NCP1651DR2G【PDF 13/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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NCP1651

http://onsemi.com

THEORY OF OPERATION

Introduction

Optimizing the power factor of units operating off of AC

lines is becoming more and more important. There are a

number of reasons for this.

There are a growing number of government regulations

requiring Power Factor Correction PFC. Many of these are

originating in Europe. Regulations such as IEC1000- -3- -2

areforcingequipmenttoutilizeinputstageswithtopologies

other than a simple off- -line front end which contains a

bridge rectifier and capacitor.

Therearealsosystemrequirementsthatdictatetheuseof

PFC. In order to obtain the maximum power from an

existingcircuitinabuilding,thepowerfactorisverycritical.

The real power available from such a circuit is:

real

= V

rms

?PF

A typical off- -line converter will have a power factor of

0.5to0.6,whichmeansthatforagivencircuitbreakerrating

only50%to60%ofthemaximumpowerisavailable.Ifthe

power factor is increased to unity, the maximum available

power can be obtained.

There is a similar situation in aircraft systems, where a

limited supply of power is available from the on- -board

generators. Increasing the power factor will increase the

load on the aircraft without the need for a larger generator.

Figure 25. Voltage and Current Waveforms

v, i

OFF- -LINE CONVERTER

PFC CONVERTER

Unity power factor is defined as the current waveform

beinginphasewiththevoltage,andundistorted.Therefore,

there are two causes of power factor degradation phase

shift and distortion. Phase shift is normally caused by

reactive loads such as motors which are inductive, or

electroluminescent lighting which is highly capacitive. In

such a case the power factor is relatively simple to analyze,

and is determined by the phase shift.

PF = cos ?/DIV>

Where ?is the phase angle between the voltage and the

current.

Reduced power factor due to distortion is more

complicated to analyze and is normally measured with AC

analyzers, although most circuit simulation programs can

also calculate power factor. One of the major causes of

distortion is rectification of the line into a capacitive filter.

This causes current spikes that do not follow the input

voltage waveform. An example of this type of waveform is

shown in the upper diagram in Figure 25.

A power converter with PFC forces the current to follow

the input waveform. This reduces the peak current, the rms

current and eliminates any phase shift.

In most modern PFC circuits, to lower the input current

harmonics, and improve the input power factor, designers

havehistoricallyusedaboosttopology.Theboosttopology

can operate in the Continuous (CCM), Discontinuous

(DCM), or Critical Conduction Mode.

Most PFC applications using the boost topology are

designedtousetheuniversalinputacpower85- -265Vac,50

or 60 Hz, and provide a regulated DC bus (typically

400 Vdc). In most applications, the load cannot operateoff

the high voltage DC bus, so a DC- -DC converter is used to

provide isolation between the AC source and load, and

providealowvoltageoutput.Theadvantagestothissystem

configuration are, low THD, a power factor close to unity,

excellent voltage regulation, and transient response on the

isolated DC output. The major disadvantage of the boost

topologyisthattwopowerstagesarerequiredwhichlowers

the systems efficiency, increases components count, cost,

and increases the size of the power supply.

ON Semiconductors NCP1651 offers a unique

alternative for Power Factor Correction designs, where the

NCP1651 has been designed to control a PFC circuit

operating in a flyback topology. There are several major

advantagestousingtheflybacktopology.First,theusercan

createalowvoltageisolatedsecondaryoutput,withasingle

powerstage,andstillachievealowinputcurrentdistortion,

and a power factor close to unity. A second advantage,

comparedtotheboosttopologywithaDC- -DCconverter,is

alowercomponentcountwhichreducesthesizeandthecost

of the power supply.

The NCP1651 can operate in either the Continuous or

Discontinuous Mode of operation, the following analysis

will help to highlight the advantages of Continuous versus

Discontinuous Mode of operation.

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