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AN1317 - APPLICATION NOTE

low switching frequency and always in discontinuous mode.

As an example, to get a 13V output voltage with a 265 Vrms input voltage, the maximum duty cycle

would be less than 5%. With a switching frequency of 100 kHz, the maximum conduction

practice the switching frequency is chosen just above the audio ones, in the 20 kHz to 30 kHz range and

the VIPer20 will work in discontinuous mode with a duty cycle of about 2% to 3% at high line and about

6% to 10% at low line.

From the VIPer20 datasheet, the switching frequency is given here below:

On the schematic of fig. 1 and fig. 5, R1=10k

Ω and C3=10nF have been chosen to get a switching

frequency near by 20 kHz (21.7 kHz typical).

3.2 Inductor

In normal operation, for both topologies, the switching cycle consists of two phases. First, the Power

switching cycle is shown on fig. 6.

Knowing the output power, the switching frequency and the maximum VIPer20 peak current, L1 can be

computed as follow:

for the Buck:

with

and

d

V

ou t

V

in

-----------

=

F

s

2.3

R

1

C

3

⋅

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

1

550

R

1

150

–

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

–





⋅

=

P

out

1

2

---L

1

1

2

---I

p

V

out

dI

DD

V

⋅

out

–

⋅⋅

+

⋅⋅

=

dt

on

F

s

⋅

=

t

V

out

–

()

L

1

I

p

⋅

=

P

ou t

1

2

---L

1

1

V

out

V

in

V

out

–

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

+





I

DD

V

⋅

ou t

–

⋅⋅

⋅

=

L

1

2

P

out

I

DD

V

⋅

out

+

1

V

out

V

in

V

out

–

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

+





⋅⋅

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

⋅

=