Thyristors are what? Principle of operation and characteristics of thyristors

Thyristors are power electronic keys that are not completely controlled. Often in technical books you can see one more name of this device - a single-operation thyristor. In other words, under the influence of a control signal it is translated into one state - a conducting state. If to specify, it includes a chain. To turn it off, it is necessary to create special conditions that ensure a drop in the forward current in the circuit to zero.

Features thyristors

Thyristor switches conduct electricity only in the forward direction, and in the closed state it can withstand not only the direct voltage, but also the reverse voltage. The structure of the thyristor is four-layer, there are three conclusions:

1. Anode (indicated by the letter A).
2. Cathode (letter C or K).
3. Control electrode (Y or G).

Thyristors have a whole family of volt-ampere characteristics, they can be used to judge the state of an element. Thyristors are very powerful electronic switches, they are capable of switching circuits in which the voltage can reach 5000 volts, and the current strength is 5000 amperes (the frequency does not exceed 1000 Hz).

Thyristor operation in DC circuits

A conventional thyristor is turned on by applying a current pulse to the control terminal. Moreover, it must be positive (with respect to the cathode). The duration of the transient process depends on the nature of the load (inductive, active), the amplitude and the rate of growth in the control circuit of the current pulse, the temperature of the semiconductor crystal, and also the applied current and voltage on the thyristors available in the circuit. The characteristics of the circuit depend directly on the type of semiconductor element used.

In the circuit in which the thyristor is located, the occurrence of a high rate of voltage rise is unacceptable. Namely, the value at which the element switches on spontaneously (even if there is no signal in the control circuit). But at the same time, the control signal should have a very high slope of the characteristic.

Ways to turn off

There are two types of commutation of thyristors:

1. Natural.
2. Forced.

And now more about each kind. The natural occurs when the thyristor operates in an alternating current circuit. And this switching occurs when the current drops to zero. But you can implement forced switching in many different ways. What kind of thyristor control to choose, to solve the scheme designer, but it is worth talking about each type separately.

The most characteristic way of forced switching is to connect a capacitor that was pre-charged with a button (key). The LC circuit is included in the thyristor control circuit. This chain also contains a fully charged capacitor. During the transient process, current fluctuations occur in the load circuit.

Methods of forced switching

There are several more types of forced switching. Often a circuit is used in which a switching capacitor having a reverse polarity is used. For example, this capacitor can be connected to the circuit by means of some auxiliary thyristor. In this case, a discharge to the main (working) thyristor occurs. This will lead to the fact that the capacitor current, directed against the direct current of the main thyristor, will help reduce the current in the circuit down to zero. Consequently, the thyristor will shutdown. This happens for the reason that the thyristor device has its own characteristics, characteristic only for it.

There are also circuits in which LC chains are connected. They are discharged (and with fluctuations). At the very beginning, the discharge current flows towards the worker, and after equalization of their values, the thyristor turns off. After that, the current flows from the oscillating circuit through the thyristor to the semiconductor diode. At the same time, as long as the current flows, a voltage is applied to the thyristor. It is modulo equal to the voltage drop across the diode.

Thyristor operation in AC circuits

If the thyristor is included in the AC circuit, the following operations can be performed:

1. Enable or disable the electrical circuit with active-resistive or active load.
2. Change the average and the actual value of the current that passes through the load, thanks to the ability to adjust the timing of the control signal.

Thyristor keys have one feature - they conduct current in only one direction. Consequently, if it is necessary to use them in alternating-current circuits, it is necessary to apply counter-parallel inclusion. Actual and average current values can vary due to the fact that the timing of the signal to the thyristors is different. At the same time, the power of the thyristor must meet the minimum requirements.

Phase Control Method

With a phase control method with forced-type commutation, the load is adjusted by changing the angles between the phases. Artificial commutation can be done with the help of special circuits, or it is necessary to use fully controlled (locked) thyristors. On their basis, as a rule, a charger is built on a thyristor, which allows you to adjust the current depending on the level of charging of the battery.

Pulse width control

It is also called its PWM modulation. During the opening of the thyristors, a control signal is applied. Transitions are open, and there is some voltage on the load. During closing (during the entire transient process), no control signal is given, hence, the thyristors do not conduct a current. When the phase control is performed, the current curve is not sinusoidal, the voltage waveform changes. Consequently, there is also a malfunction of consumers, which are sensitive to high-frequency interference (incompatibility occurs). A simple design has a regulator on the thyristor, which without problems will allow to change the necessary value. And you do not need to use massive LATRs.

Thyristors, lockable

Thyristors are very powerful electronic switches used for switching high voltages and currents. But they have one huge drawback - management is incomplete. And more specifically, this is manifested by the fact that to turn off the thyristor, it is necessary to create conditions under which the forward current will decrease to zero.

It is this feature that imposes some restrictions on the use of thyristors, and also complicates the circuits based on them. In order to get rid of these drawbacks, special thyristor designs have been developed, which are locked by a signal from one control electrode. They are called two-operative, or lockable, thyristors.

Locked Thyristor Design

The four-layer structure of p-p-p-p y thyristors has its own peculiarities. They make them different from conventional thyristors. It is now about the complete controllability of the element. Volt-ampere characteristic (static) in the forward direction is the same as for simple thyristors. That's just a direct current thyristor can pass much more in value. But there are no functions for blocking large reverse voltages for locked thyristors. Therefore, it is necessary to connect it counter-parallel with the semiconductor diode.

A characteristic feature of a locked thyristor is a significant drop in direct voltages. To make a trip, it is necessary to apply a powerful current pulse (negative, in the ratio 1: 5 to the direct current value) to the control terminal. But only the pulse duration should be as small as possible - 10 ... 100 μs. Locked thyristors have a lower value of the limiting voltage and current than the conventional ones. The difference is about 25-30%.

Types of thyristors

Above we considered locked, but there are still many types of semiconductor thyristors, which are also worth mentioning. In a variety of designs (chargers, switches, power controllers), certain types of thyristors are used. Somewhere it is required that control is carried out by supplying a stream of light, hence, an optiostyristor is used. Its peculiarity lies in the fact that a semiconductor crystal sensitive to light is used in the control circuit. The parameters of thyristors are different, all have their own characteristics, characteristic only for them. Therefore, it is necessary at least in general terms to imagine what kinds of these semiconductors exist and where they can be applied. So, here's the entire list and the main features of each type:

1. Diode-thyristor. The equivalent of this element is a thyristor, to which is connected a counter-parallel semiconductor diode.
2. Dinistor (diode thyristor). It can go into a state of total conductivity if a certain voltage level is exceeded.
3. Triac (symmetric thyristor). Its equivalent is two thyristors connected in the opposite direction.
4. Thyristor inverter high-speed differs high speed of commutation (5 ... 50 μs).
5. Thyristors with FET control . It is often possible to meet designs based on MOSFETs.
6. Optical thyristors, which are controlled by light streams.

Implementing element protection

Thyristors are devices that are critical to the rates of direct current and direct voltage rise. For them, as well as for semiconductor diodes, such phenomenon as the flow of reverse recovery currents is characteristic, which very quickly and sharply drops to zero value, aggravating the probability of overvoltage. This overvoltage is a consequence of the fact that the current in all circuit elements that have an inductance (even the very small inductors characteristic for mounting - wires, paths of the board) abruptly ceases to exist. To implement the protection, it is necessary to use a variety of circuits, which, in dynamic modes of operation, can be protected from high voltages and currents.

Typically, the inductive resistance of the voltage source that enters the circuit of the operating thyristor has such a value that it is more than sufficient to not further include some additional inductance in the circuit. For this reason, in practice, a chain of shaping the switching path is often used, which significantly reduces the speed and level of overvoltage in the circuit when the thyristor is disconnected. Capacitive-resistive chains are most often used for these purposes. They are connected in parallel with the thyristor. There are quite a few types of circuit modification of such circuits, as well as the methods of their calculation, parameters for the operation of thyristors in different modes and conditions. But the chain of forming the switching path of the locked thyristor will be the same as for the transistors.