AN1889 STMicroelectronics, AN1889 Datasheet - Page 15

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

Where I
From a bench verification it is convenient to choose
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 1500V.
For the demo board, wires with the following section have been used:
d
At primary side it is suggested to use an insulated wire capable to sustain 3500V.
Once defined the current transformer we still need to determine the zener diode, the capacitor C and the
resistor R
initial base peak current and its duration t
A good choice for R
peak, and at the same time, it generates negligible power dissipation.
Being the minimum ON time 1.4 µ s, tpeak should be less than 0.7 µ s. In this particular case it has been
fixed at 400ns. Hence C
I
diode DZ that clamps the voltage across the small capacitor C
following formula:
For the diode D in figure 8a the BA159 has been selected.
10. PWM DRIVER
As already mentioned, we still need to correctly drive the gate of ESBT. A simple PWM driver can both
drive the gate of ESBT and supervise the fly-back operations requested by the converter. The common
UC3842 provides a cost effective current mode control.
As already mentioned in chapter 5, the UC3845 needs 15V for its correct biasing and this voltage
reference is supplied by the auxiliary output of the transformer. Neglecting for the moment the start-up
circuit, next step is to correctly design the network for the PWM driver terminals.
Calculation and considerations made on the following steps are related to the complete schematic
diagram of the realized demo board reported in figure 9.
1. Primary side regulation.
It can be achieved in a simple way just fixing a voltage divider. The advantages of primary side
regulation are essentially linked to its simplicity and low cost, by the way it cannot supply an accurate
control on output voltage at load variation. It is generally chosen for low power SMPS. The voltage
reference of the internal error amplifier is set in the UC3842 at 2.5V, consequently the values chosen for
the network resistors are:
peak
P
=0.5mm (Primary winding)
must be limited in order to avoid an extra saturation of the device. This action is made by the zener
Mmax
B
. As already mentioned in chapter 8, the turn-on performance of the ESBT is related to the
is the magnetization current related to I
b
is 0.56 Ω . This value allows us to eliminate the ringing on the base current after the
b
= 238nF (the nearest 220nF has been used).
R
12
=22k Ω
and
d
peak
S
=0.25mm (Secondary winding).
R
13
that can be given by:
=3.9k Ω
Cmax
N
N
2
1
T
T
=
= I
4
, and than
P
b
. The zener is designed according to the
AN1889 - APPLICATION NOTE
N
1
T
=
3
N
2
T
=
12
.
15/25