AN2623

Design circuit

13/25

3.5

Feedback loop

Since current mode control is employed using the L5991 current mode controller, the power

stage of the forward converter exhibits a single output pole due to the output capacitor and

load combination, along with a zero due to the ESR of the output capacitor. The goal of the

compensator is to achieve a slope of -20 db/decade for the closed loop gain, with a phase

margin greater than 45 degrees at the crossover frequency. To achieve good dc regulation, a

high low-frequency gain is another requirement for the compensator. For continuous

conduction mode operation, the transfer function of the forward converter (power stage) is:

Equation 39

where (referring to Figure 4)

●

●

●

●

n is the turn ratio between the primary and secondary side

●

●

In order to reach the objective previously stated at the beginning of this section, the

feedback compensation network transfer function, using L5991, is obtained as:

Equation 40

where (referring to Figure 4)

●

●

CTR is the current transfer ratio of the optocoupler

●

●

●

●

ω

to compensate

ω

p

●

ω

to compensate

ω

z

●

G

()

G

1o

1

s

ω

z

-----

+

⎝⎠

⎛⎞

1

s

ω

p

-----

+

⎝⎠

⎛⎞

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

⋅

=

G

1o

nR

0

⋅

3R

9

⋅

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

=

R

0

V

2

out

P

0

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

=

ω

z

1

ESR

cout

C

out

⋅

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

=

ω

p

1

R

0

C

out

⋅

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

=

Cs

()

C

0

1

s

ω

zc

--------

+

1

s

ω

pc

--------

+

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

1

s

---

⋅⋅

=

C

0

12

10

3

CTR

⋅⋅

R

5

C

8

R

3

⋅⋅

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

=

ω

zc

1

R

7

C

8

⋅

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

=

ω

pc

1

12

10

3

C

12

⋅⋅

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

=