NCP1651DR2G【PDF 17/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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NCP1651

http://onsemi.com

Thekeytounderstandinghowtheinputcurrentisshaped

intoahighqualitysinewaveistheoperationoftheACerror

amplifier. The inputs of an operational amplifier operating

in its linear range, must be equal. There are several

secondaryeffects,thatcreatesmalldifferencesbetweenthe

inverting and non- -inverting inputs, but for the purpose of

this analysis they can be considered to be equal.

The fullwave rectified sinewave output of the reference

multiplierisfedintothenon- -invertinginputoftheACerror

amplifier. The inverting input to the AC error amplifier

receives a signal that is comprised of the input fullwave

rectified sinewave (which is not modified by the reference

multiplier), and summed with the filtered input current.

Since the two inputs to this amplifier will be at the same

potential,thecomplexsignalatthenon- -invertinginputwill

have the same waveshape as the AC reference signal. The

AC reference signal (V

ref

) is a fullwave rectified sinewave,

and the AC input signal (V

line

) is also a fullwave rectified

sinewave,therefore,theACcurrentsignal(I

),mustalsobe

a fullwave rectified sinewave. This relationship gives the

formula:

ref

= .75 ?V

ref

+ (k ?I

)

TheIinsignalhasawidebandwidth,anditsinstantaneous

value will not follow the low frequency fullwave rectified

sinewave exactly, however, the output of the AC error

amplifier has a low frequency pole that allows the average

valueof the.75V

line

+(kxI

) tofollowV

ref

.SincetheAC

erroramplifierisatransconductanceamplifier,itisfollowed

byaninvertingunitygainbufferstagewithalowimpedance

output so that the signal can be summed with the

instantaneousinputswitchingcurrent(I

).Theoutputofthe

buffer is still V

error(ac)

The difference between V

error(ac)

and the 4.0 volt

reference, sets the window that the instantaneous current

willmodulatein,todeterminewhentoturnthepowerswitch

off.

Since the input current has a fundamental frequency that

is twice that of the line, the output filter must have poles

lower than the input current to create a reasonable DC

waveform. The DC output voltage is compared to a

reference voltage by a secondary side error amplifier, and

the error signal out of the secondary side amplifier is fed

back into the Feedback input through an optocoupler.

Theswitchisturnedonbytheoscillator,whichmakesthis

a fixed frequency controller. Under normal operation, the

switch will remain on until the instantaneous value of

error(ac)

?reachesthe 4.0 volt reference level, at whichtime

the switch will turn off.

Figure 31. Typical Signals for PFC Circuit

AC Input

ref

line

k ?I

.75 ?V

line

+ k ?I

error(ac)

error(ac)?/DIV>

error(ac)

ref

OSC

4 V ref

GND

4 V ref

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