AN-6961 datasheet(7/8 Pages) FAIRCHILD

Part #	AN-6961

Description	Critical Conduction Mode PFC Controller

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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AN-6961

APPLICATION NOTE

www.fairchildsemi.com

Rev. 1.0.2 • 4/8/09

7

Determine the Output Capacitor Co

The output capacitor is determined by the requirement of

sufficient hold-up time tHold:

η

•

⎟

⎠

⎞

⎜

⎝

⎛

−

•

=

2

min

.

o

2

o

hold

o

V

t

P

2

C

(11)

where tHold is the output capacitor hold-up time, which is

measured from the time the AC input turns off to before the

output voltage falls below the minimum operating voltage of

the following DC/DC stage.

The output ripple voltage ΔVO is expressed as:

o

V

C

P

V

•

=

Δ

ω

(12)

where:

ω= 2 π f,

f is AC line frequency,

IO is the output current.

Determine the Compensation Capacitor CEA

As discussed previously, to achieve a low input current THD,

the variation of the on-time within one input AC cycle should

be very small. To achieve this, the bandwidth should be

lower than 20Hz. The capacitance CEA connected between

COMP and GND can be obtained as:

BW

2

g

C

m

EA

•

=

π

(13)

where BW is PFC control loop bandwidth and establishes it

as 20Hz. The error amplifier is a trans-conductance

amplifier that converts voltage to current with a 125µmho

output conductance.

Design Example

This section shows a design example of a 90W (19V/4.74A)

adaptor. From the specification, all critical components are

treated and final measurement results are given.

The basic design specification are shown as following:

•

AC Input Voltage Range Vrms: 90 ~ 264 VAC

•

Rated Output Power PO: 90 W

•

Minimum Switching Frequency fs,min: 35KHz

•

High Regulated Output Voltage: 400V (at 180 ~ 264VAC)

•

Low Regulated Output Voltage: 250V (at 90 ~ 132VAC)

Based on the given design guideline, the critical parameters

are calculated and summarized in Table 1:

Table 1. Critical System Parameters

Lb

530µH

ton,fix(90Vrms)

13.86µs

ipk

3.327A

ton,fix(132Vrms)

6.44µs

Nb

65T

ton,fix(180Vrms)

3.46µs

Naux

7T

ton,fix(264Vrms)

1.61µs

CO

68µF/450V

ΔVO(VO = 250V)

14.043V

CEA

1μF

ΔVO(VO = 400V)

8.77V

Reference Circuit

The complete circuit diagram is shown in Figure 19 and

the Bill of Materials for the PFC stage is shown in Table 2.

Table 2. BOM List of PFC Stage

Reference

Components

Reference

Components

F1

4A/250V

C2

0.33μ/275V

R1

510K

C3

Open

R2

510K

C4

Open

R3

10K

C5

Open

R4

1M

C7

0.47μF/400V

R5

18.7K

C8

0.47μF/400V

R6

1M

C11

2.2μF/50V

R7

430K

C12

68μF/450V

R8

1M

C24

104pF

R13

Open

C25

Open

R14

24K

C27

Open

R15

68K(47K)

C28

Open(22p)

R16

10

Ω

C29

221pF

R17

0

Ω

Q1

2N-7002

R18

0.18

Ω/2W

Q2

2SK-2482/

TO-220

R55

Open

BD1

KBP205G

R56

Open

ZD1

ZD24V

R57

Open

D2

R860/TO-220

R58

Open

D3

1N4148

R59

0

Ω

L1

1mH

MOV1

470V/7D

L2

13mH

TR1

055

L4

RM-10/

530µH

U1

FAN6961

L5

400μH

C1

224pF

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