FAN6862L Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Constant Output Power Limit
FAN6862R/L 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. FAN6862R/L 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 27.
Leading-Edge Blanking (t_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 27. Sawtooth Current Limiter
Figure 28. Current Sense R-C Filter
Soft-Start
The FAN6862R/L 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 5ms. 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
FAN6862R/L • Rev. 1.0.0
12
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