AN1889

Application description

11/33

Equation 10

2.2

Fly-back transformer design

Once defined the turn ratio, the needed primary inductance and the peak current, to

complete the design of the transformer we still need to determine the magnetic core

material, its geometry, and the exact number of primary turns. The choice of correct size

and material of transformer is often an iterative process that may require several try and

error steps before finding the optimal choice. Standard soft ferrite with gaped core and E-

type geometry is a common choice for fly-back operation. The calculation of the product of

the areas AP (cross sectional active area of the core multiplied by window area available for

Equation 11

Where

∆T is the maximum temperature variation with respect to the ambient temperature,

KU is the utilization factor of the window (say the portion of the window used for winding that

By the way all ferrite manufacturers report tables with the suggested core type and size for

given output power and frequency.

For our project the type ED2924-PC40 ferrite material from TDK has been chosen. Next

The core must not saturate even at high temperature and in overload conditions (like start-

up or secondary short circuit), the level of this current in the present project can reach 1.6 A.

following values for the selected ED2924-PC40 core:

●

●

calculated.

In fact, being the primary inductance:

Equation 12

At this point we can also calculate the auxiliary winding needed to supply the driver. In our

case the driver used is the UC3842 that from its specification can be driven with 15 V. By

I

ms primary

()

I

P

3

-------

T

onmax

T

S

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

0.38A

=

=

A

P

10

()

∆T

1

2

---

K

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

1.316

]

[

=

L

P

N

2

A

L

N

P

150

=

⇒

=