FSQ0465RS

Manufacturer Part NumberFSQ0465RS
DescriptionA Quasi-Resonant Converter (QRC) generally shows lower EMI and higher power conversion efficiency than a conventional hard-switched converter with a fixed switching frequency
ManufacturerFairchild Semiconductor
FSQ0465RS datasheet
 
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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
) decreases below the set
O
voltage. This reduces the current through the opto-
coupler LED, which also reduces the opto-coupler
transistor current, thus increasing the feedback voltage
(V
). If V
exceeds 2.5V, D1 is blocked and the 5µA
FB
FB
current source starts to charge C
this condition, V
continues increasing until it reaches
FB
6V, when the switching operation is terminated, as
shown in Figure 29. The delay time for shutdown is the
time required to charge C
from 2.5V to 6V with 5µA. A
FB
20 ~ 50ms delay time is typical for most applications.
V
F B
O ve rlo a d p ro te c tio n
6 .0 V
2 .5 V
t
= C
*(6 .0 -2 .5 )/I
1 2
fb
T
1
Figure 29. Overload Protection
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 30. 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.
© 2008 Fairchild Semiconductor Corporation
FSQ0465RS/RB Rev. 1.0.1
3R
R
FSQ0465 Rev.00
Figure 30. Abnormal Over-Current Protection
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
slowly up to V
. In
B
CC
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 31.
MOSFET
F S Q 0 4 6 5 R e v .0 0
Drain
Current
V
FB
0
V
o
d e la y
0
I
o
T
t
2
0
Figure 31. Output Short Waveforms
4.4 Over-Voltage Protection (OVP): 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. Then, V
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
proportional to the output voltage and the FSQ-series
uses V
CC
voltage. If V
14
OSC
S
Q
PWM
Gate
LEB
driver
R
Q
200ns
R
sense
+
AOCP
V
-
OCP
and SenseFET
FB
is higher than 2V and the
FB
reaches V
again. An
CC
start
Turn-off delay
Rectifier
Diode
Current
Minimum turn-on time
D
1.2µs
output short occurs
FSQ0465 Rev. 00
climbs up in a similar manner to the
fb
instead of directly monitoring the output
exceeds 19V, an OVP circuit is activated,
CC
www.fairchildsemi.com
2
GND
I
LIM
is
CC