FAN6921ML Fairchild Semiconductor, FAN6921ML Datasheet - Page 18

• Manufacturer Part Number: FAN6921ML
• Description: The highly integrated FAN6921ML combines a Power Factor Correction (PFC) controller and a Quasi-Resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6921ML • Rev. 1.0.2
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 power
system, FAN6921ML provides a high current to charge
external V
time and build up normal rated output voltage within
three seconds. To save power consumption, after V
voltage exceeds turn-on voltage and enters normal
operation; this high voltage startup circuit is shut down
to avoid power loss from startup resistor.
Figure 36 shows the characteristic curve of V
and operating current I
than V
operation and turns off some unnecessary internal
circuit to reduce operating current. By doing so, the
period from V
the hiccup mode frequency can be decreased to reduce
the input power in case of output short circuit. Figure 37
shows the typical waveforms of V
signal at hiccup mode operation.
Green-Mode Operation and PFC-ON / OFF Control
(FB Pin)
Green mode is used to further reduce power loss in the
system (e.g. switching loss). It uses an off-time
modulation technique to regulate switching frequency
according to FB pin voltage. When output loading is
decreased, FB voltage becomes lower due to secondary
feedback movement and the t
t
detection circuit is activated to detect the valley on the
drain voltage of the PWM switch. When the valley signal
OFF-MIN
V
V
V
Figure 37. Typical Waveform of V
DD-OFF
DD-PWM-OFF
DD-ON
Gate
Figure 36. V
(determined by FB voltage), the internal valley
DD-PWM-OFF
DD
Gate Signal in Hiccup Mode Operation
capacitor to accelerate controller’s startup
DD-PWM-OFF
DD
, FAN6921ML stops all switching
vs. I
I
DD-PWM-OFF
DD
to V
DD-OP
. When V
I
DD-OP
DD-OFF
Characteristic Curve
OFF-MIN
I
can be extended and
DD-ST
DD
DD
DD
is extended. After
voltage and gate
voltage is lower
Voltage and
DD
voltage
DD
is detected, FAN6921ML outputs PWM gate signal to
turn on the switch and begin a new switching cycle.
With green mode and valley detection, at light load
condition; power system can perform extended valley
switching at DCM operation and further reduce
switching loss for better conversion efficiency. The FB
pin voltage versus t
shown in Figure 38. As Figure 38 shows, FAN6921ML
can extend t
400Hz switching frequency.
Referring to Figure 1 and Figure 2, FB pin voltage is not
only used to receive secondary feedback signal to
determine gate on time, but also determines PFC stage
on or off status. At no-load or light-load conditions, if PFC
stage is set to be off; that can reduce power consumption
from
conversion efficiency. When output loading is decreased,
the FB pin voltage becomes lower and, therefore, the
FAN6921ML can detect the output loading level
according to the FB pin voltage to control the on / off
status of the PFC part.
Valley Detection (DET Pin)
When FAN6921ML operates in green mode, t
determined by the green mode circuit according to FB
pin voltage level. After t
detection circuit is activated. During the off time of the
PWM
discharges to zero; the transformer inductor and
parasitic capacitor of PWM switch start to resonate
concurrently. When the drain voltage on the PWM
switch falls, the voltage across on auxiliary winding V
also decreases since auxiliary winding is coupled to
primary winding. Once the V
falls to negative, V
(refer to Figure 39) and FAN6921ML is forced to flow
out a current I
this I
threshold current, PWM gate signal is sent out after a
fixed delay time (200ns typical).
18
DET
PFC
Figure 38. V
switch,
current. If this source current rises to a
OFF
stage
DET
when
time up to 2.5ms, which is around
. FAN6921ML reflects and compares
DET
FB
OFF-MIN
switching
voltage is clamped by the DET pin
Characteristic Curve
Voltage vs. t
transformer
OFF-MIN
time characteristic curve is
AUX
device
voltage resonates and
, the internal valley-
OFF-MIN
inductor
and
Time
www.fairchildsemi.com
OFF-MIN
increase
current
AUX
is