FAN4800CUN【PDF 13/19 页】IC PWM/PFC CTLR COMBO 16-MDIP

?2011 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN4800AU/CU " Rev. 1.0.1

13

Functional Description

Oscillator

The internal oscillator frequency is determined by the

timing resistor and capacitor on the RT/CT pins as

shown in Figure 25. The frequency of the internal

oscillator is given:

T

O SC

C

R

360

56

.

0

1

+

?/DIV>

=

(1)

Because the PWM stage generally uses a forward

converter, it is necessary to limit the maximum duty

cycle at 50%. To have a small tolerance of the

maximum duty cycle, a frequency divider with toggle

flip-flops is used, as illustrated in Figure 25. The

operation frequency of PFC and PWM stage is 1/4 of

oscillator frequency. (For FAN4800CU, the operation

frequencies for PFC and PWM stages are 1/4 and 1/2

of oscillator frequency, respectively).

The dead time for the PFC gate drive signal is

determined by:

T

D E A D

C

3 6 0

=

(2)

The dead time should be smaller than 2% of the

switching period to minimize line current distortion

around the line zero crossing.

RT /C T

V

RE F

O S C

T Q

T -F F

T

Q

O P W M (F A N 4 8 0 0 C U )

O P F C , O P W M

T -F F

Figure 25. Oscillator Configuration

R T / C T

O P F C

O P W M

O P W M ( F A N 4 8 0 0 C U )

P F C D e a d - T i m e

Figure 26. Timing Diagram

I

D

I

D S

V

G . P F C

V

G .P W M

V

G . P F C

V

G . P W M

I

D S

I

D

Figure 27. Interleaved Leading / Trailing

Edge Modulation

Figure 27 shows the interleaved leading / trailing edge

modulation, where the turn-off of the PFC drive signal is

synchronized to the turn-on of the PWM drive signal.

This technique allows the PFC output diode current to

flow directly into the downstream DC/DC converter,

minimizing the current ripple of PFC output capacitor.

Gain Modulator

Gain modulator is the key block for the PFC stage

because it provides the reference to the current control

error amplifier for the input current shaping, as shown in

Figure 28. The output current of the gain modulator is a

function of V

EA

, I

AC,

and V

RMS

. The gain of the gain

modulator is given as a ratio between I

MO

and I

AC

with a

given V

RMS

when V

EA

is saturated to HIGH. The gain is

inversely proportional to V

RMS

2

, as shown in Figure 29,

to implement line feed-forward. This automatically

adjusts the reference of current control error amplifier

according to the line voltage, such that the input power

of PFC converter is not changed with line voltage (as

shown in Figure 30).

IS E N S E

IA C

V R M S

V E A

=

?/DIV>

?nbsp -

= ?/DIV>

-

2

( 0

. 6

)

.

M O A

C

E

A

C

M

A

X

G

I

K

V

I

V V

IE A

R M

R

M

G a in

M o d u la to r

R R M S 1

R R M S 2

R

R M S 3

C R M S 1

C

R M S 2

R

I A C

V

I N

I

L

x

2

k

Figure 28. Gain Modulator Block

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