AN1606 APPLICATION NOTE

8/18

Voltage feed-forward.

Voltage feed-forward, a useful function in wide range applications, requires a DC voltage proportional to the rms.

value of the input mains. For the L4981, this value must be between 1.5V to 5.5V so that it can mirror over a

wide range.

Since the rectified mains frequency is 100 -120Hz, we need a large rejection for this frequency and because the

feed-forward reaction time is proportional to the bandwidth, we introduce a second order filter that allows good

compromise between attenuation of the fundamental frequency and response time.

The circuit (Fig.8) is similar to the fig.7 described earlier.

Figure 8.

c)

The time constants cannot be expressed in simple way and so that the position of poles can be numerically cal-

culated.

d)

Choosing to calculate this value at the midpoint of the allowed values:

e)

Voltage feed-forward Circuit (A) and equivalent circuit (B).

A

R

a

R

b

R

c

C

a

v

R

a

C

b

B

R

a

R

c

C

a

+

v

R

b

R

a

C

b

Coupled

Inductor

H

LP

K

LP

1

1st

1

+

() 1st

2

+

()

⋅

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

=

K

LP

R

c

Ra

2Rb

2Rc

++

()

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

=

V

LP

V

RMS

22

π

-----------

K

LP

⋅

=

22

π

-----------

88

264

+

2

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

K

LP

1.5

5.5

+

2

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

=

⋅⋅