?/DIV>
=
(1)
Because the PWM stage generally uses a forward
converter, it is necessary to limit the maximum duty
cycle at 50%. To have a small tolerance of the
maximum duty cycle, a frequency divider with toggle
flip-flops is used, as illustrated in Figure 25. The
operation frequency of PFC and PWM stage is 1/4 of
oscillator frequency. (For FAN4800CU, the operation
frequencies for PFC and PWM stages are 1/4 and 1/2
of oscillator frequency, respectively).
The   dead   time for   the PFC   gate   drive signal   is
determined by:
T
D E A D
C
3 6 0
=
(2)
The dead time should be smaller than 2% of the
switching period to minimize line current distortion
around the line zero crossing.
RT /C T
V
RE F
O S C
T  Q
T -F F
T
Q
O P W M (F A N 4 8 0 0 C U )
O P F C , O P W M
T -F F
Figure 25. Oscillator Configuration
R T  / C  T
O  P  F  C
O  P  W   M
O  P  W   M   ( F  A  N  4  8  0  0  C  U  )
P  F  C   D  e  a  d  - T  i m  e
Figure 26. Timing Diagram
I
D
I
D S
V
G . P F C
V
G .P W M
V
G . P F C
V
G . P W M
I
D S
I
D
Figure 27. Interleaved Leading / Trailing
Edge Modulation
Figure 27 shows the interleaved leading / trailing edge
modulation, where the turn-off of the PFC drive signal is
synchronized to the turn-on of the PWM drive signal.
This technique allows the PFC output diode current to
flow directly into the downstream DC/DC converter,
minimizing the current ripple of PFC output capacitor.
Gain Modulator
Gain modulator is the key block for the PFC stage
because it provides the reference to the current control
error amplifier for the input current shaping, as shown in
Figure 28. The output current of the gain modulator is a
function of V
EA
, I
AC,
and V
RMS
. The gain of the gain
modulator is given as a ratio between I
MO
and I
AC
with a
given V
RMS
when V
EA
is saturated to HIGH. The gain is
inversely proportional to V
RMS
2
, as shown in Figure 29,
to   implement   line   feed-forward.   This   automatically
adjusts the reference of current control error amplifier
according to the line voltage, such that the input power
of PFC converter is not changed with line voltage (as
shown in Figure 30).
IS E N S E
IA C
V R M S
V E A
=
?/DIV>
?nbsp  -
=   ?/DIV>
-
2
(  0
.    6
)
.
M   O A
C
E
A
A
C
M
A
X
G
I
K
V
I
V    V
IE A
R  M
R
M
G a in
M o d u la to r
R  R M  S 1
R  R M  S 2
R
R M  S 3
C  R M  S 1
C
R M  S 2
R
I A C
I A C
V
I N
I
L
x
2
k
Figure 28. Gain Modulator Block
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
FAN4800C |
制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:PFC/PWM Controller Combination |
FAN4800CMY |
功能描述:電流型 PWM 控制器 PWM/PFC, Avg Curr 30uA SU, 2.6mA Op RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
FAN4800CNY |
功能描述:電流型 PWM 控制器 PWM/PFC, Avg Curr 30uA SU, 2.6mA Op RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
FAN4800CS |
制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:PFC/PWM Controller Combination |
FAN4800CSMY |
功能描述:電流型 PWM 控制器 PFC/PWM Controller Combination RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |