An Introduction to Thyristors and Their Applications

There are many industrial applications for which control of power input and/or output is required. Examples of such applications are variable speed drives, illumination controllers, and temperature regulators. The initial developments in power-control schemes were based on variable tap-changing transformers and series and shunt regulators (consisting of resistors or reactors) which produce a change in the applied voltage, and thereby vary the power. These schemes either were found to be inefficient or involved costly equipment. Further, the power control was usually in steps. Such drawbacks have since been partially overcome by saturable-core reactors. The advent of magnetic amplifiers paved the way for complete static control of power without moving parts. During the period before World War II, tremendous improvements were made in the performance and design of magnetic power controllers, and the evolution of square-loop materials improved their range of amplification. Magnetic control of power is still adopted for many military and industrial applications where reliability and sturdiness are of prime importance. The major limitation of this method is the bulkiness of the controller which prohibits its use in certain applications. Also, the core-magnetising current and iron losses result in low input power factor and low efficiency.

The basic properties and applications of thyristors have been explained in Chapter 1. Since the SCR is the most widely-used member of the family of thyristors, a more detailed analysis of its operation and characteristics is given here. The other devices will be discussed in Chapter 4.

Silicon-controlled rectifiers are now available with a voltage rating upto 10 kV and a current rating upto 1200 A. In many power control applications, the required voltage and current ratings are lower than these maximum limits. Therefore, even though it may be possible to obtain a single SCR of proper voltage and current ratings, on many occasions the designer is forced to use lower-rated SCRs for reasons of economy and availability. In such a situation, lower-rated SCRs hive to be connected in series and parallel combinations to suit the voltage and current requirements of the circuit for a particular application. Series and parallel coinbinations are also often used when it is required to control power in low-voltage high-current circuits or high-voltage low-current circuits because an SCR of suitable voltage and current ratings may not be available.

As already mentioned in Chapter 1 the term ‘thyristor’ is used for the family of semiconductor devices that exhibit bistable characteristics and can be switched on and off like the gas-tube thyratron. The SCR, which has been discussed in detail in Chapter 2, is a member of this family, and is widely used because of its large power-handling capacity. The operating characteristics of some of the other important thyristors are described in this chapter. All, except the triac, are low-power devices often used for switching in control circuits. They also find applications as efficient triggering devices for SCRs and triacs, and in digital and pulse circuits.

Because of the bistable characteristics of semiconductor devices, whereby they can be switched on and off, and the efficiency of gate control to trigger such devices, thyristors have been found to be ideally suited for many industrial applications. As already pointed out in Sections 1.2 and 1.3, their compactness, reliability, low losses, and fast turn-on and turn-off times have given them specific advantages over saturable-core reactors and gas tubes. Out of the several members of the thyristor family described in Chapters 1 and 4, the SCR and the triac have high ratings for voltage and current, and are widely used for power control. The other thyristors are used for low-power applications, and as switching devices in control and digital circuits.

In Chapter 2, we considered various methods for triggering SCRs. Of these, the most efficient method for power modulation with thyristors uses gate control. In AC circuits, the SCR can be turned on by the gate at any angle α with respect to the applied voltage. This angle α is the firing angle. Power control is obtained by varying the firing angle, and this is known as phase control.

The method by which AC supply voltage is used for the commutation of conducting SCRs has been discussed in Chapter 6; it is very convenient for both rectification and inversion in phase-controlled bridge circuits and midpoint-connection circuits. This method of commutation is referred to as class F type of commutation (see Chapter 8), and is also known as line commutation. All types of AC to variable voltage DC converters used for motor control and regulated power supplies, in both of which the AC input current is made continuous by a large reactor on the DC side, make use of line commutation without any external commutating components. The input-output characteristics of such line-commutated converters will be considered in detail in this chapter. The effect of source impedance of various types (R, L, or C) on the output voltage, and the minimum value of inductance required on the DC side to provide continuous load current for both active and passive loads will also be discussed.

As explained in Chapter 2, the SCR is turned off when its forward current is reduced below the level of the holding current. Forward voltage can be reapplied after allowing a certain time for the excess carriers in the outer and inner layers of the SCR to decay. The decay and recombination of these excess carriers may be accelerated by applying a reverse voltage across the SCR.

In the on-off method of power control, voltage is applied to the load for a specified period called on-time (T_on), and the load is open-circuited or the applied voltage is removed for a duration known as off-time (T_off). The load power can be controlled by varying the on- and off-time. The ratio T_on to (T_on + T_off) is known as the duty cycle. This method can be applied to both AC and DC circuits. In AC circuits with direct on-off control, the load current is alternating, and therefore a triac can be very conveniently used.