FAN100 datasheet(10/21 Pages) FAIRCHILD

Part #	FAN100

Description	Design and Application of Primary-Side Regulation (PSR) PWM Controller

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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10 / 21 page

AN-6067

APPLICATION NOTE

www.fairchildsemi.com

Rev. 1.0.1 • 1/26/10

10

Application Information

Transformer Design

The transformer inductor current must operate in DCM

under any conditions. A typical output V-I curve is shown in

Figure 24. For discontinuous current mode operation, the

transformer inductor should be small enough to meet this

condition. Point “B” is the lowest output voltage within the

CC regulation and the widest discharge time of the

transformer inductor due to the reflected voltage on the

primary inductor. It is the easiest into CCM condition for

transformer inductor.

Point “A” is the maximum output power of the power

system. Ensure that the magnetic flux density falls within

0.25~0.3 Tesla, considered a safe range. The number of

turns for primary transformer inductor can be determined on

point “A.” Figure 25 shows the characteristic curve of turn

ratio and transformer inductance.

o

V

o

I

(

)

maximum output power

determine turn number

A

B

determine

primary inductor

Figure 24. Critical Operating Points to Determine

the Transformer

0

0.5

1

1.5

2

2.5

3

3.5

5

6

7

8

9

10

11

12

13

14

15

n(turn ratio)

B=0.5V

B=1V

B=1.5V

B=2V

1 ,

0.45

IoA Vf

V

==

Figure 25. Characteristic Curve of Turn Ratio

and Inductance

Determine Maximum and Minimum Input Voltage

Figure 26 shows the corrected input voltage waveform. The

red line shows ripple voltage on the bulk capacitor and the

minimum and maximum voltage on the bulk capacitor is

expressed in equations 7 and 8, respectively. The CBULK is

the input capacitor and a typical value is 2-3µF per watt of

output power for wide range input voltage (90-264V).

2.5

Assume

mS

.min

in

V

Figure 26. Bridge Rectifier and Bulk Capacitor Voltage

Waveform

2

.min

,min

2(1- 0.3)

2-

120

oo

in

ac

bulk

VI

VV

C

η

⋅⋅ ⋅

=⋅

⋅⋅

(7)

.max

2

in

ac

VV

=⋅

(8)

Determine the Turn Ratio

The transformer turn ratio (n

p=Npri/Nsec) is an important

parameter of the flyback converter; it affects the maximum

duty ratio when the input voltage is at a minimum value. It

also influences the voltage stresses on the MOSFET and the

secondary rectifier. The permissible voltage stresses and the

maximum voltage stresses on the MOSFET, as well as the

secondary rectifier, can be expressed as:

.max

()

DS

in

p

o

f

VV

n

V

=+

⋅

+

(9)

.max

in

Fo

p

V

VV

n

=

+

(10)

The leakage spike due to leakage inductance on the

MOSFET and rectifier must also be taken into account.

Determine Transformer Inductance

Determine the VDD voltage level and if the output voltage is

defined. The turn ratio between auxiliary winding and

secondary winding can be calculated as:

DD

fa

a

Of

VV

n

VV

+

=

+

(11)

where VDD is voltage on VDD cap, usually ranging from

about 15V~20V.

In the CC regulation region, on point “B,” the power system

shuts down if the output voltage is too low and the VDD

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