fsq0465rb Fairchild Semiconductor, fsq0465rb Datasheet - Page 12

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fsq0465rb

Manufacturer Part Number
fsq0465rb
Description
Fsq0465rs/rb- Green-mode Fairchild Power Switch Fps? For Quasi-resonant Operation - Low Emi And High Efficiency
Manufacturer
Fairchild Semiconductor
Datasheet
FSQ0465RS/RB Rev. 1.0.0
© 2008 Fairchild Semiconductor Corporation
Functional Description
1. Startup: At startup, an internal high-voltage current
source supplies the internal bias and charges the
external capacitor (C
illustrated in Figure 22. When V
FPS™ begins switching and the internal high-voltage
current source is disabled. The FPS continues its normal
switching operation and the power is supplied from the
auxiliary transformer winding unless V
stop voltage of 8V.
2. Feedback Control: FPS employs current-mode
control, as shown in Figure 23. An opto-coupler (such as
the FOD817A) and shunt regulator (such as the KA431)
are typically used to implement the feedback network.
Comparing the feedback voltage with the voltage across
the R
switching duty cycle. When the reference pin voltage of
the shunt regulator exceeds the internal reference
voltage of 2.5V, the opto-coupler LED current increases,
pulling down the feedback voltage and reducing the duty
cycle. This typically happens when the input voltage is
increased or the output load is decreased.
V
Figure 23. Pulse-Width-Modulation (PWM) Circuit
O
FSQ0465 Rev.00
sense
FSQ0465 Rev.00
8V/12V
H11A817A
KA431
resistor makes it possible to control the
V
FB
Figure 22. Startup Circuit
3
C
V
B
CC
4
I
delay
V
a
V
SD
) connected to the VCC pin, as
CC
C
D1
VCC
V
V
REF
V
cc good
I
FB
D2
+
FB
-
*
R
3R
CC
OSC
CC
reaches 12V, the
OLP
I
start
goes below the
Internal
Bias
driver
V
Gate
REF
6
R
SenseFET
sense
V
V
STR
DC
12
2.1 Pulse-by-Pulse Current Limit: Because current-
mode control is employed, the peak current through the
SenseFET is limited by the inverting input of PWM
comparator (V
that the 0.9mA current source flows only through the
internal resistor (3R + R = 2.8k), the cathode voltage of
diode D2 is about 2.5V. Since D1 is blocked when the
feedback voltage (V
voltage of the cathode of D2 is clamped at this voltage,
clamping V
through the SenseFET is limited.
2.2 Leading-Edge Blanking (LEB): At the instant the
internal SenseFET is turned on, a high-current spike
usually occurs through the SenseFET, caused by
primary-side capacitance and secondary-side rectifier
reverse recovery. Excessive voltage across the R
resistor would lead to incorrect feedback operation in the
current-mode PWM control. To counter this effect, the
FPS employs a leading-edge blanking (LEB) circuit. This
circuit inhibits the PWM comparator for a short time
(t
Modulation (PWM) Circuit
3. Synchronization: The FSQ-series employs a quasi-
resonant switching technique to minimize the switching
noise and loss. The basic waveforms of the quasi-
resonant converter are shown in Figure 24. To minimize
the MOSFET's switching loss, the MOSFET should be
turned on when the drain voltage reaches its minimum
value, which is indirectly detected by monitoring the V
winding voltage, as shown in Figure 24.
LEB
Figure 24. Quasi-Resonant Switching Waveforms
MOSFET Gate
V
V
) after the SenseFET is turned on Pulse-Width-
sync
ds
ON
FB
V
*. Therefore, the peak value of the current
1.2V
DC
FB
*), as shown in Figure 23. Assuming
T
F
FB
) exceeds 2.5V, the maximum
230ns Delay
1.0V
V
V
V
RO
RO
ovp
ON
(8V)
FSQ0465 Rev.00
www.fairchildsemi.com
sense
CC

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