FSQ0565RS Fairchild Semiconductor, FSQ0565RS Datasheet - Page 14

• Manufacturer Part Number: FSQ0565RS
• 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

FSQ0565RS/RQ Rev. 1.0.3
© 2008 Fairchild Semiconductor Corporation
4.1 Overload Protection (OLP): Overload is defined as
the load current exceeding its normal level due to an
unexpected abnormal event. In this situation, the
protection circuit should trigger to protect the SMPS.
However, even when the SMPS is in the normal
operation, the overload protection circuit can be
triggered during the load transition. To avoid this
undesired operation, the overload protection circuit is
designed to trigger only after a specified time to
determine whether it is a transient situation or a true
overload situation. Because of the pulse-by-pulse
current limit capability, the maximum peak current
through the SenseFET is limited, and therefore the
maximum input power is restricted with a given input
voltage. If the output consumes more than this maximum
power, the output voltage (V
voltage. This reduces the current through the opto-
coupler LED, which also reduces the opto-coupler
transistor current, thus increasing the feedback voltage
(V
current source starts to charge C
this condition, V
6V, when the switching operation is terminated, as
shown in Figure 30. The delay time for shutdown is the
time required to charge C
20 ~ 50ms delay time is typical for most applications.
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 FSQ-series has overload protection, it is not
enough to protect the FSQ-series in that abnormal case,
since severe current stress is imposed on the SenseFET
until OLP triggers. The FSQ-series has an internal
AOCP circuit, shown in Figure 31. 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
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
the SMPS.
FB
6 .0 V
2 .5 V
V
). If V
F B
FB
Figure 30. Overload Protection
exceeds 2.5V, D1 is blocked and the 5µA
T
1
FB
continues increasing until it reaches
O ve rlo a d p ro te c tio n
t
1 2
= C
FB
fb
from 2.5V to 6V with 5µA. A
O
*(6 .0 -2 .5 )/I
) decreases below the set
B
slowly up to V
d e la y
F S Q 0 5 6 5 R e v .0 0
T
2
CC
t
. In
14
4.3 Output-Short Protection (OSP): If the output is
shorted, steep current with extremely high di/dt can flow
through the SenseFET during the LEB time. Such a
steep current brings high voltage stress on the drain of
SenseFET when turned off. To protect the device from
such an abnormal condition, OSP is included in the FSQ-
series. It is comprised of detecting V
turn-on time. When the V
SenseFET turn-on time is lower than 1.2µs, the FPS
recognizes this condition as an abnormal error and shuts
down PWM switching until V
abnormal condition output short is shown in Figure 32.
4.4.1
FSQ0565RS: If the secondary-side feedback circuit
malfunctions or a solder defect causes an opening in the
feedback path, the current through the opto-coupler
transistor becomes almost zero. In this case, V
up in a similar manner to the overload situation, forcing
the preset maximum current to be supplied to the SMPS
until overload protection is activated. Because more
energy than required is provided to the output, the output
voltage may exceed the rated voltage before overload
protection is activated, resulting in the breakdown of the
devices in the secondary side. To prevent this situation,
an over-voltage protection (OVP) circuit is employed. In
general, V
V
FSQ0765R Rev.00
V
FB
Figure 31. Abnormal Over-Current Protection
I
0
0
o
o
3R
0
R
MOSFET
V
Current
Drain
Figure 32. Output Short Waveforms
CC
CC
AOCP
is proportional to the output voltage and the
PWM
Over-Voltage
OSC
D
250ns
Rectifier
LEB
Current
Diode
FB
S
R
Minimum turn-on time
CC
Q
Q
is higher than 2V and the
output short occurs
Turn-off delay
+
-
Protection
reaches V
FB
driver
FSQ0565 Rev. 00
V
Gate
OCP
and SenseFET
1.2µs
R
start
www.fairchildsemi.com
sense
(OVP)
again. An
fb
I
LIM
climbs
2
GND
of