Page 172 - Power Electronic Control in Electrical Systems

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160 Power semiconductor devices and converter hardware issues

be broadly classified as voltage controlled or current controlled. In the first case, and
in simple terms, a voltage signal between two terminals controls the on and off state,
whereas in the second case, the injection of current through the third terminal
provides such control.
Simplified and linearized voltage and current waveforms during the turn-on and
turn-off interval are shown in Figure 5.6. In reality, these waveforms are shaped with
snubber networks added in the power circuit to protect the main semiconductor
device and to reduce or minimize the switching losses. The overlap between the
voltage and current waveforms therefore is greatly dependent upon not only the
switching characteristics of the device itself but also on the way the power circuit is
designed and controlled.
For instance, there is a family of converters based on resonant concepts where the
voltage and current waveforms not only have the shape of sinusoidal signals as
opposed to linear waveforms shown in Figure 5.6 but also the overlap is minimal
and the respective switching losses quite low.
In the last fifteen years such resonant concepts have been extensively applied in
the converter technology and many ideas from the thyristor converters have been
used to control the shape of the switching waveforms and reduce the losses. This way
the switching frequency of the system can be increased with a number of benefits
attached to such improvement. The new family of converters known as soft-switching
converters or quasi-resonant converters with control techniques modified or based on
PWM concepts have been the focus of R&D (Divan, 1989; Divan, 1991; Divan et al.,
1989; Divan et al., 1993). There are already many products in this area in the market
mainly for adjustable speed motor drives and medium power converters for power
systems applications.
It is beyond the scope of this book to provide further information on such
technology. A review paper of the developments of this technology has been recently
written by Bellar et al., 1998.
Before presenting the main semiconductor devices, we will discuss the desired
characteristics of the power switches.
The `perfect' fully controlled power switch would have the following characteristics:
1. High forward and reverse voltage blocking ratings. In order to achieve higher power
ratings for a given converter, many switches are connected in series to build a

Fig. 5.6 Linear switching voltage and current waveforms for a semiconductor switch.