AN1262 APPLICATION NOTE

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must be accounted for as well (the minimum specified value will be considered) and the turn number may result

quite high.

17

PRIMARY FEEDBACK IMPLEMENTATION

In this approach, which will be considered with regards to the L6590 and the L6590D only, the voltage generated

by the self-supply winding is sensed and regulated. This solution, shown in fig. 16, is cheap because no opto-

coupler is needed, but provides poor regulation, especially as a result of load changes.

Ns turn ratio only. Actually, numerous non-idealities, mainly transformer's parasitics, cause the actual ratio to

deviate from the ideal one. Line regulation is quite good, in the range of ± 2%, whereas load regulation is about

±5%. Output voltage tolerance is instead in the range of ±10%. The resulting transfer function is:

Table 21 shows how its quantities are defined. As to the selection of Vcc, the same considerations concerning

the circuit of fig. 15 apply to the circuit in fig. 16a. Such limitation is not in the circuit of fig. 16b.

Figure 16. Primary feedback: circuits

Table 21. G1(j

ω) Implementation: Primary Feedback

The value of the resistor Rs (R’s for the circuit of fig. 16b) in series to the bias diode will be selected to achieve

minimum load regulation and its value may range from few units to some hundred ohm.

Symbol

Definition

G1 j

ω

()

V

COMP

∆

V

out

∆

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

N

aux

N

S

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

V

CO MP

∆

V

CC

∆

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

⋅

=

N

aux

N

S

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

1

R

H

C

Fs

C

Fp

+

()

⋅

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

1

j

ω

-----

1j

+

ω R

F

C

Fs

⋅⋅

1j

+

ω R

F

C

Fs

C

Fp

⋅

C

Fs

C

Fp

+

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

⋅⋅

-----------------------------------------------------------.

⋅⋅

⋅

==

a)

b)

L6590

L6590D

Vcc

VFB

COMP

22 µF

L6590

L6590D

Vcc

VFB

COMP

22 µF

220 nF

R

L

15 V

V

CC

-----------

k

Ω

[]

>

R

H

V

CC

2.5

–

2.5

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

L

⋅

=

C

Fp

N

aux

N

S

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

f

Z

f

P

-----

1

G1

0

R

H

⋅

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

⋅⋅

=

C

Fs

C

Fp

f

P

f

Z

-----

1

–





⋅

=

R

F

1

2

π f

Z

C

Fs

⋅⋅

⋅

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

=