Application Note 1612

2

AN1612.1

November 28, 2011

Transformer Core Selection

From Figure 2, the RMS current in the transformer primary side

can be calculated from:

The RMS current in each transformer secondary side can also be

computed from:

The transformer used in this design is Pulse’s PA3374Nl. It is a

gapped ferrite toroid core, which has the following parameters:

This section provides general guideline to calculate the number

of turn and wire size. For more details on designing transformer

parameters, please contact a Pulse representative.

The number of turns on the primary side, Np, can be determined

from:

Therefore, the primary side has 26 turns. With the turn ratio of 1,

the secondary side and the auxiliary primary side also have 26

turns.

Next the calculate the maximum flux density to make sure that it

is below the saturation limit.Where:

For the operating power level, the wire sizes of the primary,

secondary, and auxiliary windings are selected such that the

circular mil/A).

To simplify transformer winding, AWG#37 is used for all primary,

secondary and auxiliary windings.

Primary MOSFET Selection

The primary MOSFET needs to be able to handle the voltage

stress, given by:

As a good design practice, some margin is provided to this peak

stress voltage to accommodate transient spikes and for a good

reliable performance over time. Providing a 30% design margin

as a rule of thumb, the minimum rating on the primary MOSFET

needs to be 54.6V.

FIGURE 2. TYPICAL OPERATIONAL CURRENT WAVEFORMS

MOSFET ON

DIODES (D1 AND

D2) OFF

MOSFET OFF

DIODES (D1 AND

D2) ON

MOSFET OFF

DIODES (D1

AND D2) OFF

I

rms pri

,

I

pk

d

3

---

⋅

=

(EQ. 3)

1.06

0.35

3

-----------

0.362A

=

⋅

=

I

rms sec

,

I

pk

2

-------

d

2

3

------

⋅

=

(EQ. 4)

1.06

2

-----------

0.5

3

--------

0.216A

=

⋅⋅

=

N

p

LuH

[] 1000

×

A

L

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

=

(EQ. 5)

23.8

1000

×

35

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

26.07

=

=

B

max

L

,

⋅

N

⋅

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

=

(EQ. 6)

23.8

6

–

×10

1.06

⋅

26 4.3

2

–

×10

⋅

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

4

×10

0.226T

=

=

A

wpri

,

0.25335 0.362

0.0917cm

2

=

⋅

≥

(EQ. 7)

V

DSFET

V

=

IN MAX

,

nV

out

V

f

+

()

×

[]

+

(EQ. 8)

26.4

1

15

0.6

+

()

×

[] 42V

=

+

=