© 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com
FSQ-Series • Rev. 1.1.3 16
FSCQ-Series — Green Mode Fairchild Power Switch (FPS™)
Fault
situation
9V
15V
Vcc
Vds
ICC
IOP
t
Fault
occurs Fault
removed
Normal
operation Normal
operation
Power
on
ISTART
Figure 33. Auto Restart Mode Protection
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 over load protection circuit can be
triggered during the load transition. To avoid this
undesired operation, the overload protection circuit is
designed to trigger after a specified 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 maximum 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 opto-coupler LED,
which also reduces the opto-coupler transistor current,
thus increasing the feedback voltage (Vfb). If Vfb
exceeds 2.8 V, D1 is blocked, and the 5 μA current
source starts to charge CB slowly up to VCC. In this
condition, Vfb continues increasing until it reaches 7.5 V,
then the switching operation is terminated as shown in
Figure 34. The delay for shutdown is the time required
to charge CB from 2.8 V to 7.5 V with 5 μA. In general, a
20~50 ms delay is typical for most applications. OLP is
implemented in auto restart mode.
VFB
t
2.8V
7.5V
Overload Protection
t12= CB*(7.5-2.8)/Idelay
t1t2
Figure 34. 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 FSCQ series has OLP (Overload
Protection), 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 35.
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.
2
S
Q
Q
R
OSC
R
2.5R
GND
Gate
Driver
LEB
PWM
+
-
VAOCP
AOCP
Rsense
Figure 35. AOCP Block
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 opto-coupler transistor becomes almost
zero. Then, Vfb climbs up in a similar manner to the over
load situation, forcing the preset maximum current to be
supplied to the SMPS until the over load 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.
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 12 V, an OVP is triggered resulting
in a shutdown of SMPS. In order to avoid undesired
triggering of OVP during normal operation, the peak
voltage of the sync signal should be designed to be
below 12 V. 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
approximately 150°C, the thermal shutdown triggers.
This protection is implemented in the latch mode.