Page 94 - Power Electronics Handbook

Smartpower 87

Usually these small transformers have a primary inductance less than about
2mH and a maximum voltage-time product of about 10 V ps. To get a
good span ratio requires a larger core size, and to resolve adjacent pulses
the positive and negative halves of the waveform must have equal
voltage-time products. This usually places a requirement for a high
backswing, although it is usual to limit this in circuit applications, as it adds
to the rating of semiconductors which are driving the transformer. The
primary inductance will be high if a large stepup ratio is used in the
transformer and this should be avoided if a high bandwidth and span ratio
are required. Therefore, isolating transformers used for driving power
circuits usually have a 1:l ratio.
Figure 3.12 shows examples of circuits using isolating transformers to
control power semiconductors. In all instances the drive current is derived
from a separate power supply since the main supply cannot be used, as was
done for optical isolators illustrated in Figures 3.10(b) and 3.1qc). Two
seriesconnected thyristors are controlled in Figure 3.12(a), the resistors
and capacitors across the gates of the power devices preventing spurious
triggering, and the capacitor also improving their dv/dt rating. The diode
and zener diode, D1 and D2 respectively, present a negative voltage across
the primary during its off period, so enabling the transformer to recover
quickly, increasing the pulse-repetitive frequency. However, this also applies
a negative voltage across the secondary, which is prevented from reaching
the gates of the thyristors by the series-connected diodes D, and D,. This
voltage does appear at the collector of the drive transistor TR, and increases
its rating.
The circuit of Figure 3.12(b) shows how a simple arrangement can be
used to obtain a two-step output waveform, of the type shown in Figure
3.1. Capacitor C1 initially provides a low-impedance path to enable a high
output pulse, which reduces in amplitude once the capacitor charges and
resistor R1 limits the output voltage.
Optical couplers are better suited than transformers in applications
requiring continuous drive, such as for transistors, since they do not
saturate. However, transformers can be used by operating them in an
oscillatory circuit, the output being rectified to give d.c. drive. Such a
circuit is shown in Figure 3.12(c), where the two transistors form a
push-pull oscillator, and the output is rectified by diodes D1 and D2 before
being applied to the base of the power transistor TR3.

3.5 Smart power

As mentioned earlier, it is common practice to include control circuitry
within the same package as the power devices, and this combination is often
referred to as smart power components. The amount of integration can vary
and may be in various forms. The very basic combination may consist of the
power devices with protection components, such as diodes and temperature
sensors. However the term smart power usually refers to devices with a much
greater level of integration, such as logic circuitry and memory (Clarke,
1991; Murari, B. et al., 1995; Rubenstein, 1996).