FL7930 FAIRCHILD [Fairchild Semiconductor], FL7930 Datasheet - Page 17

• Manufacturer Part Number: FL7930
• Description: Single-Stage Flyback and Boundary-Mode PFC
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2011 Fairchild Semiconductor Corporation
FL7930 • Rev. 1.0.0
By THD optimizer, turn-on time over one AC line period
is proportionally changed, depending on input voltage.
Near the zero cross, lengthened turn-on time improves
THD performance.
11. Input Voltage Absent Detection: To save power
loss caused by input voltage-sensing resistors and to
optimize THD, FL7930 omits AC input voltage detection.
Therefore, no information about AC input is available
from the internal controller. In many cases, the V
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
voltage drops because there is no input power source.
Consequently, the control loop tries to compensate for
the output voltage drop and V
maximum. This lasts until AC input voltage is live again.
When AC input voltage is live again, high V
high switching current and more stress is put on the
MOSFET and diode. To protect against this, FL7930
internally checks if the input AC voltage exists. If input
Figure 44. Circuit of THD Optimizer
Figure 45. Effect of THD Optimizer
CC
is still alive during that time, but output
COMP
reaches its
COMP
allows
CC
of
17
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,
which applies less voltage and current stress on startup.
Figure 46. Operation without Input Voltage
Figure 47. Operation with Input Voltage
Absent Circuit
Absent Circuit
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