fs8s0765rcb Fairchild Semiconductor, fs8s0765rcb Datasheet - Page 11

• Manufacturer Part Number: fs8s0765rcb
• Description: 7a/650v Snychronized Power Switch, Primary Side Regulation
• Manufacturer: Fairchild Semiconductor

FS8S0765RCB Rev. 1.0.2
Functional Description
1. Start up: To guarantee stable operation of the control
IC, FS8S0765RCB has UVLO circuit with 6V hysteresis
band. Figure 22 shows the relation between the supply
current (Icc) and the supply voltage (Vcc). Before Vcc
reaches 15V, the FPS consumes only startup current of
80mA, which is usually provided by the DC link through
the 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 normal operation until Vcc goes below the
stop voltage of 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 23 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 comparator (Vfb*). When
MOSFET turns on, usually there exists high current
spike in the MOSFET current caused by primary-side
capacitance
recovery. To prevent premature termination of the
switching pulse due to the current spike, the FPS
employs leading edge blanking (LEB). The leading edge
blanking circuit inhibits the PWM comparator for a short
time after the MOSFET is turned on.
15mA
80uA
Figure 23. Pulse-Width Modulation (PWM)
Figure 22. High Voltage Current Source
V
Icc
psr
D
R
Z
Z
Vfb
and
Cfb
Vstop=9V
FB
4
secondary-side
2uA
V
Vcc
SD
Circuit
D1
Vref
Vfb*
Vstart=15V
0.9m
D2
A
2.5R
R
OSC
rectifier
OLP
driver
Gate
OVP
reverse
Vcc
11
3. Protection function: FS8S0765RCB has four self-
protective functions such as abnormal over current
protection (AOCP), overload protection (OLP), over-
voltage protection (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
μA. Then, Vcc begin to increase with the current
provided through the start-up resistor. When Vcc
reaches 15V, the FPS resumes 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 24.
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 25. It turns off the
SenseFET within 300ns after the abnormal over-current
condition is sensed.
Figure 24. Auto restart operation after
Vcc
15V
OVP
OLP
TSD
9V
V
V
Vds
Vcc
Figure 25. AOCP block
UVLO
S Q'
R
protection
(OCP,OLP,OVP or TSD)
Protection is activated
AOCP COMP.
DRIVER
PWM Comp
GATE
Vsense : 1V
Auto restart
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time
time
SenseFE
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