參數(shù)資料
型號(hào): FSCQ
廠商: Fairchild Semiconductor Corporation
英文描述: Green Mode Fairchild Power Switch (FPS)
中文描述: 綠色模式飛兆半導(dǎo)體功率開(kāi)關(guān)(FPS)
文件頁(yè)數(shù): 16/44頁(yè)
文件大?。?/td> 1204K
代理商: FSCQ
FSCQ-SERIES
16
AC power line.
Figure 12. Auto Restart Mode Protection
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 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 over load 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 2.8V, D1 is blocked
,
and the
5uA current source starts to charge C
B
slowly up to Vcc. In
this condition, 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
B
from 2.8V to 7.5V with 5uA. In general, a 20 ~
50 ms delay time is typical for most applications.
OLP
is
implemented in auto restart mode.
Figure 13. Over Load 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 (Over Load 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 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.
Figure 14. 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
Fault
situation
9V
15V
Vcc
Vds
I
CC
I
OP
t
Fault
occurs
Fault
removed
Normal
operation
Normal
operation
Power
on
I
START
V
FB
t
2.8V
7.5V
Over load protection
T
12
= C
B
*(7.5-2.8)/I
delay
T
1
T
2
2
S
Q
Q
R
OSC
R
2.5R
GND
Gate
Driver
LEB
PWM
+
-
Vaocp
AOCP
R
sense
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