FAN6861 Fairchild Semiconductor, FAN6861 Datasheet - Page 10

• 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
Operation Description
Startup Operation
Figure 17 shows the typical startup circuit and
transformer auxiliary winding for FAN6861 application.
Before
consumes only startup current (typically 8μA) and the
current supplied through the startup resistor charges
the V
voltage of 17.5V (V
and the current consumed increases to 3mA. Then, the
power required is supplied from the transformer
auxiliary winding. The large hysteresis of V
provides more holdup time, which allows using small
capacitor for V
connected to AC line for a fast reset of latch protection.
Green-Mode Operation
The FAN6861 uses feedback voltage (V
indicator of the output load and modulates the PWM
frequency, as shown in Figure 18, 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 22kHz to reduce the switching losses. As V
decreases below V
is fixed at 22.5kHz and FAN6861 enters into deep
green mode, where the operating current reduces to
2.2mA (maximum), further reducing the standby power
consumption. As V
FAN6861 enters into burst-mode operation. When V
drops below V
output voltage starts to drop, which causes the
feedback voltage to rise. Once V
switching resumes. Burst mode alternately enables and
disables switching, thereby reducing switching loss in
standby mode, as shown in Figure 19.
DD
FAN6861
capacitor (C
Figure 17. Startup Circuit
FB-ZDC
FB
DD
. The startup resistor is typically
decreases below V
FB-G
FB
, FAN6861 stops switching and the
DD-ON
begins
DD
decreases below V
(2.2V), the switching frequency
). When V
), FAN6861 begins switching
switching
FB
rises above V
DD
FB-N
reaches turn-on
operation,
FB-ZDC
(2.85V), the
FB
) as an
DD
(1.9V),
FB-ZDC
(8V)
FB
FB
it
,
10
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 20.
Figure 19. Burst Mode Operation
Figure 20. Frequency Hopping
Figure 18. PWM Frequency
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