IW1689 datasheet(11/17 Pages) DIALOG

Part #	IW1689

Description	Low-Power Off-line Digital PWM Controller

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Manufacturer	DIALOG [Dialog Semiconductor]

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iW1689

Low-Power Off-line Digital PWM Controller

Rev 2.0

iW1689

Page 11

NovembeR 7, 2012

()

900

82

10.9

ON

BULK MIN

ON

Vs

T

V

Vs

T

V

Ts

m

=

From 10.8:

85% 25

10.9

10.35

RESET

T

ss

Ts

mm

m

=

×

−

=

Allowing a maximum forward voltage drop at the Schottky

diode of 0.7 V, substitute T

ON and

T

RESET

into equation

(10.10):

900

15.2

5.7 10.35

N =

=

×

Secondary and axuiliary turn:

PR

SEC

N

=

(10.11)

PR

AUX

OUT

NV

N

V

×

=

(10.12)

With NPR = 167, and N = 15, from equation 10.10:

167 11

15

SEC

N

turns

=

From equation 10.11:

13

11

5.7

25

AUX

V

N

V

N

turns

=

Primary RMS current: Primary current is a triangle of peak

amplitude I

PRPK at maximum duration TON.

It’s RMS on a

period T

P is:

3

PRPK

ON

PRRMS

P

I

T

I

T



= 



(10.13)

Substitute I

PRPK = 304 mA into equation (10.13)

304

10.9 115

25

3

PRRMS

I

mA

=

×

=

Selecting a switching MOSFET:

For a flyback converter, the MOSFET will be selected so that

V

DS is higher than the switching voltage plus the reflecting

secondary and spike voltage.

()

DS

BULK MAX

OUT

SPIKE

V

NV

V

>

++

(10.14)

If V

SPIKE = 100V, then from equation (10.14):

372 15 5.7 100 557

DS

VV

>

+×

+

=

And I

DS > IPRPK = 304mA

Therefore, select a MOSFET = 600 V/0.4 A

The secondary current is a triangle of peak amplitude

I

SECPK = IPRPK x N at a maximum duration of TRESET. It’s RMS

value during a period T

P is:

3

RESET

SECRMS

PRPK

P

NT

I

T





=

×









(10.15)

Substituting I

PRPK = 304 mA into equation (10.15) yields:

0.304 15

10.35 1.694

25

3

SECRMS

IA

×

=

×=

The working voltage of the rectifier diode includes the output

voltage plus margin reverse voltage during the T

ON period. A

20% marginal voltage is suitable for safety:

()

120%

BULK MAX

DIODE

OUT

V

VV

N



=

+





372

1.2

5 35

15

DIODE

V



=

+=





Typically, we select the rated current of a Schottky diode

rectifier that is twice the output current value. With a known

value for I

SECRMS and VDIODE the rectifier diode will be selected

with ratings of 40 V and 3 A :

3 / 40

Diode

A

V

=

This maximum RMS current occurs at 130% of the

rated output load. Therefore, the wire size will based on

I

SECRMS/1.3.

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