Fairchild FSDH321, Руководство пользователя

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FSDH321, FSDL321
10
Figure 6. Over Load Protection (OLP)
4.2 Thermal Shutdown (TSD) : The Sense FET and the
control IC are integrated, making it easier for the control IC
to detect the temperature of the Sense FET. When the tem-
perature exceeds approximately 145
°
C, thermal shutdown is
activated.
4.3 Abnormal Over Current Protection (AOCP) : Even
though the FPS has OLP (Over Load Protection) and current
mode PWM feedback, these are not enough to protect the
FPS when a secondary side diode short or a transformer pin
short occurs. In addition to start-up, soft-start is also
activated at each restart attempt during auto-restart and when
restarting after latch mode is activated. The FPS has an
internal AOCP (Abnormal Over Current Protection) circuit,
as shown in Figure 7. When the gate turn-on signal is applied
to the power Sense FET, 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, pulse-by-pulse AOCP is triggered regardless of
uncontrollable LEB time. Here, pulse-by-pulse AOCP stops
the Sense FET within 350ns after it is activated.
Figure 7. Abnormal Over Current Protection (AOCP)
4.4 Over Voltage Protection (OVP) : In the event of a mal-
function in the secondary side feedback circuit, or an open
feedback loop caused by a soldering defect, the current
through the opto-coupler transistor becomes almost zero
(refer to Figure 5). Then, V
FB
climbs up in a similar manner
to the over load situation, forcing the preset maximum cur-
rent to be supplied to the SMPS until the over load protection
is activated. Because excess energy is provided to the output,
the output voltage may exceed the rated voltage before the
over load protection is activated, 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, Vcc is proportional to the output volt-
age and the FPS uses Vcc instead of directly monitoring the
output voltage. If V
CC
exceeds 19V, OVP circuit is activated
resulting in termination of the switching operation. In order
to avoid undesired activation of OVP during normal opera-
tion, Vcc should be properly designed to be below 19V.
V
FB
t
3V
6V
Over Load Protection
t
12
= C
FB
× (V(t
2
)-V(t
1
)) / I
DELAY
t
1
t
2
V t V V t V A I
I
t V t V
C t
DELAY
DELAY
FB
6 ) ( , 3 ) ( , 5 ;
) ( ) (
2 1
1 2
12
= = =
−
= µ
Vsense
V
FB,in
Gate Driver
Rsense
CLK
Drain
V
AOCP
PWM
COMPARATOR
AOCP
COMPARATOR
LEB
R
S Q