AN1889 Datasheet, PDF (21/33 Pages) STMicroelectronics – The need to choose a high value of the fly-back voltage

AN1889
Current transformer core selection
Once fixed both core material and size, the turn ratio must be adjusted to get the
desiderated I_C/I_B ratio according to the below equation:
Equation 31
N_eff
=
I_--P--
I_B
=
I_{--C----m----a---x----–-----}I_{--M----m----a---x-}
I_--c-
=
0_----._-9_{-----–-----}0_---._--2_---7_--
0.18
=
3.5
5
Where I_Mmax is the magnetization current related to I_Cmax = I_P
From a bench verification it is convenient to choose:
Equation 32
N_-----2---T-
N_1T
=
∆
and than N_1T =3 and N_2T =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:
d_P=0.5 mm (primary winding) and d_S=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 t_peak that
can be given by:
Equation 33
t_peak = 3 • R_b • C_b
A good choice for R_b 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 C_b= 238 nF (the nearest 220 nF has been used).
I_peak 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 C_b. The zener is
designed according to the following formula:
Equation 34
V_Z = 2 • (I_peakR_b + 1) • R_bC_b= 3.12V ⇒3.3V
For the diode D in Figure 9 the BA159 has been selected.
21/33