NCP1651DR2G【PDF 15/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

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NCP1651

http://onsemi.com

Results

ItisclearfromtheresultofouranalysisthataflybackPFC

converter operating in CCM has half the peak current and

one tenth the fundamental (100 kHz) harmonic current

compared to a flyback PFC converter operating in DCM.

TheresultsarelowerconductionlossesintheMOSFET,and

secondaryrectifyingdiode,andasmallerinputEMIfilterif

the designer needs to meet the requirements C.I.S.P.R.

conducted emission levels. On the down side to CCM

operation, the flyback transformer will belarger becauseof

the required higher primary inductance.

The advantages to operating in DCM include lower

switchinglossesbecausethecurrentfallstozeropriortothe

next switching cycle, and smaller transformer size.

It will ultimately be up to the designer to perform a

trade- -off study to determine which topology, Boost versus

Flyback, Continuous versus Discontinuous Mode of

operation will meet all the system performance

requirements. But the recent introduction of the NCP1651

allows the system designer one additional option.

ForanaveragecurrentmodeflybacktopologybasedPFC

converter,determiningthetransformerparameters(primary

inductance and turns ratio) involves several trade- -offs.

These include peak- -to- -average current ratio (higher

inductance or turns ratio result in lower peak current),

switching losses (higher turns ratio leads to higher peak

voltage and higher switching losses), CCM vs. DCM

operation (lower values of turns ratio or higher values of

inductanceextendtheCCMrange)andrangeofdutycycles

overtheoperationallineandloadrange.ONSemiconductor

has designed an Excel- -based spreadsheet to help design

withtheNCP1651andbalancethesetrade- -offs.Thedesign

aid is downloadable free- -of- -charge from our website

(www.onsemi.com).

The ideal solution depends on the specific application

requirements and the relative priority between factors such

as THD performance, cost, size and efficiency. The design

aid allows the designer to consider different scenarios and

settleonthebestsolutionfoeagivenapplication.Following

guidelines will help in settling towards the most feasible

solution.

1. Turns Ratio Limitations: While higher turns ratio

can limit the reflected primary voltage and current,

it is constrained by the inherent limitations of the

flyback topology. A turns ratio of higher than 20:1

will result in very high leakage inductance and

lead to high leakage spikes on the primary switch.

Thus, practical application of this approach is

restricted to output voltages 12 V and above.

2. CCM Operation: The NCP1651 is designed to

operate in both CCM and DCM modes. However,

the CCM operation results in much better THD

than the DCM operation. Thus, it is recommended

that the circuit be designed to operate in CCM at

the specified test condition for harmonics

compliance (typically at 230 V, full load). Please

keep in mind that at or near zero crossing

(<10 deg angle), it is neither necessary nor feasible

to maintain CCM operation.

3. Following key governing equations have been

incorporated in the design aid:

PFC Operation

ThebasicPWMfunctionoftheNCP1651iscontrolledby

a small block of circuitry, which comprises the DC

regulation loop and the PFC circuit. These components are

shown in Figure 30.

There are three inputs to this loop. They are the fullwave

rectified sinewave, the instantaneous input current and the

error signal at the FB/SD pin.

Theinputcurrentisforcedtomaintainanearunitypower

factor due to the control of the AC error amplifier. This

amplifier uses information from the AC input voltage and

theACinputcurrenttocontrolthepowerswitchinamanner

that gives good DC regulation as well as excellent power

factor.

Thereferencemultipliersetsareferencelevelfortheinput

fullwave rectified sinewave. One of its inputs is connected

to a scaled down fullwave rectified sinewave, and the other

receives the error signal which has been converted to a

current. The error signal adjusts the level of the fullwave

rectified sinewave on the multipliers output without

distorting it. To accomplish this, it is necessary for the

bandwidth of the DC error amp to be less than twice the

lowestlinefrequency.Typicallyitissetatafactoroftenless

than the rectified frequency (e.g. for a 60 Hz input, the

bandwidth would be 12 Hz).

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