FAN6861 Fairchild Semiconductor, FAN6861 Datasheet - Page 12

• Manufacturer Part Number: FAN6861
• Description: A highly integrated PWM controller, FAN6861 provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6861 • Rev. 1.0.1
V
V
by over voltage on the V
when V
(typically 250µs) to prevent false trigger by switching
noise.
Over-Temperature Protection (OTP)
The OTP circuit is composed of current source and
voltage comparators. Typically NTC thermistor is
connected between the RT pin and the GND pin. Once
the voltage of this pin drops below a threshold of 0.7V,
PWM output is disabled. Another comparator with 1V
threshold is used to introduce hysteresis of OTP.
Constant Output Power Limit
FAN6861 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 higher power limit for high
line voltage. FAN6861 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 24.
DD
DD
over-voltage protection prevents IC damage caused
Over-Voltage Protection (OVP)
conventional
DD
Figure 24. Sawtooth Current Limiter
voltage reaches 25V. It has a debounce time
pulse-by-pulse
DD
pin. The OVP is triggered
current
limiting
12
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
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 FAN6861 has an internal soft-start circuit that
increases
inverting input voltage slowly after it starts. The typical
soft-start time is 10ms. 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 to prevent transformer saturation and reduce
the stress on the secondary diode during startup.
this
Figure 25. Current Sense R-C Filter
pulse-by-pulse
blanking
period
current-limit
(360ns),
www.fairchildsemi.com
comparator
the
PWM