AN1889 STMicroelectronics, AN1889 Datasheet - Page 14

• Manufacturer Part Number: AN1889
• Description: ESBT STC03DE170 IN 3-PHASE AUXILIARY POWER SUPPLY
• Manufacturer: STMicroelectronics

Figure 8: ESBT Current Transformer Driver and Equivalent Circuit
In figure 8b 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 8a 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:
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:
The magnetization current for the two cores under analysis will be:
Being the I
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.
Once fixed both core material and size, the turn ratio must be adjusted to get the desiderated I
according to the below equation:
14/25
Pmax
R
B
=0.9A in steady state, the N67 core with 2 turns at primary side must obviously be
Fig. 8a
C
D
B
For N30:
For N67:
I
M
max
=
V
1
T
L
I
V
ON
C
TP
1
max
=
0
5 .
X
I
M
M
. 4
⋅
10
28
Fig. 8b
.
I
66
X
P
P
=
. 1
24
V
S
C
/I
B
ratio