AN-8027

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 8/26/09

2

Functional Description

Gain Modulator

The 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

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.

ISENS

E

IA

C

VRMS

VEA

2

(0.7)

(0.7)

MO

AC

EA

AC

MAX

RMS

EA

IG I

KV

I

VV

=⋅

⋅−

=⋅

−

IEA

R

M

R

M

Gain

Modulator

R

RMS1

R

RMS2

R

RMS3

C

RMS1

C

RMS2

R

IAC

I

AC

V

IN

I

L

x

2

k

Figure 2. Gain Modulator Block

V

RMS

V

RMS-UVP

2

1

RMS

G

V

∝

Figure 3. Modulation Gain Characteristics

To sense the RMS value of the line voltage, an averaging

circuit with two poles is typically employed, as shown in

Figure 2. The voltage of VRMS pin in normal PFC

operation is given as:

3

12

3

2

2

π

=⋅

++

RMS

RMS

LINE

RMS

RMS

RMS

R

VV

RR

R

(1)

However, once PFC stops switching operation, the junction

Figure 2 is clamped at the peak of the line voltage. Then,

the voltage of VRMS pin is given by:

3

12

3

2

NS

RMS

RMS

LINE

RMS

RMS

RMS

R

VV

RR

R

=

++

(2)

Therefore, the voltage divider for VRMS should be

designed considering the brownout protection trip point

and minimum operation line voltage.

PFC runs

PFC stops

V

IN

V

RMS

The rectified sinusoidal signal is obtained by the current

enough to prevent saturation of the gain modulator as:

.

2

159

MAX

LINE BO

IAC

V

GA

R

μ

⋅<

(3)

is 1.08V (which can be found in the datasheet), and 159µA is

the maximum output current of the gain modulator.

Current and Voltage Control of Boost Stage

As shown in Figure 5, the FAN480X employs two control

loops for power factor correction: a current control loop

and a voltage control loop. The current control loop shapes

inductor current, as shown in Figure 6, based on the

reference signal obtained at the IAC pin as:

1

LCS

MO

M

AC

M

I RI

R

I

G R

⋅

=⋅

=

⋅ ⋅

(4)