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參數(shù)資料
| 型號： | SG6902SZ |
| 廠商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Green-Mode PFC / Flyback-PWM Controller |
| 中文描述： | POWER FACTOR CONTROLLER, 68 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | 0.300 INCH, ROHS COMPLIANT, MS-013AC, SOP-20 |
| 文件頁數(shù)： | 16/22頁 |
| 文件大?。?/td> | 863K |
| 代理商： | SG6902SZ |
Product Specification
Green-Mode PFC / Flyback-PWM Controller
SG6902
System General Corp.
Version 1.3.1 (IAO33.0022.B3)
- 16 -
www.sg.com.tw www.fairchildsemi.com
September, 2007
FIG.5 Average-Current-Mode Control Loop
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 5
shows the total control loop for the average-current-mode
control circuit of SG6902.
The current source output from the switching charge
multiplier-divider can be expressed as:
(μA)
V
V
I
K
I
2
RMS
EA
AC
MO
×
×
=
------------
(4)
I
IMP
, the current output from the IMP pin, is the
summation of I
MO
and I
MR1
. I
MR1
and I
MR2
are identical
fixed-current sources. R
2
and R
3
are also identical. They
are used to pull high the operating point of the IMP and
IPFC pins since the voltage across R
S
goes negative with
respect to ground. The constant current sources I
MR1
and
I
MR2
are typically 60μA.
Through the differential amplification of the signal across
R
S
, better noise immunity is achieved. The output of I
EA
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
S,
is proportional to I
MO:
S
S
2
MO
I
R
I
R
×
=
×
--------------------
(5)
According to Equation 5, the minimum value of R
2
and
maximum of R
S
can be determined, because I
MO
should
not exceed the specified maximum value.
There are different concerns in determining the value of
the sense resistor R
S
. The value of R
S
should be small
enough to reduce power consumption, but large enough to
maintain the resolution. A current transformer (CT) can
improve the efficiency of high-power converters.
To achieve good power factor, the voltage for V
RMS
and
V
EA
should be kept as DC as possible, according to
Equation 4. Good RC filtering for V
RMS
and narrow
bandwidth (lower than the line frequency) for voltage
loop are suggested for better input current shaping. The
transconductance error amplifier has output impedance
Z
O
(>90k
Ω
) and a capacitor C
EA
(1μF ~ 10μF) that should
be connected to ground (as shown in Figure 5). This
establishes a dominant pole f1 for the voltage loop:
EA
0
1
C
×
R
2
1
f
×
=
π
----------------------
(6)
The average total input power can be expressed as:
AC
EA
2
RMS
V
EA
AC
RMS
V
2
RMS
V
EA
AC
RMS
V
MO
I
RMS
V
IN(rms)
I
IN(rms)
I
×
R
V
2
V
R
Vin
V
V
Pin
∝
×
=
×
×
∝
×
×
∝
×
=
----------------
(7)
V
EA
, the output of the voltage error amplifier, controls the
total input power and the power delivered to the load.
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| SG6931DZ | 制造商:Fairchild Semiconductor Corporation 功能描述: |
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