25/42

AN1262 APPLICATION NOTE

Figure 12. Secondary feedback: TL431 + optocoupler circuit (I)

Table 16. G1(j

ω) Implementation: secondary feedback (I)

This technique provides very good regulation of the output voltage and galvanic isolation from the primary side

at the same time.

In Table 16 it is possible to find the design relationships useful to derive the part values. Icmax is specified in

the Datasheet (2.5mA).

The following condition should be met:

,

(20)

should be selected and the calculations from step a) to step e) redone.

Symbol

Definition

Vout

TL431

L6590

L6590D

L6590A

COMP

1.5

3.5

1 mA

VFB

Not needed

in the L6590A

R

H

V

out

2.5

–

2.5

-------------------------

R

L

⋅

=

R

B

CTR

min

V

out

3.5

–

I

Cmax

-------------------------

⋅

<

C

F

CTR

max

R

COMP

⋅

R

B

R

H

G1

0

⋅⋅

-----------------------------------------------

=

R

F

1

2

π f

Z

C

F

⋅⋅

⋅

--------------------------------

R

H

–

=

C

CO M P

1

2

π f

P

R

COMP

⋅⋅

⋅

---------------------------------------------

=

CTR

max

CTR

min

-----------------------

G1

c

π

180

----------

Φ1

c

⋅





tan

R

CO MP

-------------------------------------------

V

out

3.5

–

I

Cmax

-------------------------

⋅⋅

≤