NCP1651DR2G【PDF 25/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

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NCP1651

http://onsemi.com

Basic Specifications

Thedesignofanypowerconverterbeginswithabasicset

of specifications. The following parameters should be

known before you begin:

max

(Maximum rated output power)

Vrms

min

(Minimum operational line voltage)

Vrms

max

(Maximum operational line voltage)

switch

(Nominal switching frequency)

out

(Nominal regulated output voltage)

Most of these parameters will be dictated by system

requirements.

Transformer

For an average current mode, fixed frequency PFC

converter, there is no magic formula to determine the

optimum value of the transformers primary inductance.

There are several trade- -offs that should be considered.

These include peak current vs. average current, switching

losses vs. core losses and range of duty cycles over the

operational line and load range. All of these are a function

of inductance, line and load. These parameters determine

when the converter is operating in the continuous

conduction mode and when it is operating in the

discontinuous conduction mode.

If you are designing your own transformer, the

ON Semiconductor spreadsheet (NCP1651_Design.xls)

that is available as a design aid for this part can be of help.

Enter various values of inductance as well asthe turnsratio

and observe the variation in duty cycle and peak current vs.

average current.

The transformers duty cycle is an important parameter.

There are two main limitations for the duty cycle. The first

is the output voltage reflected back to the primary, which is

scaled by the turns ratio. This means that with a 10:1

(pri:sec) turns ratio, and a 12 volt output, the power switch

willseetheinputvoltage plus120 volts(10 x12 volts) plus

leakageinductancespike.Thisreflectedvoltagedetermines

the maximum voltage rating of the power switch.

The second, there are practical limits to the turns ratio.

Given the flyback converter transfer function, continuous

conduction mode,

= V

n (D1 D)

Itisevidentthatthereisadirectrelationshipbetweenduty

cycle and the turns ratio. In general, 10:1 is about the

maximum,althoughsome transformer manufacturersgoas

highas12:1oreven15:1.Turnsratiosof20:1andaboveare

not normally practical as they result in very high values of

leakageinductance,whichcreateslargespikesonthepower

switch. They also have a very large reflectovoltage

associated with them.

Theotheroptionistocontactatransformermanufacturer

such as Coiltronics (www.cooperet.com/

) or Coilcraft

(www.coilcraft.com/

). These companies will design and

manufacture transformers to your requirements.

Using the available spreadsheet, with the following

parameters,aprimaryinductanceof330mHandaturnsratio

of 10:1 would be a good choice.

Limits

max

= 100 W

Vin

max

= 265 V

rms

Vin

min

= 85 V

rms

= 12 V

= 330 mH

switch

= 100 kHz

= 10

Figure 39. Switching Current versus Phase Angle

PHASE ANGLE (?

135

180

PEAK CURRENT

PEDESTAL

CURRENT

Figure 40. Continuous/Discontinuous and Duty Cycle

DEGREES (?

135

180

100

100% = Discontinuous

50% = Continuous

DUTY CYCLE

MODE

Ifanauxiliarywindingisdesiredtoprovideabiassupply,

it should provide a minimum of 12.1 volts (to exceed the

UVLO spec) and a maximum of 18 volts. The auxiliary

windingshouldbeconnectedsuchthatitconductswhenthe

power switch is off. Near the zero crossings of the line

frequency, the voltage will have a peak voltage equal to the

regulatedoutputvoltagedividedbytheturnsratio.Thefilter

cap on the V

pin needs to be of sufficient size to hold the

voltage up over between the zero crossings.

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