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AN-6920MR

Integrated Critical-Mode PFC / Quasi-Resonant Current-Mode PWM Controller FAN6920

Fairchild Semiconductor 

AN-6920

(Design Example)

0.7 > 30µ A ,

RDET 2

RDET 2 < 23.3kΩ

Setting the OVP trip point at 22.5V,

K DET

=

RDET 1

RDET 2

=

N AUX

NS

⋅ VOVP

2.5

−1=

3 ⋅ 22.5 −1 = 5.75

4 2.5

Then RDET1 = K DET⋅R ⋅ RDET 2 < 134kΩ

I PK.L

DS

I PK.K

DS

= VO.PFC.H

VO.PFC.L

⋅ VO.PFC.L + VRO

VO.PFC.H + VRO

= 400 ⋅ 300 + 240 = 1.125

300 400 + 240

Using 113% of 1.125,

VLIMIT . L

= 1.27 =

− 994 ⋅VO.PFC.L

NA

NP

+ RDET1

VLIMIT . H

− 994 ⋅VO.PFC.H

NA

NP

+ RDET1

=

− 994 ⋅ 300 ⋅

− 994 ⋅ 400 ⋅

3

48

3

48

+

+

RDET1

RDET 1

=

−18637.5 + RDET1

− 24850 + RDET1

Then, RDET1 = 47.5KΩ and REDT2=8.25KΩ.

RDET1 and RDET2 are selected from the off-the-shelf

components as 150kΩ and 18kΩ, respectively.

Then, the pulse by pulse current limit threshold voltage

is obtained as:

VLIMIT

=

VO.PFC.L

−877 ⋅ (

NA

NP

RDET1

− 0.7

+

0.7

) + 0.882

RDET 2

= 0.474V

To set current limit level at low line as 115% of IDSPK

0.63 = 0.27Ω

1.53A ×1.15

APPLICATION NOTE

[STEP-B4] Design the Feedback Circuit

Figure 19 is a typical feedback circuit mainly consisting of a

shunt regulator and a photo-coupler. R01 and R02 form a

voltage divider for output voltage regulation. RF and CF are

adjusted for control-loop compensation. A small-value RC

filter (e.g. RFB = 100Ω, CFB = 1nF) placed from the FB pin

to GND can increase stability substantially. The maximum

source current of the FB pin is about 1.2mA. The

phototransistor must be capable of sinking this current to

pull the FB level down at no load. The value of the biasing

resistor, RBIAS, is determined as:

VO − VOPD − VKA ⋅ CTR > 1.2 ×10−3

RBIAS

(40)

where VOPD is the drop voltage of photodiode, about 1.2V;

VKA is the minimum cathode to anode voltage of shunt

regulator (2.5V); and CTR is the current transfer rate of the

opto-coupler.

Figure 19. Feedback Circuit

(Design Example) Assuming CTR is 100%;

VO −VOPD −VKA ⋅ CTR > 1.2 ×10−3

RBIAS

RBIAS

<

VO

− VOPD − VKA

1.2 ×10−3

19 −1.2 − 2.5

= 1.2 ×10−3

= 12.75kΩ

330Ω resistor is selected for RBIAS.

The voltage divider resistors for VO sensing are selected as

66.5kΩ and 10kΩ.

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.0 • March 10, 2011

12

www.fairchildsemi.com

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