NCP1651DR2G【PDF 30/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

26页 27页 28页 29页 [30页]31页 32页

NCP1651

http://onsemi.com

Optocoupler Transfer

Theoptocouplerisusedtoallowforgalvanicisolationfor

the error signal fromthe secondary toprimary sidecircuits.

The gain is based on the Current Transfer Ratio of the

device.Thiscanchangeovertemperatureandtime,butwill

not result in a large change in dB.

Therecommendedcapacitoratpin8is0.022mF.Ifalarger

capacitor is used, the pole may become low enough that it

willhaveaneffectonthegainphaseplotsneartheunitygain

crossoverfrequency.Inthiscaseandadditionalzerowillbe

required in the error amplifier bias circuitry.

Reference Signal

The error signal is transmitted to the primary side circuit

via. the optocoupler, is converted to a current by the V- -I

converter and is then used as an input to the reference

multiplier. The gain of this block is dependent on the AC

input voltage, because of the multiplier which requires two

inputs for one output.

Modulator and Output Stage

The modulator receives an input from the reference

multiplier and forces the current to follow the shape and

amplitude. The is an internal loop within thissection due to

thecurrentsenseamplifier.Basedontheassumptionslisted

in the introduction to this analysis, this is not analyzed

separately.

The equation for the gain is good for frequencies below

thepole.Thereisasinglepoleduetotheoutputfilter.Since

theNCP1651isacurrentmodeconverter,theinductorisnot

part of the output pole as can be seen in that equation.

The modulator and output stage transfer functions have

been split into two sets of equations. The first defines the

relationship between the input current and AC reference

signal, and the later, define the output stage gain and pole.

Due to the nature of a flyback transformer, the gain of the

output stage is dependant on the duty cycle (t

/T). For

continuous mode operation, the on- -time is:

?/DIV>

rms

out

+ 1

Calculating the Loop Gain

At this point in the design process, all of the parameters

involved in this calculation have been determined with the

exception of the pole- -zero pair on the output of the voltage

error amplifier.

All equations give gains in absolute numbers. It is

necessary to convert these to the decibel format using the

following formula:

A(dB) = 20 Log

(A)

For example, the voltage divider would be:

A =

5.6 k

560 k + 5.6 k

= 0.0099

A(dB) = 20 Log

0.0099 = - -40 dB

Thegainoftheloopwillvaryastheinputvoltagechanges.

It is recommended that the compensation for the error

amplifierbecalculatedunderhighline,fullloadconditions.

Thisshould be the greatest bandwidth that the unit will see.

By necessity, the unity gain (0 dB) loop bandwidth for a

PFC unit, must be less than the line frequency. If the

bandwidth approaches or exceeds the line frequency, the

voltage error amplifier signal will have frequency

components in its output that are greater than the line

frequency. These components will cause distortion in the

outputofthereferenceamplifier,whichisusedtoshapethe

current waveform. This in turn will cause distortion in the

current and reduce the power factor.

Typically the maximum bandwidth for a 60 Hz PFC

converter is 10 Hz, and slightly less for a 50 Hz system. This

canbeadjustedtomeettheparticularrequirementsofasystem.

The unity gain bandwidth is determined by the frequency at

which the loop gain passes through the 0 dB level.

For stability purposes, the gain should pass through 0 dB

withaslopeof- -20dBforapproximatelyondecadeoneither

sideoftheunitygainfrequency.Thisassuresaphasemargin

of greater than 45?

Thegaincanbecalculatedgraphicallyusingtheequations

of Figure 18 as follows:

Divider:

Calculate V?V

in dB, this value is constant so it

will not change with frequency.

OptocouplerTransfer:

CalculateV

usingtheequation

provided. Convert this value into dB.

ReferenceSignal:

CalculateV

ref

usingthepeaklevelof

the AC input signal at high line that will be seen on pin 9.

Convert this to dB. This is also a constant value.

Modulator and Output Stage:

Calculate the gain in dB for

/DV

ref

for the modulator, and also the gain in dB for the

outputstage(DV

out

/DI

).Calculatethepolefrequency.The

gainwillbeconstantforallfrequencieslessthanf

.Starting

at the pole frequency, this gain will drop off at a rate of

20 dB/decade.

Plot the sum of all of the calculated values. Be sure to

include the output pole. It should resemble the plot of

Figure 45. This plot shows a gain of 34 dB until the pole of

the output filter is reached at 3 Hz. After that, the gain is

reduced at a rate of 20 dB/decade.

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