NCP1651DR2G【PDF 20/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

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NCP1651

http://onsemi.com

Figure 35. Ramp Compensation Circuit

Oscillator

16 k

AC Ref

Buffer

Current

Sense

Amp

PWM

Comparator

Ramp Compensation

1.6i

The current mirror is designed with a 1:1.6 current ratio.

Therampsignalinjectedcanbecalculatedbythefollowing

formula:

ramp

1.6 Vosc

16 k

102,400

(eq. 1)

Where:

ramp

= Peak injected current signal (4 V)

= Ramp compensation resistor (k?

Oscillator

Theoscillatorgeneratesthesawtoothrampsignalthatsets

the switching frequency, as well as sets the gain for the

multipliers. Both the frequency and the peak- -to- -peak

amplitude are important parameters.

The oscillator uses a current source for charging the

capacitor on the C

pin. The charge rate is approximately

200 mA and is trimmed to maintain an accurate, repeatable

frequency. Discharge is accomplished by grounding the C

pin with a saturated transistor. A hysteretic comparator

monitorsthatrampsignalandisusedtoswitchbetweenthe

current source and discharge transistor. While the cap is

charging, the comparator has a reference voltage of

4.0 volts. When the ramp reaches that voltage, the

comparator switches from the charging circuit to the

dischargecircuit,anditsreferencechangesfrom4.0to~0.5

volts (overshoot and delays will allow the valley voltage to

reach 0 volts).

The relationship between the frequency and timing

capacitor is:

= 47,000f

Where C

is in pF and f is in kHz.

It is important not to load the capacitor on this pin, since

thiscouldaffecttheaccuracyofthefrequencyaswellasthat

ofthemultiplierswhichusetherampsignal.Anyuseofthis

signal should incorporate a high impedance buffer.

Due to the requiredaccuracy of the peakand valleyramp

voltages, the NCP1651 is not designed to be synchronized

to the frequency of another oscillator.

Average Current Compensation

The input signal to this amplifier is the input fullwave

rectified sinewave. The amplifier is a unity gain amplifier,

withavoltagedividerontheoutputthatattenuatesthesignal

by a factor of 0.75. This scaled down fullwave rectified

sinewave is summed with the low frequency current signal

out of the current sense amplifier.

The sum of these signals must equal the signal at the

non- -inverting input to the AC error amplifier, which is the

outputofthereferencemultiplier.Sincethereisahardlimit

of 4.5 volts at the non- -inverting input, the sum of the line

voltage plus the current cannot exceed this level.

A typical universal input design operates from 85 to

265vac,whichisarangeof3.1:1.TheoutputoftheCurrent

Compensationamplifierwillchangebythisamounttoallow

the maximum current to vary inversely to the line voltage.

AC Error Amplifier

The AC error amplifier is a transconductance amplifier.

Thisamplifierforcesasignalwhichisthesumofthecurrent

and input voltage to equal the AC reference signal fromthe

reference multiplier.

Transconductance amplifiers differ from voltage

amplifiers in that the output is a high impedance with a

controlled voltage- -to- -current gain. This amplifier has a

nominal gain of 100 umhos (or 0.0001 amps/volt). This

means that an input voltage differential of 10 mv would

cause the output current to change by 1.0 mA. Itsmaximum

output current is 30 mA.

Current Sense Amplifier

Thecurrentsenseamplifierisawidebandwidthamplifier

with a differential input. It consists of a differential input

stage, a high frequency current mirror (PWM output) and a

low frequency current mirror (AC error amp output).

Figure 36. Current Sense Amplifier

CURRENT

MIRROR

avg

Error

Amp

avg fltr

CURRENT

MIRROR

PWM

3 k

30 k

3 k

S+

LEB

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