FSQ0465RS Fairchild Semiconductor, FSQ0465RS Datasheet - Page 15

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FSQ0465RS

Manufacturer Part Number
FSQ0465RS
Description
A Quasi-Resonant Converter (QRC) generally shows lower EMI and higher power conversion efficiency than a conventional hard-switched converter with a fixed switching frequency
Manufacturer
Fairchild Semiconductor
Datasheet

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FSQ0465RS/RB Rev. 1.0.1
© 2008 Fairchild Semiconductor Corporation
resulting in the termination of the switching operation. To
avoid undesired activation of OVP during normal
operation, V
4.5 Thermal Shutdown with Hysteresis (TSD): The
SenseFET and the control IC are built in one package.
This enables the control IC to detect the abnormally high
temperature of the SenseFET. If the temperature
exceeds approximately 140°C, the thermal shutdown
triggers IC shutdown. The IC recovers its operation when
the junction temperature decreases 60°C from TSD
temperature and V
5. Soft-Start: The FPS has an internal soft-start circuit
that increases PWM comparator inverting input voltage
with the SenseFET current slowly after it starts up. The
typical soft-start time is 17.5ms. The pulse width to the
power switching device is progressively increased to
establish the correct working conditions for transformers,
inductors, and capacitors. The voltage on the output
capacitors is progressively increased with the intention of
smoothly establishing the required output voltage. This
mode helps prevent transformer saturation and reduces
stress on the secondary diode during startup.
6. Burst Operation: To minimize power dissipation in
standby mode, the FPS enters burst-mode operation. As
the load decreases, the feedback voltage decreases. As
shown in Figure 32, the device automatically enters
burst-mode when the feedback voltage drops below
V
output voltages start to drop at a rate dependent on
standby current load. This causes the feedback voltage
to rise. Once it passes V
resumes. The feedback voltage then falls and the
process repeats. Burst-mode operation alternately
enables and disables switching of the power SenseFET,
thereby reducing switching loss in standby mode.
BURL
(350mV). At this point, switching stops and the
CC
should be designed below 19V.
CC
reaches startup voltage (V
BURH
(550mV), switching
start
).
15
7. Switching Frequency Limit: To minimize switching
loss and Electromagnetic Interference (EMI), the
MOSFET turns on when the drain voltage reaches its
minimum value in quasi-resonant operation. However,
this causes switching frequency to increases at light load
conditions. As the load decreases or input voltage
increases, the peak drain current diminishes and the
switching frequency increases. This results in severe
switching losses at light-load condition, as well as
intermittent switching and audible noise. These problems
create limitations for the quasi-resonant converter
topology in a wide range of applications.
To overcome these problems, FSQ-series employs a
frequency-limit function, as shown in Figures 33 and
Figure 34. Once the SenseFET is turned on, the next
turn-on is prohibited during the blanking time (t
the blanking time, the controller finds the valley within
the detection time window (t
MOSFET, as shown in Figures 33 and Figure 34 (Cases
A, B, and C). If no valley is found during t
SenseFET is forced to turn on at the end of t
Therefore, the devices have a minimum switching
frequency of 48kHz and a maximum switching frequency
of 67kHz.
0.55V
0.35V
Vo
V
V
I
V
DS
FB
DS
set
O
Figure 32. Waveforms of Burst Operation
FSQ0465 Rev. 00
t1
Switching
disabled
t2 t3
W
) and turns on the
Switching
disabled
W
www.fairchildsemi.com
, the internal
t4
W
(Case D).
B
). After
time

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