FAN6920MR Fairchild Semiconductor, FAN6920MR Datasheet - Page 19

• Manufacturer Part Number: FAN6920MR
• Description: The highly integrated FAN6920MR combines Power Factor Correction (PFC) controller and quasi-resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6920MR • Rev. 1.0.5
PFC Burst Mode
To minimize the power dissipation at light-load
condition, the FAN6920MR PFC control enters burst-
mode operation. As the load decreases, the PWM
feedback voltage (V
PFC-BM
V
increases. When the PFC feedback voltage on INV pin
(V
V
stops and the PFC output voltages start to drop. Once
the V
voltage (V
starts switching again. Burst-mode operation alternately
enables and disables switching of the power MOSFET
to reduce the switching loss at light-load condition.
The V
shown in Figure 1. Since the VIN pin is connected to
rectified AC input line voltage through the resistive
divider, a higher line voltage generates a higher VIN pin
voltage. The V
increases, making the PFC choke current be limited at a
higher input voltage to reduce acoustic noise. If the
V
shuts down at light load with high line voltage input
condition.
COMP
COMP
COMP-H
INV
) triggers the OVP threshold voltage (V
INV
COMP-H
for 100ms, the device enters PFC burst mode, the
pulls high to V
pulls low to V
drops below the feedback comparator reference
Figure 36. PFC Burst Mode Behavior
is below the PFC V
REF
is adjusted by the voltage of the VIN pin, as
), V
COMP-H
COMP
FB
) decreases. When V
COMP-L
pulls high to V
COMP-H
decreases as VIN pin voltage
, the OPFC pin switching
, and PFC output voltage
OZ
, the PFC automatically
COMP-H
FB
and OPFC
< V
INV-OVP
CTRL-
),
PWM Stage
HV Startup and Operating Current (HV Pin)
The HV pin is connected to the AC line through a
resistor (refer to Figure 1). With a built-in high-voltage
startup circuit, when AC voltage is applied to the power
system, FAN6920MR provides a high current to charge
the external V
startup time and build up normal rated output voltage
within three seconds. To save power consumption, after
V
operation; this high-voltage startup circuit is shut down
to avoid power loss from startup resistor.
Figure 1 shows the characteristic curve of V
and operating current I
than V
operation and turns off unnecessary internal circuits to
reduce operating current. By doing so, the period from
V
mode frequency can be decreased to reduce the input
power in case of output short circuit. Figure 39 shows
the typical waveforms of V
with hiccup mode operation.
19
DD
DD-PWM-OFF
Figure 38. V
voltage exceeds turn-on voltage and enters normal
Figure 37. V
DD-PWM-OFF
to V
DD
DD-OFF
DD
, FAN6920MR stops all switching
COMP-H
Characteristic Curve
capacitor to speed up controller’s
vs. I
can be extended and the hiccup
DD
DD-OP
Voltage vs. V
. When V
DD
Characteristic Curve
voltage and gate signal
DD
VIN
voltage is lower
Voltage
www.fairchildsemi.com
DD
voltage