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FSBH0270ANY View Datasheet(PDF) - Fairchild Semiconductor

Part Name
Description
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FSBH0270ANY
Fairchild
Fairchild Semiconductor Fairchild
FSBH0270ANY Datasheet PDF : 16 Pages
First Prev 11 12 13 14 15 16
Functional Description
Startup Operation
The HV pin is connected to bulk voltage through an
external resistor, RHV, as shown in Figure 26. When AC
voltage is applied to power system, an internal HV
startup circuit provides a high current (around 3.5mA) to
charge an external VDD capacitor until VDD voltage
exceeds the turn-on threshold voltage (VDD-ON). For
better power consumption, the HV startup circuit shuts
down during normal operation. The external VDD
capacitor and auxiliary winding maintain the VDD voltage
and provide operating current to controller.
Brown-In/Out Function
FSBH0x70 has a built-in internal brown-in/out protection
comparator monitoring voltage of VIN pin. Figure 28
shows a resistive divider with low-pass filtering for line-
voltage detection on the VIN pin.
Figure 26. Startup Circuit
Slope Compensation
The FSBH-series is designed for flyback power
converters. The peak-current-mode control is used to
optimize system performance. Slope compensation is
added to reduce current loop gain and improve power
system stability. The FSBH-series has a built-in,
synchronized, positive slope for each switching cycle.
Soft-Start
The FSBH-series has an internal soft-start circuit that
reduces the SenseFET switching current during power
system startup. The characteristic curve of soft-start
time versus VLMT level is shown in Figure 27. The VLMT
level slopes up like a six-step staircase. In doing so,
power system can smoothly build up the rated output
voltage and effectively reduce voltage stress on the
PWM switch and output diode.
Figure 28. Brown-In/Out Function on VIN Pin
Once the VIN pin voltage is lower than 0.6V and lasts
for 500ms, the PWM gate is disabled to protect the
system from over current. FSBH0x70 starts up as VIN
increases above 1.1V. Because the divider resistors of
the VIN pin are connected behind the bridge, the ratio
calculation for brownout in PFC and non-PFC system
are different, as shown in Figure 29. The formulas are
provided in the following equations:
Brownout with PFC:
RC
RA + RB + RC
2VAC _ OUT
2
π
= 0.6
(1)
Brownout with non-PFC:
RC
RA + RB + RC
2VAC _OUT = 0.6
(2)
Brown-in level is determined by:
VAC _ IN
=
1.1 RA
2
+ RB
RC
+ RC
(3)
Figure 27. Soft-Start Function
© 2009 Fairchild Semiconductor Corporation
FSBH0F70A, FSBH0170/A, FSBH0270/A, FSBH0370 • Rev. 1.0.3
Figure 29. VIN Level According to PFC Operation
www.fairchildsemi.com
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