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參數資料
| 型號： | FAN4800 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | Low Start-Up Current PFC/PWM Controller Combos |
| 中文描述： | 低啟動電流式PFC / PWM控制器功放音箱 |
| 文件頁數： | 12/19頁 |
| 文件大小： | 369K |
| 代理商： | FAN4800 |
FAN4800
12
calculate the Z
CI
Rs: Sensing resistor of the boost converter
2.5V: Amplitude of the PFC leading modulation ramp
L: boost inductor
A modest degree of gain contouring is applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current loop pertur-
bations.
However, the boost inductor will usually be the domi-
nant factor in overall current loop response.
Therefore, this contouring is significantly less marked
than that of the voltage error amplifier. This is illustrated
in the Figure A of the Typical Performance Characteris-
tics.
I
SENSE
filter, the RC filter between Rs and I
ENSE:
There are two reasons to add a filter at I
SENSE
pin:
1) Protection: During start up or in-rush current condi-
tions, it will have a large voltage cross, Rs, which is the
sensing resistor of the PFC boost converter. It requires
the I
SENSE
filter to attenuate the energy.
2) To reduce L, the Boost Inductor: The I
SENSE
filter
also can reduce the Boost Inductor value since the
I
SENSE
filter behaves like an integrator before I
SENSE
pin
which is the input of the current error amplifier, IEAO.
The I
SENSE
filter is an RC filter. The resistor value of the
I
SENSE
filter is between 100ohm and 50ohm because
I
OFFSET
X Rs can generate an offset voltage of IEAO.
Selecting an R
FILTER
which is equal to 50ohm, will
keep the offset of the IEAO less than 5mV. Usually, we
design the pole of I
SENSE
filter at Fpfc/6, one sixth of
the PFC switching frequency. Therefore, the boost in-
ductor can be reduced 6 times without disturbing the
stability. Thus the capacitor of the I
SENSE
Filter, C
FIL-
TER
, will be around 283nF.
Osillator(RAMP1)
The oscillator frequency is determined by the values of
R
T
and C
T
, which determine the ramp and off-time of
the oscillator output clock
:
The dead time of the oscillator is derived from the fol-
lowing equation
:
at V
REF
=7.5V and t
RAMP
= C
T
* R
T
* 0.55
The dead time of the oscillator may be determined
using:
12.11mA
The dead time is so small (t
RAMP
>>t
DEAD TIME
) that the
operating frequency can typically be approximated by:
Figure 2. Compensation Network Connection for the
Voltage and Current Error Amplifiers
V
RMS
4
3
15
2
Gain
Modulator
V
FB
I
SENSE
I
AC
IEAO
VEAO
3.5k
3.5k
2.5V
PFC CMP
1
16
PFC
Output
Vref
FAN4800
Vcc
GND
V
BIAS
R
BIAS
0.22uF
Ceramic
15V
Zener
Figure 3. External Component Connection to Vcc
fOSC
tRAMP
tDEADTIME
+
-------------------------------------------------------------
=
tRAMP
CT
RT
×
In
VREF
3.75
–
-----------------–
×
=
tDEADTIME
------------------------
=
CT
×
227
CT
×
=
fOSC
-------------------
=
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