AN1889 datasheet(20/33 Pages) STMICROELECTRONICS

20 / 33 page

Current transformer core selection

AN1889

20/33

In Figure 10 the driving circuit as been modeled with the equivalent schematic diagram of

the transformer with its secondary closed with a voltage generator. In fact, looking at

Figure 9 we must note that during on time, the base of ESBT can be seen as a forward

biased diode, to this we have to add the voltage drop on both diode D and resistor RB in

series with the base of the ESBT to complete the secondary side load model. With these

premises in a first order of analysis, the secondary side can be simply modeled as a voltage

source given by:

Equation 27

At primary side both primary inductance and magnetization inductance have been reported.

Considering that only the IP fraction of the total collector current will be transferred to the

secondary, the magnetization current has to be firstly as low as possible, secondly, its value

must been taken into account for the proper calculation of transformer turn ratio. The

magnetization voltage drop (that is the voltage at the primary of the current transformer) can

be now easily calculate:

Equation 28

The magnetization current for the two cores under analysis will be:

Equation 29

and

Equation 30

Being the IPmax=0.9 A in steady state, the N67 core with 2 turns at primary side must

obviously be excluded, unless increasing the number of turns, but this would lead to fill all

the available window area and consequently to the necessity of choosing a bigger core size.

Figure 9.

ESBT current transformer

driver

Figure 10.

ESBT equivalent circuit

BEon

2.5V

≅

----------

•

2.5

---

0.5V

•

Mmax

1TONmax

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

0.5

10.66

•

19.8

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

0.27

For N30

Mmax

1TONmax

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

0.5

10.66

•

4.28

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

1.24

For N67

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