AN-6902

APPLICATION NOTE

© 2007 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.2.1 • 5/1/08

7

Figure 9. Multi-vector Error Amplifier

Equation 22 shows the output of the voltage error

power delivered to the load.

Although the PFC stage has a low bandwidth voltage loop

for better input power factor, the innovative multi-vector

error amplifier provides a fast transient response to clamp

the overshoot and undershoot of PFC output voltage.

Figure 10 shows the block diagram of the multi-vector

error amplifier. When the variation of the feedback

voltage (FBPFC) exceeds ±5% of the reference voltage

(3V), the trans-conductance error amplifier programs its

open circuit, SG6902 is turned off immediately to prevent

over-voltage on the output capacitor.

Two-level PFC Output voltage

of PFC is usually designed to 250V at low line and 400V

at high line. This improves efficiency of the power

converter for low-line input. The RANGE pin (open-

drain) is used for the two-level output voltage setting.

Figure 10 shows the RANGE output that programs the

PFC output voltage. The RANGE output is shorted to

drops below 1.6V. The output voltages can be determined

using below equations:

V

3

R

R

R

V

Open

Range

B

B

A

O

×

+

=

⇒

=

(23)

()

()

V

3

R

//

R

R

//

R

R

V

GND

Range

C

B

C

B

A

O

×

+

=

⇒

=

(24)

1

3

V

R

R

O

B

A

−

=

(25)

33

.

82

1

3

250

R

R

B

A

=

−

=

(26)

Figure 10. At high line input, maximum output voltage is:

V

420

1

R

//

R

R

15

.

3

V

C

B

A

)

MAX

(

O

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

×

=

(27)

Another circuit provides further over-voltage protection

to inhibit the PFC switching once the feedback voltage

exceeds the 3.25V the output voltage is clamped at:

V

433

1

R

//

R

R

25

.

3

V

C

B

A

)

OVP

(

O

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

×

=

(28)

Figure 10.

Feedback Voltage of PFC

PWM SECTION

Soft-starting the PWM stage

The soft-start pin controls the rising time of the output

voltage and prevents the overshoot during power on. The

given by:

OZ

SS

SS

SS

V

I

t

C

×

=

(29)

VEA

FBPFC

RANGE

3V

+

SG69XX

(PFC)