FSB117H Fairchild Semiconductor, FSB117H Datasheet - Page 12

• Manufacturer Part Number: FSB117H
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FSB117H / FSB127H / FSB147H • Rev. 1.0.4
Functional Description
Startup Operation
The HV pin is typically connected to the AC line input
through two external diodes and one resistor (R
shown in Figure 18. When the AC line voltage is
applied, the V
voltage through the diodes and resistor. After V
voltage reaches the turn-on threshold voltage (V
the startup circuit charging V
and V
transformer. Once the FSB-series starts, it continues
operation until V
startup time with a given AC line input voltage is:
Brown-in/out Function
The HV pin can detect the AC line voltage using a
switched voltage divider that consists of external
resistor (R
Figure 18. The internal line sensing circuit detects the
real RMS value of the line voltage using sampling circuit
and peak detection circuit. Since the voltage divider
causes power consumption when it is switched on, the
switching is driven by a signal with a very narrow pulse
width to minimize power loss. The sampling frequency
is adaptively changed according to the load condition to
minimize the power consumption in light-load condition.
Based on the detected line voltage, brown-in and
brownout thresholds are determined. Since the internal
resistor (R
R
V
V
t
HV
STARTUP
BROWN IN
BROWN OUT
, the thresholds are given as:
DD


is supplied by the auxiliary winding of the
LS
HV

Figure 18. Startup Circuit
) of the voltage divider is much smaller than
(
R
) and internal resistor (R
RMS
DD
HV
(
RMS
hold-up capacitor is charged by the line
DD

)
C

DD
drops below 6V (V
)
200

R

HV
ln
200
R
k
HV
V

AC IN
k
V
DD
AC ON


V
V
capacitor is switched off
AC OFF
AC IN

2

2 2


2


LS
2 2

DD-OFF1

V
), as shown in
DD ON

). The IC
HV
DD-ON
(2)
(3)
), as
(1)
DD
),
12
PWM Control
The FSB-series employs current-mode control, as
shown in Figure 19. An opto-coupler (such as the
H11A817A) 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. A synchronized positive slope is
added to the SenseFET current information to guarantee
stable current-mode control over a wide range of input
voltage. The built-in slope compensation stabilizes the
current loop and prevents sub-harmonic oscillation.
Soft-Start
The FSB-series has an internal soft-start circuit that
progressively increases the pulse-by-pulse current limit
level of MOSFET during startup to establish the correct
working conditions for transformers and capacitors, as
shown in Figure 20. The current limit levels have nine
steps, as shown in Figure 21. This prevents transformer
saturation and reduces stress on the secondary diode
during startup.
Figure 20. Soft-Start and Current-Limit Circuit
sense
Figure 19. Current Mode Control
resistor makes it possible to control the
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