FSL137H Fairchild Semiconductor, FSL137H Datasheet - Page 10

• Manufacturer Part Number: FSL137H
• Description: The highly integrated FSL137H consists of an integrated current mode Pulse Width Modulator (PWM) and an avalanche-rugged 700V SenseFET
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FSL137H Rev. 1.0.3
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
FSL137H is designed for flyback power converters. The
peak-current-mode control is used to optimize system
performance. Slope compensation is added to stabilize
the current loop. FSL137H inserts a synchronized,
positively sloped ramp at each switching cycle.
Soft-Start
The FSL137H has internal soft-start circuit that slowly
increases the SenseFET current after startup. The
typical soft-start time is 5ms during which the V
is increased in six steps to smoothly establish the
required output voltage, as shown in Figure 24. It also
helps to prevent transformer saturation and reduce the
stress on the secondary diode during startup.
DD
capacitor maintains V
Figure 24.
DD
Figure 23.
capacitor voltage reaches V
HV
Soft-Start Function
. The startup current turns off
DD
Startup Circuit
until the auxiliary winding
DD
DD-ON
capacitor
Limit
HV
. The
level
, as
10
Green-Mode Operation
The FSL137H 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 FSL137H enters into “deep” green
mode to reduce the standby power consumption. As
V
into burst-mode operation. When V
ZDC
starts to drop, which causes the feedback voltage to
rise. Once V
Burst mode alternately enables and disables switching,
thereby reducing switching loss to improve power
saving, as shown in Figure 26.
FB
, FSL137H stops switching and the output voltage
decreases below V
Figure 26.
FB
Figure 25.
rises above V
FB
FB-G
decreases below V
Burst Mode Operation
(2.4V), the switching frequency
FB-ZDC
PWM Frequency
FB-ZDC
(2.1V), FSL137H enters
, switching resumes.
FB
drops below V
FB-N
www.fairchildsemi.com
(2.5V), the
FB
) as an
FB-
FB