AN1621 APPLICATION NOTE

8/14

The primary peak current is:

A good condition is when Im 10% of Ipk; in this case Im is imposed lower than 6% of Ip.

2.8

Output inductor & auxiliary supply

To calculate the inductor value, the max. current ripple has been set at 2.6A, 20% of Iomax.

The max. current ripple occurs at max. input voltage; in this operating condition the min duty cycle is:

The inductor value is defined taking into account the current downslope which corresponds to the off-time of the

power switches:

Toff max. = T · (1-Dmin) = 5 · (1-0.22) = 3.9

µs

that yields:

To realise the inductor, a Magnetic's Kool Mm Core 77930 has been used. Since the inductor has to assure the

requested value at maximum load current, the turns number must be calculated taking into account the roll-off

of the initial permeability.

This results in 24 turns, that leads to 90

µH at zero load.

We can take advantage of this non-linear characteristic to keep a good output regulation and stability when

working at min. load current( around 0.5A for this case).From the output inductor it is drown also the energy

necessary to supply the L5991A controller by introducing a 9 turns auxiliary winding.

2.9

Input capacitor

The input filtering capacitor has to be dimensioned in order to deliver the requested max. output power, at min.

mains value, with a reasonable ripple voltage at 100Hz.

The design process has to consider also the rms current flowing into the capacitor ( a major reason of stress)

and the voltage rating.

The minimum capacitor value is defined by:

where:

Vcp is the peak value at minimum mains voltage, 248V

Vcm is the minimum value at minimum mains voltage, 200V

The chosen capacitors are 2 x 220

µF - 400V EYS 06.

At this point the new Vcm value is 215V.

The conduction time is:

I

pk

P

o

η V

⋅⋅

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

312

0.9 200 0.48

⋅⋅

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

3.22A

==

=

D

min

nV

o

V

loss

+

()

⋅

V

inmax

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

3.2

24

1.5

+

()

⋅

375

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

0.22

==

=

L

o

V

o

V

loss

+

() T

offmax

⋅

I

max

∆

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

24

1.5

+

() 3.9 10

6

–

⋅⋅

2.6

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

39

µH

==

=

C

P

o

η F

50

V

Cp

2

V

Cm

2

–

()

⋅⋅

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

312

0.9 50

248

2

200

2

–

()

⋅⋅

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

310

µF

==

=

t

c

cos

1

–

V

cm

V

cp

-----------







2

π F

50

⋅⋅

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

cos

1

–

215

248

----------





23.1450

⋅⋅

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

1.68 10

3

–

sec

⋅

==

=