FAN6862_10 Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Constant Output Power Limit
FAN6862 has saw-limiter for pulse-by-pulse current
limit, which guarantees almost constant power limit over
different line voltages of universal input range.
The conventional pulse-by-pulse current limiting
scheme has a constant threshold for current limit
comparator, which results in a higher power limit for
high line voltage. FAN6862 has a sawtooth current limit
threshold that increases progressively within a switching
cycle, which provides lower current limit for high line
and makes the actual power limit level almost constant
over different line voltages of universal input range, as
shown in Figure 28.
Leading-Edge Blanking (LEB)
Each time the power MOSFET is switched on, a turn-on
spike occurs across the sense-resistor caused by
primary-side capacitance and secondary-side rectifier
reverse recovery. To avoid premature termination of the
switching pulse, a leading-edge blanking time is built in.
During this blanking period (360ns), the PWM
comparator is disabled and cannot switch off the gate
driver. Thus, RC filter with a small RC time constant is
enough for current sensing.
Figure 28. Sawtooth Current Limiter
Figure 29. Current Sense R-C Filter
Soft-Start
The FAN6862 has an internal soft-start circuit that
increases pulse-by-pulse current-limit comparator
inverting input voltage slowly after it starts. The typical
soft-start time is 4ms. The pulsewidth to the power
MOSFET is progressively increased to establish the
correct working conditions for transformers, rectifier
diodes, and capacitors. The voltage on the output
capacitors is progressively increased with the intention
of smoothly establishing the required output voltage. It
also helps prevent transformer saturation and reduces
the stress on the secondary diode during startup.
© 2009 Fairchild Semiconductor Corporation
FAN6862 • Rev. 1.0.2
12
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