AN1262 APPLICATION NOTE

24/42

The synthesis of G1(j

ω) can be done by following the following step-by-step procedure:

a) Calculate gain and phase of G2(j

;

G2(j

ω) will be calculated at maximum input voltage and maximum load, where the gain-bandwidth product is

maximum.

b) Calculate gain and phase of G1(j

;

,

c) Cancel the pole of G2(j

ω) by placing the zero of G1(jω) in the neighborhood:

(

α = 1 to 5)

d) Place the pole of G1(j

ω) so as to get the desired phase margin:

,

The synthesis of G1(j

ω) is completed. The following step will concern the practical implementation of such func-

tion, that is the realization of a Type 2 amplifier. This will be done considering two cases, the secondary and the

primary sensing feedback.

16

SECONDARY FEEDBACK IMPLEMENTATION

This kind of feedback, shown in fig. 12, uses a popular arrangement with a TL431 as secondary reference/error

amplifier and an optocoupler to transfer the control signal to the primary side. The error amplifier of the IC is

(and the duty cycle is controlled) by modulating the current Ic sunk from the pin. A change of Ic causes a change

and table 16 shows how its quantities are defined

G2

c

G2 2

π f

c

⋅⋅

()

=

Φ2

c

180

π

----------

G2 2

π f

c

⋅⋅

()

[]

arg

⋅

=

G1

c

G1 2

π f

c

⋅⋅

()

1

G2

c

-----------

==

Φ1

c

180

π

----------

G1 2

π f

c

⋅⋅

()

[]

arg

⋅

180

–

Φ

m

Φ2

c

–

+

==

f

Z

ω

Z

2

π

⋅

-----------

α

ω

out

2

π

⋅

-----------

⋅

==

f

P

ω

P

2

π

⋅

-----------

=

f

C

π

180

----------

Φ1

c

⋅





tan

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

≈

G1

0

2

π G1

c

f

c

f

Z

⋅

f

P

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

⋅⋅

⋅

≈

G1 j

ω

()

V

COMP

∆

V

out

∆

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

V

COMP

∆

I

C

∆

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

I

C

∆

I

F

∆

--------

I

F

∆

V

K

∆

-----------

V

K

∆

V

out

∆

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

⋅⋅

⋅

CTR

max

R

COMP

⋅

R

B

R

H

C

F

⋅⋅

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

1

j

ω

-----

1j

ω R

H

R

F

+

() C

F

⋅⋅

+

1j

ω R

CO MP

C

COMP

⋅⋅

+

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

⋅⋅

==

=