ML4812CQX 查看數據表（PDF） - Fairchild Semiconductor

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ML4812CQX

Power Factor Controller

Fairchild Semiconductor 

ML4812

PRODUCT SPECIFICATION

Where RS is the current sense resistor and NC is the turns

ratio of the current transformer (T1) used. In general, current

transformers simplify the sensing of switch currents (espe-

cially at high power levels where the use of sense resistors is

complicated by the amount of power they have to dissipate).

Normally the primary side of the transformer consists of a

single turn and the secondary consists of several turns of

either enameled magnet wire or insulated wire. The diameter

of the ferrite core used in this example is 0.5" (SPANG/Mag-

netics F41206-TC). The rectifying diode at the output of the

current transformer can be a 1N4148 for secondary currents

up to 75mA average.

Sense FETs or resistive sensing can also be used to sense the

switch current. The sensed signal has to be amplified to the

proper level before it is applied to the ML4812.

The value of the ramp compensation (SCPWM) as seen at the

inverting terminal of the PWM comparator is:

SCPWM = -R----T---2-×--.-5--C---×-T----R×----M-R-----S--C--

(11)

The required value for RSC can therefore be found by equat-

ing: SCPWM = ASC x SPWM, where ASC is the amount of

slope compensation and solving for RSC. The value of

GM OUT depends on the selection of RAMP COMP.

RP

=

V-----I--N----(--m-----a---x----)--P---E---A----K--

ISINE(PEAK)

=

2----6--0-0--.--5×---m--1---.A-4---1---4--

=

750kΩ

(12)

RM

=

V-----C----L---A---M-----P----×-----R----P-

VIN(PEAK)

=

4---9-.-9-0----×-×---7--1-5--.-40---1-k--4-Ω---

=

28.8kΩ

(13)

The peak of the inductor current can be found approximately

by:

ILPEAK

=

1----.-4---1---4-----×----P----P---O----U---T--

VIN(RMS)

=

1----.-4---1---4-----×----2---0---0--

90

=

3.14A

(14)

Selection of NC which depends on the maximum switch

current, assume 4A for this example is 80 turns.

RS

=

V-----C----L---A---M-----P----×-----N----C--

ILPEAK

=

4----.-9-----×----8---0--

4

=

100Ω

(15)

Where RS is the sense resistor, and VCLAMP is the current

clamp at the inverting input of the PWM comparator. This

clamp is internally set to 5V. In actual application it is a good

idea to assume a value less than 5V to avoid unwanted cur-

rent limiting action due to component tolerances. In this

application, VCLAMP was chosen as 4.9V.

Having calculated RS, the value SPWM and of RSC can now

be calculated:

RSC = -A----S---C-----×----S--2--P-.--5W----×-M----R-×---M--R----T-----×-----C----T-

(16)

RSC

=

------------------------2---.--5----×-----2---8---.--8---k---Ω---------------------------

0.7 × (0.225 × 106) × 14K × 1nF

=

33kΩ

The following values were used in the calculation:

RM = 28.8kΩ

RT = 14kΩ

ASC = 0.7

CT = 1nF

Voltage Regulation Components

The values of the voltage regulation loop components are

calculated based on the operating output voltage. Note that

voltage safety regulations require the use of sense resistors

that have adequate voltage rating. As a rule of thumb if 1/4W

resistors are chosen, two of them should be used in series.

The input bias current of the error amplifier is approximately

0.5µA, therefore the current available from the voltage sense

resistors should be significantly higher than this value. Since

two 1/4W resistors have to be used the total power rating is

1/2W. The operating power is set to be 0.4W then with 380V

output voltage the value can be calculated as follows:

R1 = (380V)2 ⁄ 0.4W = 360kΩ

(17)

Choose two 178kΩ, 1% connected in series. Then R2 can be

calculated using the formula below:

R2

=

-----V----R----E---F----×-----R----1-----

VOUT – VREF

=

5--3--V-8---0--×--V---3---5–---6-5--k--V-Ω---

=

4.747kΩ

(18)

Choose 4.75kΩ, 1%. One more critical component in the

voltage regulation loop is the feedback capacitor for the error

amplifier. The voltage loop bandwidth should be set such

that it rejects the 120Hz ripple which is present at the output.

If this ripple is not adequately attenuated it will cause distor-

tion on the input current waveform. Typical bandwidths

range anywhere from a few Hertz to 15Hz. The main com-

promise is between transient response and distortion. The

feedback capacitor can be calculated using the following

formula:

CF = -3---.-1---4---2-----×----R1-----1---×-----B-----W----

(19)

CF = 3----.-1---4---2-----×----3---5---16----k---Ω------×-----2---H-----z- = 0.44µF

10

REV. 1.0.4 5/31/01

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