FSFR2100U_10 FAIRCHILD [Fairchild Semiconductor], FSFR2100U_10 Datasheet - Page 11

• Manufacturer Part Number: FSFR2100U_10
• Description: Fairchild Power Switch (FPS) for Half-Bridge Resonant Converters
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2007 Fairchild Semiconductor Corporation
FSFR series Rev.1.0.9
in Figure 19. FSFR-series also has an internal soft-start
for 3ms to reduce the current overshoot during the initial
cycles, which adds 40kHz to the initial frequency of the
external soft-start circuit, as shown in Figure 20. The
initial frequency of the soft-start is given as:
It is typical to set the initial frequency of soft-start two ~
three times the resonant frequency (f
network.
The soft-start time is three to four times of the RC time
constant. The RC time constant is as follows:
4. Control Pin: The FSFR-series has a control pin for
protection, cycle skipping, and remote on/off. Figure 21
shows the internal block diagram for control pin.
Protection: When the control pin voltage exceeds 5V,
protection
described in the protection section.
Pulse Skipping: FSFR-series stops switching when the
control pin voltage drops below 0.4V and resumes
switching when the control pin voltage rises above 0.6V.
To use pulse-skipping, the control pin should be
connected to the opto-coupler collector pin. The
frequency that causes pulse skipping is given as:
T
f
SS
SKIP
ISS
=
f
Figure 20. Frequency Sweeping of Soft-start
=
ISS
R
(
=
SS
f
Figure 21. Internal Block of Control Pin
s
5.2
R
⋅
5
C
min
R
k
2 .
is
SS
Ω
min
k
+
triggered.
40kHz
5.2
+
R
. 4
SS
k
R
Ω
16
max
) 100 40 (
k
×
Detailed
x
100
+
(
kHz
Control loop
kHz
take over
O
applications
) of the resonant
)
)
time
are
(3)
(4)
(5)
11
Remote On / Off: When an auxiliary power supply is
used for standby, the main power stage using FSFR-
series can be shut down by pulling down the control pin
voltage, as shown in Figure 23. R1 and C1 are used to
ensure soft-start when switching resumes.
4. Current Sensing Method
Current Sensing Using Resistor: FSFR-series senses
drain current as a negative voltage, as shown in Figure
24 and Figure 25. Half-wave sensing allows low power
dissipation in the sensing resistor, while full-wave
sensing has less switching noise in the sensing signal.
CON
FPS
R
T
Figure 22. Control Pin Configuration for
Figure 23. Remote On / Off Circuit
R
min
OP1
Pulse Skipping
R1
C1
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OP1
Main Off
Output
Output
Aux
Main