FAN6862R Fairchild Semiconductor, FAN6862R Datasheet - Page 12

• Manufacturer Part Number: FAN6862R
• Description: A highly integrated PWM controller, FAN6862R/L provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6862R/L • Rev. 1.0.0
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
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.
conventional
Figure 27. Sawtooth Current Limiter
pulse-by-pulse
current
limiting
12
Leading-Edge Blanking (t
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
comparator is disabled and cannot switch off the gate
driver. Thus, RC filter with a small RC time constant is
enough for current sensing.
Soft-Start
The FAN6862R/L has an internal soft-start circuit that
increases
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.
this
Figure 28. Current Sense R-C Filter
pulse-by-pulse
blanking
period
current-limit
LEB
(360ns),
)
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comparator
the
PWM