AN1889

Current transformer core selection

21/33

Once fixed both core material and size, the turn ratio must be adjusted to get the

Equation 31

From a bench verification it is convenient to choose:

Equation 32

Considering the short length of wires at both primary and secondary side, the exact

calculation of their section is not of primary importance, while considerations about their

insulation are, since the voltage at primary side during turn-on can overstep 1500 V.

For the demo board, wires with the following section have been used:

At primary side it is suggested to use an insulated wire capable to sustain 3500 V.

Once defined the current transformer we still need to determine the zener diode, the

capacitor C and the resistor RB. As already mentioned in chapter 8, the turn-on

can be given by:

Equation 33

current after the peak, and at the same time, it generates negligible power dissipation.

Being the minimum ON time 1.4 ms, tpeak should be less than 0.7 µs. In this particular case

Equation 34

For the diode D in Figure 9 the BA159 has been selected.

N

eff

I

P

I

B

-----

I

Cmax

I

Mmax

–

I

c

5

----

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

0.9

0.27

–

0.18

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

3.5

=

=

==

N

2T

N

1T

----------

∆

=

t

peak

3R

b

•

C

b

•

=

V

Z

2I

1

+

()

•

R

•

3.12V

3.3V

⇒

=

=