suddenly interrupted during two or three AC line
periods; VCC 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 VCOMP 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, FAN7930C
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.
Figure 41. Effect of THD Optimizer
10. Current Sense: The MOSFET current is sensed
using an external sensing resistor for over-current
protection. If the CS pin voltage is higher than 0.8V, the
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.
over-current protection comparator generates
a
protection signal. An internal RC filter of 40kΩ and 8pF
is included to filter switching noise.
9. VIN Absent Detection: To save power loss caused by
input voltage sensing resistors and to optimize THD, the
FAN7930C omits AC input voltage detection. Therefore,
no information about AC input is available from the
internal controller. In many cases, the VCC of PFC
controller is supplied by a independent power source,
like standby power. In this scheme, some mismatch
may exist. For example, when the electric power is
11. Gate Driver Output: FAN7930C 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/-800mA peak current with a typical rise and fall
time of 50ns with 1nF load. The output voltage is
clamped to 13V to protect the MOSFET gate even if the
VCC voltage is higher than 13V.
VOUT
VIN
Though VIN is
eliminated, operation of
controller is normal due
to the large bypass
capacitor.
VAUX
DMAX
fMIN
DMIN
MOSFET gate
NewVCOMP
fMIN
VIN Absence Detected
IDS
Smooth
Soft-Start
FAN7930 Rev.00
t
Figure 43. Operation with VIN Absent Circuit
Figure 42. Operation without VIN Absent Circuit
© 2010 Fairchild Semiconductor Corporation
FAN7930C • Rev. 1.0.0
www.fairchildsemi.com
16
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