FSCQ0565RT Fairchild Semiconductor, FSCQ0565RT Datasheet - Page 18

• Manufacturer Part Number: FSCQ0565RT
• Description: A Quasi-Resonant Converter (QRC) typically shows lower EMI and higher power conversion efficiency compared to conventional hard-switched converter with a fixed switching frequency
• Manufacturer: Fairchild Semiconductor

FSCQ-Series Rev. 1.1.2
output, the output voltage may exceed the rated voltage
before the over load protection triggers, resulting in the
breakdown of the devices in the secondary side. In order
to prevent this situation, an over voltage protection
(OVP) circuit is employed. In general, the peak voltage of
the sync signal is proportional to the output voltage and
the FSCQ-Series uses a sync signal instead of directly
monitoring the output voltage. If the sync signal exceeds
12V, an OVP is triggered resulting in a shutdown of
SMPS. In order to avoid undesired triggering of OVP dur-
ing normal operation, the peak voltage of the sync signal
should be designed to be below 12V. This protection is
implemented in the auto restart mode.
4.4 Thermal Shutdown (TSD): The SenseFET and the
control IC are built in one package. This makes it easy for
the control IC to detect abnormal over temperature of the
SenseFET. When the temperature exceeds approxi-
mately 150 C, the thermal shutdown triggers. This pro-
tection is implemented in the latch mode.
5. Soft Start: The FSCQ-Series has an internal soft-start
circuit that increases PWM comparator’s inverting input
voltage together with the SenseFET current slowly after
it starts up. The typical soft start time is 20ms. The pulse
width to the power switching device is progressively
increased to establish the correct working conditions for
transformers, inductors, and capacitors. Increasing the
pulse width to the power switching device also helps pre-
vent transformer saturation and reduces the stress on
the secondary diode during startup. For a fast build up of
the output voltage, an offset is introduced in the soft-start
reference current.
6. Burst Operation: In order to minimize the power con-
sumption in the standby mode, the FSCQ-Series
employs burst operation. Once FSCQ-Series enters into
the burst mode, FSCQ-Series allows all output voltages
and effective switching frequency to be reduced. Figure
15 shows the typical feedback circuit for C-TV applica-
tions. In normal operation, the picture on signal is
applied and the transistor Q
ples R
fore, only V
normal operation and determined by R
In the standby mode, the picture ON signal is disabled
and the transistor Q
and D
mined by the zener diode breakdown voltage. Assuming
that the forward voltage drop of D
mode is approximately given by:
1
3
to the reference pin of KA431. Then, Vo2 is deter-
, D
z
o1
and D1 from the feedback network. There-
V
V
o1
is regulated by the feedback circuit in
o2
norm
stby
1
is turned off, which couples R
=
=
2.5
V
Z
1
+
is turned on, which decou-
0.7
R
------------------- -
1
1
R
+
+
is 0.7V, V
2
2.5
R
2
1
and R
o2
2
in standby
as:
3
, Dz,
18
Figure 17 shows the burst mode operation waveforms.
When the picture ON signal is disabled, Q
and R
KA431 through D
age on the reference pin of KA431 is higher than 2.5V,
which increases the current through the opto LED. This
pulls down the feedback voltage (V
and forces FSCQ-Series to stop switching. If the switch-
ing is disabled longer than 1.4ms, FSCQ-Series enters
into burst operation and the operating current is reduced
from I
decreases until it reaches V
the current through the opto LED decreases allowing the
feedback voltage to rise. When the feedback voltage
reaches 0.4V, FSCQ-Series resumes switching with a
predetermined peak drain current of 0.9A. After burst
switching for 1.4ms, FSCQ-Series stops switching and
checks the feedback voltage. If the feedback voltage is
below 0.4V, FSCQ-Series stops switching until the feed-
back voltage increases to 0.4V. If the feedback voltage is
above 0.4V, FSCQ-Series goes back to the normal oper-
ation.
The output voltage drop circuit can be implemented
alternatively as shown in Figure 16. In the circuit of Fig-
ure 16, the FSCQ-Series goes into burst mode, when
picture off signal is applied to Q1. Then, V
mined by the zener diode breakdown voltage. Assuming
that the forward voltage drop of opto LED is 1V, the
approximate value of V
R
bias
Figure 15. Typical Feedback Circuit to Drop
KA431
OP
3
and Dz are connected to the reference pin of
to 0.25mA (I
Output Voltage in Standby Mode
C
V
A
O2
R
R
C
D
F
1
V
. Before V
o2
R
OB
V
stby
F
O1
o2
). Since there is no switching, V
(B +)
in standby mode is given by:
R
R
=
2
1
D
o2
D
o2
1
V
stby
Z
Z
drops to V
+
. As V
R
Q1
1
3
Regulator
FB
Linear
o2
) of FSCQ-Series
reaches V
o2
1
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stby
is turned off
Picture ON
o2
, the volt-
is deter-
Micom
o2
stby
o2
,