FAN9612 Fairchild Semiconductor, FAN9612 Datasheet - Page 8

• Manufacturer Part Number: FAN9612
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN9611 / FAN9612 • Rev. 1.1.4
Theory of Operation
1. Boundary Conduction Mode
The boost converter is the most popular topology for
power factor correction in AC-to-DC power supplies.
This popularity can be attributed to the continuous input
current waveform provided by the boost inductor and to
the fact that the boost converter’s input voltage range
includes 0V. These fundamental properties make close
to unity power factor easier to achieve.
The boost converter can operate in continuous
conduction mode (CCM) or in boundary conduction
mode (BCM). These two descriptive names refer to the
current flowing in the energy storage inductor of the
boost power stage.
As the names indicate, the current in Continuous
Conduction Mode (CCM) is continuous in the inductor;
while in Boundary Conduction Mode (BCM), the new
switching period is initiated when the inductor current
returns to zero.
Figure 6. Basic PFC Boost Converter
Figure 7. CCM vs. BCM Control
L
8
There are many fundamental differences in CCM and
BCM operations and the respective designs of the boost
converter.
The FAN9611/12 utilizes the boundary conduction mode
control algorithm. The fundamental concept of this
operating mode is that the inductor current starts from
zero in each switching period, as shown in the lower
waveform in Figure 7. When the power transistor of the
boost converter is turned on for a fixed amount of time,
the peak inductor current is proportional to the input
voltage. Since the current waveform is triangular, the
average value in each switching period is also
proportional to the input voltage. In the case of a
sinusoidal input voltage waveform, the input current of
the converter follows the input voltage waveform with
very high accuracy and draws a sinusoidal input current
from the source. This behavior makes the boost
converter in BCM operation an ideal candidate for
power factor correction.
This mode of control of the boost converter results in a
variable switching frequency. The frequency depends
primarily
instantaneous value of the input voltage, the boost
inductor value, and the output power delivered to the
load. The operating frequency changes as the input
voltage follows the sinusoidal input voltage waveform.
The lowest frequency operation corresponds to the peak
of the sine waveform at the input of the boost converter.
Even larger frequency variation can be observed as the
output power of the converter changes, with maximum
output
frequency. Theoretically, under zero-load condition, the
operating frequency of the boost converter would
approach infinity. In practice, there are natural limits to
the highest switching frequency. One such limiting factor
is the resonance between the boost inductor and the
parasitic capacitances of the MOSFET, the diode, and
the winding of the choke, in every switching cycle.
Another important characteristic of the BCM boost
converter is the high ripple current of the boost inductor,
which goes from zero to a controlled peak value in every
switching period. Accordingly, the power switch is
stressed with high peak current. In addition, the high
ripple current must be filtered by an EMI filter to meet
high-frequency
equipment connecting to the mains. The effects usually
limit the practical output power level of the converter.
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