FAN9612 Fairchild Semiconductor, FAN9612 Datasheet - Page 19

• Manufacturer Part Number: FAN9612
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN9611 / FAN9612 • Rev. 1.1.4
3. Adjusting the Output Voltage with Load
In some applications, the output voltage of the PFC
boost converter is decreased at low power levels to
boost the light load efficiency of the power supply.
Implementing this function with a circuit external to the
FAN9611/12 is straightforward because the error
amplifier reference (the positive input) is available on
the soft-start (SS) pin, as shown in Figure 28. In the
FAN9611/12 architecture, the power of the converter is
proportional to the voltage on the COMP pin, minus a
small offset. The voltage on the COMP pin is monitored
to determine the operating power of the supply.
Therefore the voltage on the SS pin can be adjusted
lower to achieve the desired lower output voltage.
Several possible implementations to adjust the output
voltage of the boost stage at light load are described in
the application note AN-8021. It includes the universal
output
modulated by input voltage to avoid the boost converter
becoming a peak rectifier at high line and light load.
4. Adjusting the Output Voltage with Input
In some applications, the output voltage of the PFC
boost converter is adjusted based on the input voltage
only. This boost follower implementations increases the
efficiency of the downstream DC-DC converter and
therefore of the overall power supply.
Implementations for both the two-level boost and the
linear boost follower (or tracking boost) are described in
application note AN-8021.
5. Adjusting the Phase-Management
In any power converter, the switching losses become
dominant at light load. For an interleaved converter
where there are two or more phases, light-load
efficiency can be improved by shutting down one of the
phases at light load (also known as phase-shedding or
phase-dropping operating).
The initial phase-management thresholds are fixed at
approximately 13% and 18% of the maximum load
power level. This means when the output power
reaches 13%, the FAN9611/12 automatically goes from
a two-phase to a single-phase operation (phase shed or
phase drop). When the output power comes back up to
18%, the FAN9611/12 automatically goes from the
single-phase to the two-phase operation (phase-add).
Figure 28. FAN9611/12 Error Amplifier Configuration
Voltage
Thresholds
voltage
adjust
implementation
which
is
19
The default thresholds can be adjusted upward based
on the application requirement; for example, to meet the
Energy STAR 5.0 or the Climate Savers Computing
efficiency requirements at 20% of the load. The phase
drop threshold can be adjusted upward (for example to
25%) by adjusting the maximum on time.
Since the phase management threshold is fixed at 13%
and 18% of the maximum power limit level, the actual
power management threshold as a percentage of
nominal output power can be adjusted by the ratio
between nominal power and maximum power limit level
as shown in Figure 29. The second plot shows an
example where the maximum power limit level is 1.4
times of nominal output power. By adjusting the
maximum on-time (using R
thresholds can be adjusted upward.
Phase management is implemented such that the
output of the error amplifier (V
change when the system toggles between single-phase
and two-phase operations, as shown in Figure 30. The
output of the error amplifier is proportional to the output
power of the converter independently, whether one or
both phases are operating in the power supply.
Furthermore, because the maximum on-time limit is
applied independently to each pulse-width modulator,
the power handling capability of the converter with only
one phase running is approximately half of the total
output power that can be delivered when both phases
are utilized.
Additional details on adjusting phase management are
provided in the application note AN-6086.
Figure 29. Adjusting Phase Management Thresholds
Figure 30. V
COMP
MOT
), the phase management
COMP
vs. t
) does not have to
ON
MAX
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