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FAN6862W Fiches technique(PDF) 11 Page - Fairchild Semiconductor

No de pièce FAN6862W
Description  Highly Integrated Green-Mode PWM Controller
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Fabricant  FAIRCHILD [Fairchild Semiconductor]
Site Internet  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN6862W Fiches technique(HTML) 11 Page - Fairchild Semiconductor

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© 2012 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN6862W • Rev. 1.0.0
11
Operation Description
Startup Operation
Figure 24 shows a typical startup circuit and transformer
auxiliary winding for a typical application. Before
switching operation begins, FAN6862W consumes only
startup current (typically 8 µA) and the current supplied
through the startup resistor charges the VDD capacitor
(CDD). When VDD reaches turn-on voltage of 16 V (VDD-
ON),
switching begins and the current consumed
increases to 2 mA. Power is then supplied from the
transformer auxiliary winding. The large hysteresis of
VDD (7 V) provides more holdup time, which allows using
a small capacitor for VDD. The startup resistor is typically
connected to AC line for a fast reset of latch protection.
Figure 24. Startup Circuit
Green-Mode Operation
The FAN6862W uses feedback voltage (VFB) as an
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
65 kHz. Once VFB decreases below VFB-N (2.5 V), the
PWM frequency starts to linearly decrease from 65 kHz
to 22.5 kHz to reduce the switching losses. As VFB
decreases below VFB-G (2.2 V), the switching frequency
is fixed at 22.5 kHz and FAN6862W enters “deep”
Green Mode, where the operating current decreases to
600 µA (maximum), further reducing the standby power
consumption. As VFB decreases below VFB-ZDC (1.6V),
FAN6862W enters Burst-Mode operation. When VFB
drops below VFB-ZDC, switching stops and the output
voltage starts to drop, which causes the feedback
voltage to rise. Once VFB rises above VFB-ZDC, switching
resumes. Burst Mode alternately enables and disables
switching, thereby reducing switching loss in Standby
Mode, as shown in Figure 26.
Figure 25. PWM Frequency
Figure 26. Burst-Mode Operation
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 61.0 kHz and
69.0 kHz with a period of 4.4 ms, as shown in Figure 27.
This covers the whole frequency range and shrinks the
period with operation frequency proportionally.
Figure 27. Frequency Hopping


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