AN-8027

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 8/26/09

11

(Design Example)

Setting the crossover frequency

as 22Hz:

1

2

6

6

2

2.5

5(2

)

70 10

0.9 1.27

2.5

20

10

387

5 270

(2

22)

MV

BOUT

MAX

VC

BOUT

BOUT

VC

GI

K

C

V

Cf

nF

π

π

−

−

⋅⋅

=⋅

⋅⋅

×⋅

⋅

=⋅

=

×

⋅⋅

⋅

9

1

11

362

2

222 20 10

VC

VC

VC

R

k

fC

π

π

−

==

=

Ω

⋅⋅

⋅⋅ ×

Setting the pole of the compensator at 120Hz:

2

3

11

3.7

2

2120 362 10

VC

VP

VC

CnF

fR

π

π

==

=

⋅⋅

⋅⋅

×

[STEP-10] Transformer Design for PWM

Stage

Figure 19 shows the typical secondary-side circuit of

forward converter for multi-output of PC power application.

A common technique for winding multiple outputs with the

same polarity sharing a common ground is to stack the

secondary windings instead of winding each output

winding separately. This approach improves the load

Figure 19 must be sized to accommodate its output current,

plus the current of the output (+12V) stacked on top of it.

To get tight regulation of 3.3V output, magnetic amplifier

(MAG-AMP) is used. The saturable core of MAG-AMP

introducing high impedance until it is saturated. This

be regulated the output voltage.

MAG AMP

Control

MAG

AMP

N

p

+12V

+5V

+3.3

V

-12V

Additiona

l LC filter

Additiona

l LC filter

Additiona

l LC filter

Additiona

l LC filter

N

S

1

N

S

2

V

REC

+

-

D

REC

N

S

3

Figure 19. Typical Secondary-Side Circuit

Once the core for the transformer is determined, the

minimum number of turns for the transformer primary-side

to avoid saturation is given by:

MIN

MIN

BOUT

MAX

P

eSW

VD

N

Af

B

=

Δ

(44)

the switching frequency, and

ΔB is the maximum flux

density swing in Tesla for normal operation.

ΔB is typically

0.2-0.3 T for most power ferrite cores in the case of a

forward converter.

The turn ratio between the primary-side and secondary-side

winding for the first output is determined by:

11

1

()

MIN

BOUT

MAX

P

SO

F

VD

N

n

NV

V

==

+

(45)

output is determined, the number of turns of other output

(n-th output) can be determined by:

()

()

()

1

11

On

F n

Sn

S

OF

VV

N

N

VV

+

=⋅

+

(46)

The golden ratio between the secondary-side windings for

the best regulation of 3.3V, 5V, and 12V is known as

2:3:7.

(Design Example)

The minimum PFC output voltage

is 310V and the maximum duty cycle of PWM

controller is 50%. By adding 5% margin to the

core is used and

ΔB=0.28, the minimum turns for the

transformer primary side is obtained as:

63

310 0.45

72

107 10

65 10 0.28

MIN

MIN

BOUT

MAX

P

eSW

VD

N

Af

B

−

⋅

==

=

Δ

×⋅ ×

⋅

The turns ratio for 5V output is obtained as:

310 0.45

25.6

(

)

(5 0.45)

MIN

BOUT

MAX

P

SO

F

VD

N

n

NV

V

⋅

==

=

=

++

The number of turns for the primary-side winding is

determined as:

1

2 25.6 51.2

MIN

pS

P

Nn N

N

=⋅

= ×

=

<

11

3 25.6 76.8

3

MIN

pS

P

S

Nn N

N

N

= ⋅= ×

=

>

∴

=

Then, the turns ratio for 12V output is obtained as:

22

21

11

12 0.7

36.99 7

50.45

OF

SS

OF

VV

NN

VV

+

+

=

⋅=

⋅ =

≅

++

Therefore, the number of turns for each winding is

obtained as: