FAN9611MX【PDF 8/35 页】IC PFC CTLR DUAL BCM 16-SOIC

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Sheet目录38

4页 5页 6页 7页 [8页]9页 10页 11页 12页

?2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN9611 " Rev. 1.1.7

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 converters input voltage range

includes 0 V. These fundamental properties make close

to unity power factor easier to achieve.

Figure 6. Basic PFC Boost Converter

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.

Figure 7. CCM vs. BCM Control

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.

There are many fundamental differences in CCM and

BCM operations and the respective designs of the boost

converter.

The FAN9611 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 on the selected output voltage, the

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 power resulting in the lowest operating

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 noise regulations enforced for

equipment connecting to the mains. The effects usually

limit the practical output power level of the converter.

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