FSQ0765RQ Fairchild Semiconductor, FSQ0765RQ Datasheet - Page 14

• Manufacturer Part Number: FSQ0765RQ
• Description: The highly integrated FSBH series consists of an integrated current-mode Pulse Width Modulator (PWM) and an avalanche-rugged 700V SenseFET
• Manufacturer: Fairchild Semiconductor

FSQ0765RQ Rev. 1.0.1
© 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 CB slowly up to V
this condition, V
6V, when the switching operation is terminated, as
shown in Figure 29. 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 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.
6.0V
2.5V
V
FB
FB
). If V
FB
Figure 29. Overload Protection
exceeds 2.5V, D1 is blocked and the 5µA
t
1
FB
continues increasing until it reaches
Overload protection
t
12
= C
FB
FB
from 2.5V to 6V with 5µA. A
O
*(6.0-2.5)/I
) decreases below the set
FSQ0765R Rev.00
delay
t
CC
2
. In
t
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 31.
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 the overload protection
triggers. Because more energy than required is provided
to the output, the output voltage may exceed the rated
voltage before the overload protection triggers, resulting
in the breakdown of the devices in the secondary side.
To prevent this situation, an OVP circuit is employed. In
general, the peak voltage of the sync signal is
proportional to the output voltage and the FSQ-series
V
FSQ0765R Rev.00
FB
V
Figure 30. Abnormal Over-Current Protection
I
0
0
o
3R
0
o
R
MOSFET
Current
Drain
Figure 31. Output Short Waveforms
AOCP
PWM
FB
OSC
D
climbs up in a similar manner to the
Diode Current
250ns
LEB
Rectifier
FB
S
R
Minimum turn-on time
CC
Q
Q
is higher than 2V and the
Turn-off delay
+
-
output short occurs
reaches V
FB
driver
V
Gate
FSQ0765R Rev. 00
OCP
and SenseFET
1.2us
R
start
www.fairchildsemi.com
sense
again. An
I
LIM
2
GND