FAN6921MR Fairchild Semiconductor, FAN6921MR Datasheet - Page 19

• Manufacturer Part Number: FAN6921MR
• Description: The highly integrated FAN6921MR combines Power Factor Correction (PFC) controller and Quasi-Resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6921MR Rev. 1.0.2
PWM Stage
HV Startup and Operating Current (HV Pin)
The HV pin is connected to 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,
FAN6921MR provides a high current to charge external
V
build up normal rated output voltage within three
seconds. To save power consumption, after V
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 internal unnecessary
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 mechanism 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
extended. After t
internal valley detection circuit is activated to detect the
valley on the drain voltage of the PWM switch. When
DD
Figure 37. Typical Waveform of V
Figure 36. V
capacitor to speed up controller’s startup time and
DD-PWM-OFF
Gate Signal at Hiccup Mode Operation
DD-PWM-OFF
OFF-MIN
DD
, FAN6921MR stops all switching
vs. I
DD
(determined by FB voltage), the
to V
DD-OP
. When V
DD-OFF
Characteristic Curve
can be extended and
DD
DD
DD
voltage and gate
voltage is lower
Voltage and
DD
DD
OFF-MIN
voltage
voltage
is
the valley signal is detected, FAN6921MR outputs PWM
gate signal to turn on the switch and begin a new
switching cycle.
With green mode operation and valley detection, at light
load condition; power system can perform extended
valley switching at DCM operation and can further
reduce switching loss for getting better conversion
efficiency. The FB pin voltage versus t
characteristic curve is shown in Figure 38. As Figure 38
shows, FAN6921MR can narrow down to 2.25ms t
time, which is around 440Hz 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
FAN6921MR 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 FAN6921MR operates in green mode, t
time is determined by the green mode circuit according
to FB pin voltage level. After t
valley detection circuit is activated. During the t
of PWM switch, when transformer inductor current
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 FAN6921MR 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).
19
2.25ms
37µs
t
8µs
DET
OFF-MIN
PFC
Figure 38. V
current. If this source current rises to a
1.15V(V
stage
DET
V
. FAN6921MR reflects and compares
CTL-PFC-OFF
DET
PFC OFF
G
FB
)
switching
voltage is clamped by the DET pin
Characteristic Curve
Voltage vs. t
AUX
OFF-MIN
device
voltage resonates and
V
OFF-MIN
2.1V(V
PFC On
CTL-PFC-ON
time, the internal
and
Time
N
OFF-MIN
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)
increase
OFF
OFF-MIN
time
time
AUX
OFF