Desired characteristics of fully-controlled power semiconductors

In switch-mode solid-state converters, the fully controlled power semiconductors can be turned on and off with control signals applied to a third terminal and can 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.

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

The 'perfect' fully controlled power switch would have the following characteristics:

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 valve especially for high and ultra-high power applications. lf new devices become available with higher voltage ratings, the number of the required switches connected in series to produce the same valve will be reduced. This will minimize the problems with the voltage sharing of the various switches in series, will increase the reliability of the overall system and will minimize the problems with their protection.
High current during conduction state. At the moment, when the current ratings of a given converter must be met, a number of switches are connected in parallel. If a device is available with high current ratings, the need for parallel connection as well as the problem of current sharing can be eliminated.
Low off-state leakage current. In most cases such a requirement is not significant as the already available switches exhibit almost negligible off-state leakage current.
Low on-state voltage drop across the switch. Even a relatively low voltage drop of a few volts across the device at significant current flowing through the device can result in high conduction losses. It is therefore important that such an on-state voltage drop is as low as possible. This becomes more important when a number of switches are connected in series to increase the power handling capability of the converter, as the load current flows through a number of switches generating high conduction losses.
Low turn-on and turn-off losses. The ability to switch from on to off state and vice versa with minimum overlap between the current and voltage waveforms means that the switching turn-on and off losses are low. When such characteristics are combined with the low conduction losses, cooling requirements and other auxiliary components may be reduced or even eliminated in certain applications making the converter simpler, smaller, more efficient and simply less expensive.
Controlled switching characteristics during turn-on and turn-off. This means that over current control becomes simpler and easier, the stresses on the device and other parts of the converter such as load, transformers, etc. can be reduced along with EMI generation, the need for filters and snubber circuits.
Capability to handle its rated voltage and current at the same time without the need for derating. This will mean snubberless design, i.e. the required extra snubber components (resistor-inductor-capacitor-diode) to protect the switch and shape its switching waveforms, can be eliminated. Therefore, if the design does not require all these components, a simpler configuration, more efficient and more reliable will result.
High dv/dt and di/dt ratings. This will eliminate or reduce the size of the snubber circuits. Of course EMI generation will limit how fast the current and voltage waveforms can change but it is desirable that the switch has large dv/dt and di/dt ratings to eliminate the previously mentioned snubber circuitry.
Ability to operate in high temperatures. This will also eliminate the cooling requirements and simplify the converter's structure.
Short-circuit fault behavior. This will mean that the converter will still be able to operate when a number of switches are connected in series allowing designs that have redundancy factors especially in high and ultra-high power applications.
Light triggering and low power requirements to control the switch. This will allow fibre optics to be used to control the switch. In most cases the power to drive the switch is taken from the power circuit itself and the low power requirements will minimize the losses of the system.

Having discussed the desired characteristics, we will consider the various fully controlled power semiconductors and their realistic characteristics later.

Gate-turn-off thyristor

Details about Gate-turn-off thyristor are discussed here- GTO

Metal-oxide-semiconductor field effect transistor

Details about Metal-oxide-semiconductor field effect transistor are discussed here- Power MOSFETs

Insulated-gate bipolar transistor

Details about Insulated-gate bipolar transistor are discussed here- Insulated Gate Bipolar Transistor (IGBT)

MOS-controlled thyristor

Details about MOS-controlled thyristor are discussed here- MOS controlled Thyristors (MCTs)

Other semiconductor devices

There are also many other devices available in the market as a product or at R&D level. Many different names are given to them such as integrated gate-commutated thyristor (IGCT or GCT), emitter turn-off thyristor (ETO) and others. All of them are more or less hybrid versions of the existing devices and effort is spent to make them with higher ratings, better switching characteristics and with reduced conduction and switching losses.

Semiconductor switching-power performance

The power frequency range of the various semiconductors discussed in the previous sections are summarized in Figure 5.12. It is clear that the thyristor dominates the ultra-high power region for relatively low frequencies. The GTO is the next device when it comes to power handling capabilities extending to frequencies of a few hundred Hz. The IGBT occupies the area of medium power with the ability to operate at relatively higher frequencies, and finally the MOSFET extends its operation to high frequency regions for relatively low power levels. The tendency over the next few years is to have the GTO extend its power area towards the thyristor level. At the same time, the IGBT will also extend its power ability towards the GTO with higher switching frequency.