FAN6982MY【PDF 12/16 页】IC CTLR PFC CCM 14-SOICN

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Sheet目录38

8页 9页 10页 11页 [12页]13页 14页 15页 16页

?2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN6982 " Rev. 1.0.3

Functional Description

Oscillator

The internal oscillator frequency of FAN6982 is

determined by the timing resistor and capacitor on the

RT/CT pin, but note that the optimum operation for

FAN6982 is between 50 and 75kHz. The frequency of

the internal oscillator is given by:

0.56

360

OSC

?nbsp ?nbsp +

(1)

The dead time for the PFC gate drive signal is

determined by

360

DEAD

(2)

The dead time should be smaller than 2% of switching

period to minimize line current distortion around line

zero crossing.

Gain Modulator

Gain modulator is the key block for PFC stage because

it provides the reference to the current control error

amplifier for the input current shaping, as shown in

Figure 23. The output current of gain modulator is a

function of V

, I

and V

RMS

. The gain of the gain

modulator is given as a ratio between I

and I

with a

given V

RMS

when V

is saturated to high. The gain is

inversely proportional to V

RMS

, as shown in Figure 24,

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 25.

= ?/DIV>

?/DIV>

(

0.6)

(

0.6)

MAX

RMS

G I

K V

Figure 23. Gain Modulator Block

RMS

RMS-UVP

RMS

Figure 24. Modulation Gain Characteristics

Figure 25. Line Feed-Forward Operation

To sense the RMS value of the line voltage, an

averaging circuit with two poles is typically employed as

shown in Figure 23. Notice that the input voltage of

PFC is clamped at the peak of the line voltage once

PFC stops switching since the junction capacitance of

bridge diode is not discharged, as shown in Figure 26.

Therefore, the voltage divider for V

RMS

should be

designed considering the brownout protection trip point

and minimum operation line voltage.

PFC runs

PFC stops

RMS

Figure 26. V

RMS

According to the PFC Operation

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