AN1889 datasheet(21/33 Pages) STMICROELECTRONICS

21 / 33 page

AN1889

Current transformer core selection

21/33

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

desiderated IC/IB ratio according to the below equation:

Equation 31

Where IMmax is the magnetization current related to ICmax = IP

From a bench verification it is convenient to choose:

Equation 32

and than N1T =3 and N2T =12.

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:

dP=0.5 mm (primary winding) and dS=0.25 mm (secondary winding).

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

performance of the ESBT is related to the initial base peak current and its duration tpeak that

can be given by:

Equation 33

A good choice for Rb is 0.56 Ω. This value allows us to eliminate the ringing on the base

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

it has been fixed at 400 ns. Hence Cb= 238 nF (the nearest 220 nF has been used).

Ipeak must be limited in order to avoid an extra saturation of the device. This action is made

by the zener diode DZ that clamps the voltage across the small capacitor Cb. The zener is

designed according to the following formula:

Equation 34

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

eff

-----

Cmax

Mmax

–

----

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

0.9

0.27

–

0.18

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

3.5

----------

∆

peak

•

peakRb

()

•

bCb

•

3.12V

3.3V

⇒

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