FSBH0370 Fairchild Semiconductor, FSBH0370 Datasheet - Page 12

• Manufacturer Part Number: FSBH0370
• Description: The highly integrated FSBH-series consists of an integrated current-mode Pulse Width Modulator (PWM) and an avalanche-rugged 700V SenseFET
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FSBH0F70A, FSBH0170/A, FSBH0270/A, FSBH0370 • Rev. 1.0.3
Functional Description
Startup Operation
The HV pin is connected to bulk voltage through an
external resistor, R
voltage is applied to power system, an internal HV
startup circuit provides a high current (around 3.5mA) to
charge an external V
exceeds the turn-on threshold voltage (V
better power consumption, the HV startup circuit shuts
down during normal operation. The external V
capacitor and auxiliary winding maintain the V
and provide operating current to controller.
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 V
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 27.
Figure 26. Startup Circuit
LMT
level is shown in Figure 27. The V
HV
, as shown in Figure 26. When AC
Soft-Start Function
DD
capacitor until V
DD-ON
DD
DD
voltage
voltage
). For
LMT
DD
12
Figure 29.
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.
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 V
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:
Brownout with non-PFC:
Brown-in level is determined by:
Figure 28.
V
R
R
AC IN
A
A
+
+
_
R
R
R
R
C
B
C
B
=
+
+
1.1
R
R
V
C
C
2
IN
Brown-In/Out Function on VIN Pin
⋅
⋅
⋅
Level According to PFC Operation
R
2
2
A
V
V
+
AC OUT
AC OUT
R
R
_
_
C
B
+
R
C
⋅
=
π
2
0.6
=
0.6
www.fairchildsemi.com
(1)
(2)
(3)
IN