AN1262 APPLICATION NOTE

28/42

Table 19. G1(j

ω) Implementation: secondary feedback (III)

Figure 15 shows a special configuration, with the optocoupler connected in series to the supply pin of the IC that

provides the following benefits:

a) a large range of the voltage generated by the auxiliary winding can be allowed since the changes are

"damped" by the phototransistor and Vcc is stabilized by the error amplifier; this is useful with a poor quality

transformer or when the output voltage (tracked by the auxiliary voltage) may decrease because of constant

current regulation (e.g. battery chargers, see fig.40 on L6590’s datasheet).

b) during overload and short circuit the power throughput is automatically reduced because the operation of the

output voltage is too low, there will not be enough current through the photodiode at the secondary side to

c) despite the IC's OVP protection is bypassed by such configuration, the system is still protected against op-

tocoupler's failures: if that happens, the phototransistor will no longer be able to supply the IC, which will go

into UVLO just like in case of overload or short circuit.

The transfer function of the schematic of Fig. 15 is:

a low-frequency pole that causes a phase lag noxious for the phase margin. This pole needs to be compensated

pensate it. The pole at the origin and the other zero-pole pair realize a type 2 amplifier (see Table 20 to see how

Symbol

Definition

;

R

F2

2k

Ω

>

R

C

2.5 R

F2

⋅

<

R

H

V

out

2.5

–

2.5

------------------------- R

L

⋅

=

R

B

CTR

min

V

out

3.5

–

2.5

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

R

E

⋅⋅

<

C

F1

CTR

max

R

E

R

F2

⋅⋅

R

E

R

C

+

() · R

B

R

H

G1

0

⋅⋅

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

=

R

F1

1

2

π f

Z

C

F1

⋅⋅

⋅

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

R

H

–

=

C

F2

1

2

π f

P

R

F2

⋅⋅

⋅

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

=

G1 j

ω

()

V

COMP

∆

V

out

∆

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

V

CO MP

∆

V

CC

∆

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

V

CC

∆

I

C

∆

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

I

C

∆

I

F

∆

--------

I

F

∆

V

K

∆

-----------

V

K

∆

V

out

∆

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

=

⋅⋅

⋅

⋅

=

=

= CTR

ma x

R

F2

R

B

----------

1

j

ω R

H1

C

F1

⋅⋅

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

1j

ω R

c

C

s

⋅⋅

+

() 1j

+

ω R

H1

R

F1

+

() C

F1

⋅⋅

[]

⋅

1j

+

ω R

H2

R

c

+

() C

s

⋅⋅

[] 1j

+

ω R

F2

C

F2

⋅⋅

()

⋅

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

.

⋅⋅

⋅