ACT337

Rev 2, 14-Nov-12

Innovative Power

TM

- 7 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

V

7

.5

73

5

8

.

0

40

3

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0

5

(

375

V

V

)

V

V

(

V

V

OUTCV

DREV

DS

OUTCV

INDCMAX

RO

=

−

×

+

×

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+

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）

(5)

110

T

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110

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1

KHz

72

mA

634

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48

90

f

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48

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η

(6)

(7)

(8)

(9)

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(16)

F

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280

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50

kHz

72

48

.

0

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2

V

f

D

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C

RIPPLE

SW

OUTCC

OUT

=

×

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=

×

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△

(17)

(10)

8

110

3.7

1

1

N

N

N

N

P

P

S

S

=

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(11)

18

8

25

.

2

N

N

N

N

S

S

A

A

=

×

=

×

=

(13)

k

80.6

242326

62

.

0

25

.

1

110

18

K

R

L

N

N

R

CS

P

P

A

1

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(15)

()

()

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64

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0

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2

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V

9

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0

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xfm

system

SW

P

OUT

OUTMAX

OUTFL

CSLIM

CS

=

⎟

⎠

⎞

⎜

⎝

⎛

×

×

×

+

×

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

×

+

×

=

η

η

(14)

2×10µF electrolytic capacitors based on the

2.5~3µF/W rule of thumb.

When the transistor is turned off, the voltage on the

transistor’s collector consists of the input voltage

and the reflected voltage from the transformer’s

secondary winding. There is a ringing on the rising

top edge of the flyback voltage due to the leakage

inductance of the transformer. This ringing is

clamped by a RCD network if it is used. Design this

clamped voltage as 50V below the breakdown of

the NPN transistor. The flyback voltage has to be

considered with selection of the maximum reverse

voltage rating of secondary rectifier diode. If a 40V

Schottky diode is used, then the flyback voltage can

be calculated:

The maximum duty cycle is set to be 48% at low

estimated to be 77%. Then the full load input

current is:

The maximum input primary peak current at full

load base on duty of 48%:

The primary inductance of the transformer:

equation

An EPC17 transformer gapped core with an

The number of turns of the primary winding is:

The number of turns of secondary and auxiliary

windings can be derived when Np/Ns=13.7:

current limit value based on the following equation:

The voltage feedback resistors are selected

according to below equation:

Where K is IC constant and K = 242326.

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:

A 470µF electrolytic capacitor is used to keep the

ripple small.

PCB Layout Guideline

Good PCB layout is critical to have optimal

performance. Decoupling capacitor (C4), current

sense resistor (R9) and feedback resistor (R5/R6)

respectively. There are two main power path loops.

One is formed by C1/C2, primary winding, NPN

transistor and the ACT337. The other is the

secondary

winding,

rectifier

D8

and

output

capacitors (C5). Keep these loop areas as small as

possible. Connect high current ground returns, the

input capacitor ground lead, and the ACT337 G pin

TYPICAL APPLICATION CONT’D

(12)

92

000023

.

0

3000

634

.

0

001

.

0

A

B

I

L

N

E

max

peak

P

MIN

≈

×

×

=

×

×

=