NCP1651DR2G【PDF 26/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

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NCP1651

http://onsemi.com

Error Amplifier

The error amplifier resides on the secondary side of the

circuit, and therefore is not part of the chip. A minimal

solution would include either a discrete amplifier and

reference, or an integrated circuit that combines both, such

as the TL431 series of regulators.

Figure 41. Error Amplifier Circuit

FB/SD

Error

Amp

dc2

dc1

opto

V?/DIV>

out

ref2

This configuration for the error amplifier will result in a

low cost regulator, however, due to the slow loop response

of a PFC regulator it will not protect against overvoltage

conditions (e.g. load removal) or droop when a transient

load is added.

The primary side circuit has been designed such that the

PFCcontrollerwilloperateatmaximumdutycyclewiththe

optocouple in a non- -conducting state. This is necessary to

allow the unit to bring up the output when the system is

initially energized. At this time there is not output voltage

available to drive the LED in the optocoupler.

In the circuit of Figure 41, the amplifier and reference

needtoberatedatthemaximumvoltagethattheoutputwill

experience, including transient conditions. Resistors R

dc1

and R

dc2

need to be chosen such that the voltage at V?is

equal to V

ref2

when V

out

is at its regulated voltage. R

opto

acurrentlimitingresistorthatprotectstheoptocouplerfrom

current transients due to output surges.

This design also includes inherent compensation from

transients.Sincethebandwidthoftheerroramplifierisvery

low, its output can not respond rapidly to changes in the

outputvoltage.Atransientchangeintheoutputvoltagewill

change the current through R

opto

. Since the output of the

error amplifier does not change immediately, if the output

voltage increases, the voltage across R

opto

will increase.

This drives more current through the optocoupler, which in

turn reduces the output of the converter.

An alternate regulator is recommended, which is only

slightly more expensive, and offers excellent protection

from positive transients, and quick recovery from negative

transients.

Figure 42. Error Amp with Over/Undershoot

Protection

12 V

7.5 k

bias

0.01 mF

4.02 k

5.23 k

9.31 k

out

453

422

5.23 k

Undervoltage

Capacitor

3.6 k

opto

out

Error Amplifier

MC3303

Overvoltage

Comparator

TL431

The configuration shown in Figure 42, incorporates an

error amplifier with slow loop response, plus overvoltage

andundervoltagecomparators.Undernormaloperationthe

outputs of the Undervoltage and Overvoltage Comparators

are high. The Undervoltage Comparator provides drive for

theoptocoupler,whiletheOvervoltageComparatorreverse

biases the diode on its output and is out of the loop.

Thiscircuitisdesignedwith8%trippointsbothaboveand

below the regulation limit. If an overvoltage condition

exists, the Overvoltage comparator will respond very

quickly.Whenitsoutputgoeslow,itwillprovidemaximum

drivetotheoptocoupler,whichwillshutofftheoutputofthe

converter.

Iftheoutputvoltagedrops8%ormorebelowitsregulated

level, the Undervoltage Comparator will go low. This will

removethedrivefromtheoptocoupler,whichwillallowthe

regulator to increase the duty cycle and return the output to

its regulation range much faster than the error amplifier

could.

Thisconfigurationwillworkoverarangeof5to30volts,

with the appropriate changes in R

out

, R

bias

and R

opto

out

(k? = (V

out

- - 4.753) / 0.7785

bias

(k? = (V

out

- - 4.4)

opto

(k? = (V

out

- - 3) / 2

The value for R

opto

will allow a maximum of 2 mA to

drivetheoptocoupler.Ifadditionalcurrentisneeded,change

the 2 in the denominator of that equation to the current

(in mA) that is desired.

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