AN-6747

APPLICATION NOTE

© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 9/16/10

7

6

BIAS

KA

OPD

O

10

325

CTR

R

V

V

V

−

×

>

•

−

−

(28)

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

opto-coupler.

R

2

C

FB

3R

R

5.2V

Figure 9.

Feedback Circuit

The feedback compensation network transfer function of

Figure 9 is obtained as:

PC

ZC

I

O

FB

/

s

1

/

s

1

s

ˆ

ˆ

ω

+

ω

+

•

ω

−

=

ν

ν

where

F

DB

1

B

I

C

R

R

R

=

ω

;

()

O

1

F

ZC

C

R

R

1

+

=

ω

;

FB

B

PC

C

R

1

=

ω

and C

(Design Example)

Assuming CTR is 100%;

6

BIAS

KA

OPD

O

10

325

CTR

R

V

V

V

−

×

>

•

−

−

Ω

=

×

−

−

=

×

−

−

<

−

−

k

87

10

325

5

.

2

2

.

1

32

10

325

V

V

V

R

6

6

KA

OPD

O

BIAS

5.1k

selected as 120k

Ω and 10kΩ.

[STEP-11] Design the Startup Circuit

Figure 10 shows the typical startup circuit for FAN6747.

HV pin has an internal high-voltage startup circuit that is

pin is also used to obtain line voltage information for

brownout protection and power limit line compensation, it

is typical to connect the HV pin to the AC line through a

resistor and diode.

minimizing startup time. Initially, the FAN6747 HV startup

circuit is enabled before it begins normal switching

operation. Therefore, the current supplied by the HV pin

the HV startup circuit is disabled. Then the current required

by FAN6747 is supplied from the auxiliary winding of

transformer.

It is typical to use a 150~250kΩ resistor for the HV pin to

improve the immunity against line surge.

Figure 10.

Startup Circuit

Leading-Edge Blanking (LEB)

Each time the power MOSFET is switched on, a turn-on

spike occurs across the sense resistor, caused by primary-

side capacitance and secondary-side rectifier reverse

recovery. To avoid premature termination of the switching

pulse, a leading-edge blanking time is built in. During this

blanking period (270ns), the PWM comparator is disabled

and cannot switch off the gate driver. Thus, an RC filter

with a small RC time constant is enough for current sensing

(e.g. 100

Ω + 470pF). A non-inductive resistor is