NCP1651DR2G【PDF 28/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

24页 25页 26页 27页 [28页]29页 30页 31页 32页

NCP1651

http://onsemi.com

Current Scaling Resistor & Filter Capacitor

sets the gain of the averaged current signal out of the

current sense amplifier which is fed into the AC error

amplifier. R

is used to scale the current to the appropriate

level for protection purposes in the AC error amplifier

circuit.

should be calculated to limit the maximum current

signal at the input to the AC error amplifier to less than

4.5 volts at low line and full load. 4.5 volts is the clamp

voltageattheoutputofthereferenceamplifierandlimitsthe

maximum averaged current that the unit can process. The

equation for R

is:

212 k ?R

Vin

4.5 (0.75 ?AC

ratio

?Vin

?nbsp 2

)

Where: P

= rated input power (W)

Where: R

= Shunt resistance (W)

Where: V

inLL

= min. operating rms input voltage (W)

Where: AC

ratio

= AC attenuation factor at pin 9

This equation does not allow for tolerances, and it would

beadvisabletoincreasetheinputpowertoassureoperation

at maximum power over production tolerance variations.

Thecurrentsensefiltercapacitorshouldbeselectedtoset

its pole about a factor of 10 below the switching frequency.

5.3

Where: C

= Pin 6 capacitance (nF)

Where: f = pole frequency (kHz)

so, for a 100 kHz switching frequency, a 10 kHz pole is

desirable, and C

would be 0.5 nF.

Reference Multiplier

The output of the reference multiplier is a pulse width

modulatedrepresentationoftheanaloginput.Themultiplier

is internally loaded with a resistor to ground which will set

the DC gain. An external capacitor is required to filter the

signal back into one that resembles the input fullwave

rectifiedsinewave.Thepoleforthiscircuitshouldbegreater

than the line frequency and lower than the switching

frequency.

1/15th of the switching frequency is a recommended

startingvaluefora60Hzlinefrequency.Thefiltercapacitor

for pin 10 can be determined by the following equation:

2 3.14 25 k f

pole

6.366E- -6

pole

Where: C

= Pin 10 capacitance (F)

Where: f

pole

= Ref gain pole freq (Hz)

AC Error Amplifier

The AC error amplifier is a transconductance amplifier

that is terminated with a series R

impedance. This creates

a pole- -zero pair.

To determine the values of R

and C

, it is necessary to

look at the two signals that reach the PWM inputs. The

non- -inverting input is a slow loop using the averaged

current signal. Its gain is:

30 k

3 k

?/DIV>

15 k

?(g

) ?2.3

Wherethefirsttwotermsarethegainsinthecurrentsense

amplifier averaging circuit. The next term is the gain of the

transconductance amplifier and the constant is the gain of

the AC Reference Buffer.

The high frequency path is that of the instantaneous

currentsignaltothePWMnon- -invertinginput.Thisgainis

16 k/3 k = 5.33, since the input signal is converted to a

current through a 3 k resistor in the current sense amplifier,

and then terminated by the 16 k resistor at the PWM input.

For stability, the gain of the low frequency path must be

less than the gain of the high frequency path. This can be

written as:

345, 000 ?/DIV>

m ?R

< 5.3

The suggested resistor and capacitor values are:

130,000 g

and for a zero at 1/10th of the switching frequency

1.59

Where: R

& R

are in units of Ohms

Where: g

is in units of mhos

Where: C

is in Farads

Where: f

is in Hz

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