SG6905SZ

Manufacturer Part NumberSG6905SZ
DescriptionIC PWM CTLR PFC/FLYBACK 20-SOP
ManufacturerFairchild Semiconductor
SG6905SZ datasheet
 


Specifications of SG6905SZ

ModeAverage CurrentFrequency - Switching65kHz
Current - Startup10µAVoltage - Supply16 V ~ 20 V
Operating Temperature-40°C ~ 105°CMounting TypeSurface Mount
Package / Case20-SOIC (7.5mm Width)Lead Free Status / RoHS StatusLead free / RoHS Compliant
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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
is inserted between the turn-off of
OFF
the PFC gate drive and the turn-on of the PWM.
Figure 26. Line-Voltage Detection Circuit
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:
I
V
AC
EA
=
I
K
μ (
A)
MO
2
V
RMS
Refer to Figure 27, the current output from IMP pin, I
is the summation of I
and I
MO
identical fixed current sources. They are used to pull
HIGH the operating point of the IMP and IPFC pins
since the voltage across R
goes negative with respect
S
to ground. The constant current sources I
are typically 60µA.
Through the differential amplification of the signal
across R
, better noise immunity is achieved. The
S
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
is proportional to I
S
=
I
R
I
R
MO
2
S
S
There are different concerns in determining the value of
the sense resistor R
The value of R
S
to reduce power consumption, but it should be large
enough
to
maintain
the
transformer (CT) may be used to improve the efficiency
of high power converters.
© 2007 Fairchild Semiconductor Corporation
SG6905 • Rev. 1.1.2
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
) has a specially shaped non-linearity, so that
EAO
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:
Δ
V
=
OUT
Δ
V
EAO
2
V
OUTDC
where:
:
Z
C
:
GM
V
:
P
IN
V
2OUTDC
:
C
DC
The average total input power can be expressed as:
(3)
=
P
IN
,
V
MP
RMS
. I
and I
are
MR1
MR1
MR2
V
and I
RMS
MR1
MR2
From equation 6, V
amplifier, actually controls the total input power and
hence the power delivered to the load.
.
MO
(4)
should be small
S
resolution.
A
current
Figure 27. Average Current Mode Control Loop
17
error
amplifier
(V
):
EAO
steady-state
operating
conditions
of
the
voltage
error
occur
with
conventional
Δ
Δ
V
V
FB
EAO
Δ
Δ
V
V
OUT
FB
P
3
GM
Z
IN
Δ
V
C
V
S
C
EAO
DC
Compensation network for the voltage loop.
Transconductance of VEAO.
Average PFC input power.
: PFC boost output voltage
(typical designed value is 380V).
PFC boost output capacitor.
×
V
(rms)
I
(rms)
IN
IN
×
I
V
×
×
I
V
AC
EA
MO
RMS
2
V
RMS
V
×
IN
V
EA
R
V
×
=
×
AC
2
EA
2
R
V
AC
RMS
, the output of the voltage error
EA
stability
and
the
) to
FB
amplifier
linear
gain
(5)
(6)
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