19/24

AN1673 APPLICATION NOTE

Concerning the stray inductance, the models used in the standard forward application's transformers are

not precise (mainly because the mechanical position). The approximate value can be foreseen using the

formula:

[20]

In our case the calculated Lleak is 270

µH.

Where: lw = 5.6cm; bw = 0.5cm; hw = 1.55cm.

In the real transformer of the present example, the stray inductance (tested at the primary when one of the

secondary sections is shorted) is about 240

µH.

As already told, introducing an air gap in the ferrite set can control the ratio between the primary and the

stray inductance. Chose the air gap 0.33mm to get the ratio Lm/Ls=3.5,

Resonant capacitor (Cres).

For the resonant capacitor selection, it must be consider its rated current; in fact the current can be an

important limit, in particular for low capacitor values. In the polypropylene series capacitor it is usually pos-

sible to find the suitable one. On the other side the value of the Cres, for a given resonance's value (see

fig.21), profiles the sloop of the resonant curve (Q factor that is function of

), thus changing

the impedance change versus the frequency variations.

In our design it has been chosen the 22nF/630V PHE- (100°C).

Figure 21. Q factor.

Powermos.

Concerning the Powermos to be used in the half bridge, the required class of blocking voltage (Bvdss) is

500V.

Choose STP4NB50 or the equivalent STP4NK50)

The relations and parameters that can be used for calculation:

[21]

[22]

L

leak

1.3 lw Np

2

⋅⋅

bw

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

hw

3

-------- 10

8

–

⋅⋅

≈

Lres/Cres

0.01

0.1

1

D03IN1447

Aa(

ω(w))

Ab(

ω(w))

R

in

8V

o

⋅

π

2

I

o

⋅

--------------- n

2

⋅

818V

⋅

3.14 3.14 4A

⋅⋅

---------------------------------------- 144

⋅

525

==

=

I

Qpk

I

o

π

⋅

2n

⋅

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

4

π

⋅

24

-----------

0.525

≥≥

≥