AN1889

Current transformer core selection

19/33

6

Current transformer core selection

The main challenge now is to choose the core material, its shape and dimension. A correct

design of this current transformer has to take into consideration some constraints that, being

in contrast each other, lead to a few iterative design steps. The needs to have magnetic

permeability as high as possible to minimize the effect of magnetization inductance, is in

contrast with the need of a negligible inductance with respect to that of primary transformer.

In fact, being the primary winding of the fly-back transformer in series with the primary

winding of the current transformer, the relevant inductance would be seen as an increase of

the leakage inductance in the converter. Moreover a higher magnetic permeability implies a

bigger core size (to avoid the core saturation) while the dimension of the current transformer

has to be as small as possible for both space and cost reasons.

Among all possible choices a ferrite ring with 12.5 mm diameter has been selected. Starting

from the preliminarily fixed turn ratio (N=0.2), we must determine the minimum primary turns

needed to avoid the core saturation. With reference to figure 7b, by applying the Faraday's

Equation 26

inductance for some common toroidal core have been calculated and summarized in the

table below.

For the consideration previously made, the relatively high primary inductance of T38 does

not make its selection as a suitable choice, furthermore between N67 and N30, this last one,

giving the best trade off, has been selected. To deeply explain this choice, we must observe

that the magnetization inductance cannot be neglected and it also strongly contribute in

determining the real transformer transfer rate. This can be explained looking at the figure

below where the proportional driving schematic and its equivalent circuit is reported.

Table 3.

Primary Turn number and primary inductance for some common toroidal

core

Material

[nH]

[mT]

[mT]

N67

1070

480

370

1.85

◊2

4.28

N30

2200

350

225

2.53

◊3

19.8

T38

5100

350

220

2.53

◊3

49.5

V

1

N

TP

d

ϕ

dt

------

•

N

TP

A

e

∆B

∆t

--------

N

TP

⇒

•

•

2

V

1

T

onmax

•

A

e

B

sat

•

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

•

=

≅

=