FS8S0765RCB
Functional Description
1.
Start up
: To guarantee stable operation of the
control IC, FS8S0765RCB has UVLO circuit with 6V
hysteresis band. Figure 1 shows the relation between the sup-
ply current (Icc) and the supply voltage (Vcc). Before Vcc
reaches 15V, the FPS consumes only startup current of
80µA, which is usually provided by the DC link through
start-up resistor. When Vcc reaches 15V, the FPS begins
operation and the operating current increases to 15mA as
shown. Once the control IC starts operation, it continues its
normal operation until Vcc goes below the stop voltage of
9V
Icc
3. Protection function
: FS8S0765RCB has 4 self
protective functions such as abnormal over current protec-
tion (AOCP), over load protection (OLP), over voltage pro-
tection (OVP) and thermal shutdown (TSD). Because these
protection circuits are fully integrated into the IC without
external components, the reliability can be improved without
cost increase. In the event of these fault conditions, the FPS
enters into auto-restart operation. Once the fault condition
occurs, switching operation is terminated and MOSFET
remains off, which forces Vcc to be reduced. When Vcc
reaches 9V, the protection is reset and the supply current
reduces to 80 uA. Then, Vcc begin to increase with the cur-
rent provided through the start-up resistor. When Vcc
reaches 15V, the FPS resumes its normal operation if the
fault condition is removed. In this manner, the auto-restart
alternately enables and disables the switching of the power
MOSFET until the fault condition is eliminated as illustrated
in figure 3.
Prote c tion is a c tiva te d
(O CP,O LP,O VP or T SD)
15mA
V
Vds
80uA
Vcc
Vstop=9V
Vstart=15V
OVP
Figure 1. Strat up with hysteresis
tim e
V
Vcc
15
V
9V
2. Feedback Control
: FS8S0765RCB employs primary side
regulation, which permits elimination of feedback circuit
components in the secondary side such as opto coupler and
TL431. Figure 2 shows the primary side control circuit. The
primary side regulation voltage (Vpsr) is controlled to the
breakdown voltage of zener diode (Dz). Because current
mode control is employed, the drain current of the power
MOSFET is limited by the inverting input of PWM compara-
tor (Vfb*). When MOSFET turns on, usually there exists
high current spike in the MOSFET current caused by pri-
mary-side capacitance and secondary-side rectifier reverse
recovery. In order to prevent premature termination of the
switching pulse due to the current spike, the FPS employs
leading edge blanking (LEB). The leading edge blanking cir-
cuit inhibits the PWM comparator for a short time after the
MOSFET is turned on.
Vcc
Au to re sta rt
tim e
Figure 3. Auto restart operation after protection
3.1 Abnormal Over Current Protection (AOCP)
: When
the secondary rectifying diodes or the transformer pins are
shorted, a steep current with extremely high di/dt can flow
during the LEB time. Therefore, the abnormal over current
protection (AOCP) block is added to ensure the reliability as
shown in figure 4. It turns off the SenseFET within 300ns
after the abnormal over current condition is sensed.
V cc
2uA
V ref
0.9m A
OVP
OLP
S Q'
R
G ate
driver
O SC
V
p sr
D
Z
V fb
FB
4
C fb
GATE
DRIVER
SenseFET
D1
D2
2.5R
V fb*
R
TSD
PWM Comp
UVLO
R
Z
V
SD
O LP
AOCP COMP.
Vsense : 1V
Figure 2. Primary side control circuit
Figure 4. AOCP block
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FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
