FSCQ0565RT Fairchild Semiconductor, FSCQ0565RT Datasheet - Page 17

• 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
– Latch mode protection: Once this protection is trig-
4.1 Over Load Protection (OLP): Overload is defined
as the load current exceeding its normal level due to an
unexpected abnormal event. In this situation, the protec-
tion circuit should trigger to protect the SMPS. However,
even when the SMPS is in the normal operation, the over
load protection circuit can be triggered during the load
transition. To avoid this undesired operation, the over
load protection circuit is designed to trigger after a speci-
fied time to determine whether it is a transient situation
or an overload situation. Because of the pulse-by-pulse
current limit capability, the maximum peak current
through the SenseFET is limited, and therefore the maxi-
mum input power is restricted with a given input voltage.
If the output consumes more than this maximum power,
the output voltage (Vo) decreases below the set voltage.
This reduces the current through the optocoupler LED,
which also reduces the optocoupler transistor current,
thus increasing the feedback voltage (Vfb). If Vfb
exceeds 2.8V, D1 is blocked, and the 5µA current source
starts to charge C
Vfb continues increasing until it reaches 7.5V, then the
switching operation is terminated as shown in Figure 13.
The delay time for shutdown is the time required to
charge C
20~50ms delay time is typical for most applications. OLP
is implemented in auto restart mode.
I
START
gered, switching is terminated and the Sense FET
remains off until the AC power line is unplugged. Then,
V
and 15V. The latch is reset only when V
charged to 6V by unplugging the AC power line.
V
V
I
I
CC
15V
CC
CC
OP
9V
Figure 12. Auto Restart Mode Protection
ds
continues charging and discharging between 9V
Power
B
on
from 2.8V to 7.5V with 5µA. In general, a
operation
Normal
B
slowly up to V
occurs
Fault
situation
Fault
removed
Fault
CC
. In this condition,
operation
Normal
CC
is dis-
t
17
4.2 Abnormal Over Current Protection (AOCP): When
the secondary rectifier diodes or the transformer pins are
shorted, a steep current with extremely high di/dt can
flow through the SenseFET during the LEB time. Even
though the FSCQ-Series has OLP (Over Load Protec-
tion), it is not enough to protect the FSCQ-Series in that
abnormal case, since severe current stress will be
imposed on the SenseFET until the OLP triggers. The
FSCQ-Series has an internal AOCP (Abnormal Over
Current Protection) circuit as shown in Figure 14. When
the gate turn-on signal is applied to the power
SenseFET, the AOCP block is enabled and monitors the
current through the sensing resistor. The voltage across
the resistor is then compared with a preset AOCP level.
If the sensing resistor voltage is greater than the AOCP
level, the set signal is applied to the latch, resulting in the
shutdown of SMPS. This protection is implemented in
the latch mode.
4.3 Over Voltage Protection (OVP): If the secondary
side feedback circuit malfunctions or a solder defect
causes an open in the feedback path, the current
through the optocoupler transistor becomes almost zero.
Then, Vfb climbs up in a similar manner to the over load
situation, forcing the preset maximum current to be sup-
plied to the SMPS until the over load protection triggers.
Because more energy than required is provided to the
7.5V
2.8V
V
2.5R
FB
R
Figure 13. Over Load Protection
AOCP
OSC
–
+
Figure 14. AOCP Block
PWM
T
1
Over load protection
LEB
T
S
R
12
Q
Q
= C
B
+
–
* (7.5 – 2.8) / I
Driver
Vaocp
Gate
R
sense
delay
www.fairchildsemi.com
T
2
GND
2
t