FAN6863R Fairchild Semiconductor, FAN6863R Datasheet - Page 11

• Manufacturer Part Number: FAN6863R
• Description: A highly integrated PWM controller, FAN6863 provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN6863 • Rev. 1.0.1
Operation Description
Startup Operation
Figure 24 shows a typical startup circuit and transformer
auxiliary winding for a typical application. Before
switching operation begins, FAN6863 consumes only
startup current (typically 8µA) and the current supplied
through the startup resistor charges the V
(C
ON
increases to 2mA. Then power is supplied from the
transformer auxiliary winding. The large hysteresis of
V
a small capacitor for V
connected to AC line for a fast reset of latch protection.
Green-Mode Operation
The FAN6863 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
65kHz. Once V
PWM frequency starts to linearly decrease from 65kHz
to 22.5kHz to reduce the switching losses. As V
decreases below V
is fixed at 22.5kHz and FAN6863 enters “deep” Green
Mode, where the operating current decreases to 600µA
(maximum),
consumption. As V
FAN6863 enters Burst-Mode operation. When V
below V
starts to drop, which causes the feedback voltage to
rise. Once V
Burst Mode alternately enables and disables switching,
thereby reducing switching loss in Standby Mode, as
shown in Figure 26.
DD
), switching begins and the current consumed
DD
(7V) provides more holdup time, which allows using
). When V
FB-ZDC
FB
, switching stops and the output voltage
Figure 24. Startup Circuit
further
DD
rises above V
FB
reaches turn-on voltage of 16V (V
decreases below V
FB-G
FB
DD
decreases below V
reducing
(2.2V), the switching frequency
. The startup resistor is typically
FB-ZDC
the
, switching resumes.
standby
FB-N
FB-ZDC
DD
(2.5V), the
FB
capacitor
) as an
FB
(1.6V),
power
drops
DD-
FB
11
Frequency Hopping
EMI reduction is accomplished by frequency hopping,
which spreads the energy over a wider frequency range
than the bandwidth measured by the EMI test
equipment. An internal frequency hopping circuit
changes the switching frequency between 60.8kHz and
69.2kHz with a period of 4.4ms, as shown in Figure 27.
Figure 26. Burst-Mode Operation
Figure 27. Frequency Hopping
Figure 25. PWM Frequency
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