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?2010 Fairchild Semiconductor Corporation
 
www.fairchildsemi.com
FAN7930B " Rev. 1.0.3
16
 
10.   V
IN
-Absent   Detection:   To   reduce   power   loss 
caused   by   input   voltage   sensing   resistors   and   to
optimize THD, the FAN7930B omits AC input voltage
detection. Therefore, no information about AC input is
available from the internal controller. In many cases, the
V
CC
  of PFC controller is supplied by an independent
power source like standby power. In this scheme, some
mismatch may exist. For example, when the electric
power is suddenly interrupted during two or three AC
line periods; V
CC
 is still live during that time, but output
voltage drops because there is no input power source. 
Consequently, the control loop tries to compensate for
the   output   voltage   drop   and   V
COMP
   reaches   its
maximum. This lasts until AC input voltage is live again.
When AC input voltage is live again, high VCOMP allows
high switching current and more stress is put on the
MOSFET and diode. To protect against this, FAN7930B
checks if the input AC voltage exists. If input does not
exist, soft-start is reset and waits until AC input is live
again. Soft-start manages the turn-on time for smooth
operation when it detects AC input is applied again and
applies less voltage and current stress on startup.
V
IN
t
V
OU  T
V
A  U  X
O S F E T G a t e
I
D  S
f
M I N
D
M   A  X
H ig h D r a in
C u r r e n t!
V
C  O   M   P
T h o u g h V
IN
  i s e l i m i n a t e d ,
o p e r a t i o n o f c o n t r o l l e r i s
n o r m a l d u e t o t h e l a r g e
b y p a s s c a p a c i t o r .
 
Figure 41.    Without V
IN
-Absent Circuit
V
I N
t
V
O   U  T
V
A  U  X
O S F E T G a t e
I
D  S
f
M   I N
D
M   A  X
I n p u t V o lta g e A b s e n t D e te c te d
N e w V
C  O   M   P
T h o u g h V
I N
  i s e l i m i n a t e d ,
o p e r a t i o n o f c o n t r o l l e r i s
n o r m a l d u e t o t h e l a r g e
b y p a s s c a p a c i t o r .
f
M   I N
D
M   I N
S m o o th S o ft- S ta r t
 
Figure 42.    With V
IN
-Absent Circuit
 
11. Current Sense: The  MOSFET  current  is  sensed 
using an external sensing resistor for over-current
protection. If the CS pin voltage is higher than 0.8 V, the
over-current    protection    comparator    generates    a
protection signal. An internal RC filter of 40 k?and 8 pF
is included to filter switching noise.
 
12. Gate Driver Output: FAN7930B contains a single 
totem-pole output stage designed for a direct drive of
the power MOSFET. The drive output is capable of up
to +500 / -800 mA peak current with a typical rise and
fall time of 50 ns with 1 nF load. The output voltage is
clamped to 13 V to protect the MOSFET gate even if the
VCC voltage is higher than 13 V.
 
 
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