ACT412

Rev 1, 30-Oct-13

- 11 -

www.active-semi.com

Copyright © 2013 Active-Semi, Inc.

Active-Semi Proprietary―For Authorized Recipients and Customers

ActivePSR

TYPICAL APPLICATION CONT’D

An EE16 core is selected for the transformer. From

the manufacture’s catalogue recommendation, the

of the primary winding is:

The turns of secondary and auxiliary winding

can be derived accordingly:

Determining the value of the current sense resistor

(R9) uses the peak current in the design. Since the

ACT412 internal current limit is set to 1V, the

design of the current sense resistor is given by:

The voltage feedback resistors are selected

according to frequency at full load. The design

frequency at full load is given by:

The design Vo is given by:

Where k is IC constant and K=0.0000065, then we

can get the value:

When selecting the output capacitor, a low

ESR electrolytic capacitor is recommended to

minimize ripple from the current ripple. The

approximate equation for the output capacitance

value is given by:

Two 330µF electrolytic capacitors are used to keep

the ripple small.

In fact, consider of transformer production, we scale

down the turn ratio and inductance.

PCB Layout Guideline

Good PCB layout is critical to have optimal

performance. Decoupling capacitor (C4) and

feedback resistor (R5/R6) should be placed close to

VDD and FB pin respectively. There are two main

power path loops. One is formed by C1/C2, primary

winding, Mosfet transistor and current sense

resistor (R9). The other is secondary winding,

rectifier D8 and output capacitors (C5/C6). Keep

these loop areas as small as possible. Connecting

high current ground returns, the input capacitor

ground lead, and the ACT412 GND pin to a single

point (star ground configuration).

T

95

10

3000

2

.

19

00073

.

0

75

.

0

B

A

L

I

N

10

max

E

max

_

p

lim

P

=

×

×

×

=

×

×

≥

−

(13)

mA

750

mA

375

2

I

2

I

FL

_

p

lim

≈

×

=

×

=

(14)

T

24

17

4

.

1

N

N

N

N

T

17

53

.

5

/

95

N

N

/

N

N

s

S

A

A

S

p

p

s

≈

×

=

×

=

≈

=

=

(15)

Ω

.

3

.

1

75

.

0

1

I

V

R

lim

CS

CS

≈

=

=

(16)

sw

_

f

cs

cs

p

D

O

2

fb

1

fb

2

fb

1

fb

s

p

s

K

R

V

L

V

V

R

R

R

R

N

N

f

×

×

+

×

+

×

×

=

(17)

D

FB

a

s

2

fb

1

fb

o

V

V

N

N

)

R

R

1

(

V

−

×

×

+

=

(18)

K

9

.

6

R

,

K

3

.

45

R

2

fb

1

fb

=

=

(19)

F

327

mV

50

k

110

8

.

1

V

f

I

C

RIPPLE

sw

OUT

OUT

μ

=

×

=

×

=

(20)

mH

5

.

0

L

,

T

17

N

,

T

12

N

,

T

68

N

p

A

s

p

=

=

=

=

(21)