FSL127H Fairchild Semiconductor, FSL127H Datasheet - Page 10

• Manufacturer Part Number: FSL127H
• Description: The highly integrated FSL127H consists of a current mode Pulse Width Modulator (PWM) and an avalanche-rugged 700V SenseFET
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FSL127H • Rev. 1.0.2
Functional Description
Startup Operation
For startup, the HV pin is connected to the line input or
bulk capacitor through the external resistor R
shown in Figure 23. Typical startup current drawn from
the HV pin is 3.5mA and it charges the V
through the resistor R
when the V
V
of the transformer provides the operating current.
Slope Compensation
FSL127H is designed for flyback power converter. The
peak-current-mode control is used to optimize system
performance. Slope compensation is added to stabilize
the current loop. The FSL127H inserts a synchronized,
positively sloped ramp at each switching cycle.
Soft Start
The FSL127H has internal soft-start circuit that slowly
increases the SenseFET current after startup. The
typical soft-start time is 5ms, during which the V
level is increased in six steps to smoothly establish the
required output voltage, as shown in Figure 24. It also
helps prevent transformer saturation and reduces stress
on the secondary diode during startup.
DD
capacitor maintains V
AC line
Figure 24. Soft-Start Function
DD
C
DC
Figure 23. Startup Circuit
capacitor voltage reaches V
HV
R
HV
. The startup current turns off
DD
FSL127H
HV
until the auxiliary winding
VDD
DD
D
DD-ON
C
DD
DD
capacitor
HV
N
. The
A
, as
Limit
10
Green-Mode Operation
The FSL127H uses feedback voltage (V
indicator of the output load and modulates the PWM
frequency, as shown in Figure 25, such that the
switching frequency decreases as load decreases. In
heavy load conditions, the switching frequency is
100kHz. Once V
PWM frequency starts to linearly decrease from 100kHz
to 18kHz to reduce the switching losses. As V
decreases below V
is fixed at 18kHz and FSL127H enters “deep” green
mode to reduce the standby power consumption. As
V
burst-mode operation. When V
FSL127H stops switching and the output voltage starts
to drop, which causes the feedback voltage to rise.
Once V
mode alternately enables and disables switching,
thereby reducing switching loss to improve power
saving, as shown in Figure 26.
FB
decreases below V
FB
rises above V
Figure 26. Burst Mode Operation
Figure 25. PWM Frequency
FB
FB-G
decreases below V
FB-ZDC
(2.4V), the switching frequency
FB-ZDC
, switching resumes. Burst
(2.1V), FSL127H enters
FB
drops below V
FB-N
www.fairchildsemi.com
(2.5V), the
FB
) as an
FB-ZDC
FB
,