AN-8021 Datasheet, PDF (1/13 Pages) Fairchild Semiconductor – Building Variable Output Voltage Boost PFC Converters

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AN-8021
Building Variable Output Voltage Boost PFC Converters
with the FAN9612 Interleaved BCM PFC Controller
Abstract
The output voltage of a boost PFC converter has to be set above the highest input voltage of the source in order to maintain
boost operation and be able to shape the input current waveform of the power supply. For universal input voltage designs,
the appropriate output voltage selection is about 400V_DC. In many applications, it might be desirable to adjust the output
voltage according to the input AC rms voltage level, the output power of the converter, or both. The FAN9612 Interleaved
BCM PFC controller is particularly suited to implement such designs because the non-inverting input of the voltage error
amplifier is easily accessible to adjust the converter’s output voltage. This application note gives details about how to make
the boost output voltage the function of the output power of the converter. Circuits with variable level of sophistications are
shown and explained. Furthermore, it describes the design and implementation of the boost follower concept where the
output voltage becomes the function of the input AC rms voltage level.
Why Adjust V_OUT?
The output voltage of the Power Factor Correctors (PFC)
front-end is defined by many requirements and operating
parameters of the power supply. PFCs draw power from the
input source following a sine square function while their
output is loaded with a practically constant load, as depicted
in Figure 1.
Figure 1. Instantaneous Input and Output Power
of a PFC Front-End
Therefore, energy storage must take place in the power
factor corrector on the line frequency basis. This energy
storage is most efficient at high output voltage, such as
400V_DC. On the other hand, the switching losses of the
boost converter and the downstream isolated DC-DC
converter are both proportional to the output voltage of the
PFC. Higher output voltage increases switching losses,
which is most noticeable in the light-load efficiency of the
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 6/1/10
power supplies. Consequently, the output voltage should be
kept the minimum allowed by the fundamental requirements
to maintain feasible operating conditions for the boost
converter.
These opposing requirements of:
ƒ keeping the output voltage above the highest input
voltage level;
ƒ using the highest possible voltage to improve
volumetric efficiency of energy storage; and
ƒ lowering switching losses by selecting the lowest
possible output voltage;
might lead to the implementation of a system where the
boost output voltage is optimized for the momentary input
voltage and load conditions. These solutions are the focus
of this application note.
How to Adjust V_OUT?
The FAN9612 is unique among the PFC controllers because
of the full accessibility of its error amplifier. In addition to
the inverting input (FB) and the output of the error amplifier
(COMP), which must be available for output voltage
sensing and compensation, the non-inverting input (SS) is
also accessible, as shown in Figure 2.
The non-inverting input of the error amplifier is connected
to a voltage reference (V_SS) generated across the soft-start
capacitor (C_SS). The feedback divider (R_FB1 and R_FB2) is
connected to the inverting input of the error amplifier
through the FB pin. The reference voltage, together with the
feedback resistors, sets the desired output voltage according
to the expression depicted in Figure 2.
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