I
S
RMS
•
•
S
V
maintain
R
is proportional to I
EA
2
S
OFF
μ (
S
is inserted between the turn-off of
A)
The value of R
MO
the
S
and I
goes negative with respect
resolution.
MR1
MO
. I
.
S
MR1
should be small
and I
MR1
A
and I
MR2
current
are
(3)
MR2
(4)
MP
,
There are two major concerns when compensating the
voltage-loop
transient response. Optimizing interaction between
stability and transient response requires that the error
amplifier’s open-loop crossover frequency be half that
of the line frequency, or 23Hz for a 47Hz line (lowest
anticipated international power frequency). The gain vs.
input voltage of the SG6905’s voltage error amplifier
(V
under
transconductance of the error amplifier is at a local
minimum. Rapid perturbation in line or load conditions
causes the input to the voltage error amplifier (V
deviate from its 3V (nominal) value. If this happens, the
transconductance
increases significantly. This raises the gain-bandwidth
product of the voltage loop, resulting in a much more
rapid voltage loop response to such perturbations than
would
characteristics.
The voltage loop gain(s) is given by:
where:
Z
GM
P
V
C
The average total input power can be expressed as:
From equation 6, V
amplifier, actually controls the total input power and
hence the power delivered to the load.
P
∝
∝
=
≈
17
C
IN
2OUTDC
DC
IN
EAO
Δ
Δ
:
V
Figure 27. Average Current Mode Control Loop
:
V
V
V
V
OUTDC
V
2
:
=
:
RMS
RMS
OUT
EAO
) has a specially shaped non-linearity, so that
V
•
: PFC boost output voltage
•
IN
×
×
Δ
Δ
Compensation network for the voltage loop.
Transconductance of VEAO.
Average PFC input power.
(typical designed value is 380V).
PFC boost output capacitor.
P
Δ
(rms)
steady-state
occur
V
V
I
IN
V
R
MO
V
OUT
EAO
FB
•
AC
IN
V
3
∝
•
•
RMS
error
S
×
×
Δ
Δ
V
V
•
V
I
V
RMS
2
C
IN
EAO
FB
with
EA
DC
(rms)
of
EA
•
×
=
amplifier
GM
, the output of the voltage error
I
AC
the
V
2
V
operating
conventional
RMS
×
•
×
Z
V
C
R
2
V
EA
voltage
EA
AC
(V
EAO
):
conditions
error
stability
linear
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amplifier
FB
gain
and
) to
(5)
(6)
the