AN2623

Design circuit

9/25

Equation 12

and for the free wheeling diode:

Equation 13

Equation 14

In Equation 12, 13, and 14 the currents are calculated in Full Load condition considering

the worst case for each diode and can be used to calculate the maximum power dissipation

for each diode.

To reduce the number of components, the size of the board, and to minimize power losses,

the ST double fast recovery rectifier BYT16P-400 was selected.

Although the two diodes inside the same package are always working complementarily, in

order to choose the heat sink, the total power losses in the worst case can be calculated as

if, instead of two diodes there is only one that flows through the whole current of the

inductor:

Equation 15

the total thermal resistance of the diode:

Equation 16

3.4

Power transformer design and MOSFET choice

Ideally in a forward converter, the energy flows forward from the primary side to the

secondary side without any storage in the transformer. But the real transformer does not

have infinite magnetizing inductance, so during the on-time of the power MOSFET some

energy is stored in the magnetic core. The proper magnetic core and the primary winding

turn number have to be selected in order to avoid core saturation. The proper magnetic core

and primary winding turn number must be selected taking into account that some energy is

also dissipated in the magnetic core.

I

RMSdiodeR

I

out

D

max

1

1

12

------

∆I

out

I

out

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

⎝⎠

⎛⎞

2

⋅

+

⎝⎠

⎛⎞

I

RMSdiodeR

3.2 A

=

⇒

⋅⋅

=

I

AVGdiodeR

I

out

D

max

2.25 A

=

⋅

=

D

min

D

max

V

dcmin

V

dcmax

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

11.5

I

RMSdiodeF

I

out

1D

min

–

()

1

1

12

------

∆I

out

I

out

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

⎝⎠

⎛⎞

2

⋅

+

⎝⎠

⎛⎞ 4.23A

=

⋅⋅

=

⇒

=

⋅

=

%

I

AVGdiodeF

I

out

1D

min

–

() 3.825 A

=

⋅

=

P

totDiode

V

t

I

out

R

+

d

I

2

out

1

1

12

------

I

out

∆

I

out

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

⎝⎠

⎛⎞

2

⋅

+

⎝⎠

⎛⎞ 5.53 W

=

⋅⋅

⋅

=

R

thmax

T

jmax

T

AmbMax

–

P

lossesR

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

15

° C

=

=

/W