FAN7930 Fairchild Semiconductor, FAN7930 Datasheet - Page 15

• Manufacturer Part Number: FAN7930
• Description: Critical Conduction Mode PFC Controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN7930 • Rev. 1.0.1
To improve this, lengthened turn-on time near the zero
cross region is a well-known technique, though the
method may be different from company to company and
may be proprietary. FAN7930 emdodies this by sourcing
current through the ZCD pin. Auxiliary winding voltage
becomes negative when the MOSFET turns on and is
proportional to input voltage. The negative clamping
circuit of ZCD outputs the current to maintain the ZCD
voltage at a fixed value. The sourcing current from the
ZCD is directly proportional to the input voltage. Some
portion of this current is applied to the internal sawtooth
generator
Theoretically, the fixed-current source and the capacitor
at sawtooth generator decide the maximum turn-on time
when no current is sourcing at ZCD clamp circuit and
available turn-on time gets shorter proportional to the
ZCD sourcing current.
MOSFET gate
Figure 36.
Figure 37.
I
INDUCTOR
V
I
ZCD
IN
FAN7930 Rev.00
together
1.4V
Input and Output Current Near Input
Input and Output Current Near Input
150ns
ON
I
MOSFET
1.5V
Voltage Peak Zero Cross
with
I
DIODE
Voltage Peak
a
fixed-current
I
NEGATIVE
ON
source.
t
15
By THD optimizer, turn-on time over one AC line period
is proportionally changed, depending on input voltage.
Near zero cross, lengthened turn-on time improves THD
performance.
9. Input Voltage Absent Detection: To save power
loss caused by input voltage sensing resistors and to
optimize THD easily, the FAN7930 omits AC input
voltage detection. Therefore, no information about AC
input is available from the internal controller. In many
cases, the V
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
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
reaches its maximum. This lasts until AC input voltage is
Figure 38.
Figure 39.
CC
of PFC controller is supplied by a
Circuit of THD Optimizer
Effect of THD Optimizer
CC
is still alive during that time,
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