FSDM1265RB
12
Figure 6. Auto Restart Operation
3.1 Over Load Protection (OLP
):
Overload occurs when
the load current exceeds a pre-set level due to an unexpected
event.
T
he protection circuit
(OLP) is
activated to protect the
SMPS. However, even
when the SMPS is operating normally, the
OLP
circuit can become activate during the load transition. To
avoid this undesired operation, the OLP circuit is designed to
become activate after a specified time to determine whether it is in a
transient or an overload mode. 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 beyond
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 2.5V, D1 is blocked and the 3.5uA current
source slowly starts to charge C
B
up to Vcc. In this condition, Vfb
continues increasing until it reaches 6V. Then the switching
operation terminates as shown in Figure 7. The delay time for
shutdown is the time required to charge C
B
from 2.5V to 6.0V
with 3.5uA. In general, a 10 ~ 50 ms delay is typical for most
applications.
Figure 7. Ove
r
Load Protection
3.2 Over voltage Protection (OVP
)
: If the secondary side
fee
dback 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 pre-set maximum current to be
supplied to the SMPS until the OLP is activated. Because more
energy than required is provided to the output, the output voltage
may exceed the rated voltage before the OLP is activated, resulting
in the breakdown of the devices in the secondary side. In order to
prevent this situation, an OVP circuit is used. Generally, Vcc is
proportional to the output voltage and the FSDM1265RB uses Vcc
instead of directly monitoring the output voltage. If V
CC
excee
ds
19V, an OVP circuit is activated resulting in the termination
of the switching operation. In order to avoid undesired
activation of OVP during normal operation, Vcc should be
designed to be below 19V.
3.3 Thermal Shutdown (TSD
):
The
SenseFET and the control
IC are built in one package making it easy for the control IC to
detect the heat generated by the SenseFET. When the temperature
exceeds approximately
150
°
C, the thermal shutdown is
activated.
4. Soft Star
t:
The FSDM1265RB has an internal soft-start circuit,
which increases the PWM comparator and slowly inverts the input
voltage together with the SenseFET current, after it starts up. The
typical soft-start time is 10ms, The pulse width to the power
switching device is progressively increased to establish the correct
working conditions for transformers, inductors, and capacitors. The
voltage on the output capacitors is progressively increased to
smoothly establish the required output voltage. This also helps
prevent transformer saturation and reduce the stress on the
secondary dio
de during startup.
5. Burst operatio
n:
To minimize power dissipation in the
standby mode, the FSDM1265RB enters burst mode operation. As
th
e load decreases, the feedback voltage decreases. As shown
Fault
situation
8V
12V
Vcc
Vds
t
Fault
occurs
Fault
removed
Normal
operation
Normal
operation
Power
on
V
FB
t
2.5V
6.0V
Over load protection
T
1
2
1
2
/I
=
Cfb
*
(6.0-2.5)
delay
T
1
T
2