11/23

AN1317 - APPLICATION NOTE

While the output voltage grows up, the discontinuous mode is reached and the average output current

becomes the half of the maximum peak current:

and

The average output current for

can be approximate as:

, with

So:

Finally:

3.5 Front Rectifier And Filter

As single wave rectification is chosen because it allows to have the output ground connected to one of

the mains lines, as it is required in most of the non isolated applications. As the involved power is low, the

input filtering can be achieved without huge bulk capacitor. Any usual rectification diode having a reverse

voltage of 800 V will fit the needs. On the schematic of fig. 1 and 5, we used the part number 1N4007.

The energy stored in the bulk capacitor during the conduction time of the diode must be equal to the total

power dissipated when the diode is blocked.

Referring to fig. 8 and with the efficiency of the converter

, it comes:

t0

I

ou tavg

I

Dlim

≈

⇒

=

tt

ss

I

out avg

1

2

---I

Dl im

≈

⇒

=

0t

t

ss

≤≤

I

outa vg

3

4

---I

Dli m

≈

t

ss

I

DDo

C

2

⋅

V

DDhys t

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

=

V

DDhys t

V

DDon

V

DDof f

–

=

C

2

I

DD0

t

ss

⋅

V

DDhyst

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

I

DD0

C

5

V

out

⋅

4

---

I

Dli m

⋅

()

⁄

V

DDhyst

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

>

=

C

2

I

DD0

4C

5

V

out

⋅⋅

3I

Dlim

V

DDh yst

⋅⋅

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

>

η

P

out

P

in

----------

=

1

2

---C

2

i npeak

V

2

in low

–

()

t

2

t

1

–

() P

out

1

η

---

⋅⋅

=