Data Sheet

FSDM0465RE, FSDM0565RE, FSDM07652RE — Green Mode Power Switch
Figure 19. Auto Restart Operation
3.1 Overload Protection (OLP): Overload is defined as
the load current exceeding a pre-set level due to an
unexpected event. In this situation, the protection circuit
should be activated to protect the SMPS. Even when the
SMPS is in normal operation, the overload protection
circuit can be activated during the load transition. To
avoid this undesired operation, the overload protection
circuit is designed to be activated 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 beyond this maximum
power, the output voltage (V
O
) 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
(V
FB
). If V
FB
exceeds 2.5V, D1 is blocked and the 3.5µA
current source starts to charge C
B
slowly up to V
CC
. In
this condition, V
FB
continues increasing until it reaches
6V, when the switching operation is terminated, as
shown in Figure 20. The delay time for shutdown is the
time required to charge C
B
from 2.5V to 6.0V with 3.5µA.
A 10 ~ 50ms delay time is typical for most applications.
Figure 20. Overload Protection
3.2 Over-Voltage Protection (OVP): If the secondary
side feedback circuit were to malfunction or a solder
defect caused an opening in the feedback path, the
current through the opto-coupler transistor becomes
almost zero. In this event, V
FB
climbs in a similar manner
to the overload situation, forcing the preset maximum
current to be supplied to the SMPS until the overload
protection is activated. Because more energy than
required is provided to the output, the output voltage may
exceed the rated voltage before the 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
CC
is proportional to the output voltage and the
FSDM0x65RE uses V
CC
instead of directly monitoring
the output voltage. If V
CC
exceeds 19V, an OVP circuit is
activated, resulting in the termination of the switching
operation. To avoid undesired activation of OVP during
normal operation, V
CC
should be designed below 19V.
3.3 Thermal Shutdown (TSD): The SenseFET and the
control IC are built in one package. This makes it easy
for the control IC to detect the heat generation from the
SenseFET. When the temperature exceeds ~150°C, the
thermal shutdown is activated.
4. Soft-Start: The FSDM0x65RE has an internal soft-
start circuit that increases PWM comparator inverting
input voltage, together with the SenseFET current,
slowly 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 with the intention of smoothly establishing the
required output voltage. It also helps prevent transformer
saturation and reduces the stress on the secondary
diode during start-up.
Fault
situation
8V
12V
V
CC
V
DS
t
Fault
occurs
Fault
removed
Normal
operation
Normal
operation
Power
on
FSDM0565RE Rev: 00
V
FB
t
2.5V
6.0V
Overload protection
T
12
= C
FB
*(6.0-2.5)/I
delay
T
1
T
2
FSDM0565RE Rev: 00
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