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FSBH0F70WANY View Datasheet(PDF) - Fairchild Semiconductor

Part Name
Description
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FSBH0F70WANY
Fairchild
Fairchild Semiconductor Fairchild
FSBH0F70WANY Datasheet PDF : 14 Pages
First Prev 11 12 13 14
Functional Description
Startup Operation
The HV pin is connected to bulk voltage through an
external resistor, RHV, as shown in Figure 22. When AC
voltage is applied to the 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
lower 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.
Figure 22. 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 1. The VLMT
level rises in six steps. By doing so, the power system
can smoothly build up the rated output voltage and
effectively reduce voltage stress on the PWM switch
and output diode.
Figure 24. 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. FSBH0170W/0270W starts
up as VIN increases above 1.1V. Because the divider
resistors of the VIN pin are connected behind the
bridge, the ratio calculations for brownout in PFC and
non-PFC system are different, as shown in Figure 1.
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 25. VIN Level According to PFC Operation
Figure 23. Soft-Start Function
Brown-In/Out Function
FSBH0170W/0270W has a built-in internal brown-in/out
protection comparator monitoring voltage of the VIN pin.
Figure 24 shows a resistive divider with low-pass
filtering for line-voltage detection on the VIN pin.
Brown-In Function of FSBH0F70WA
The VIN pin functions are disabled from FSBH0F70WA
which still exist brown-in protection in VDD pin. There is
a discharge current internal from VDD to ground during
startup. The HV source current must be larger than IDD-
ST to charge the capacitor of VDD. The brown-in level can
be determined by RHV according to the equation:
RHV =
2VAC 12
IDDST
(4)
© 2011 Fairchild Semiconductor Corporation
FSBH0F70WA/0170W/0270W • Rev. 1.0.2
11
www.fairchildsemi.com
 

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