Thyristors are power electronic keys that are not fully controlled. Often in technical books you can see another name for this device - a single-operation thyristor. In other words, under the influence of a control signal, it is transferred to one state - conducting. More specifically, it includes a circuit. To turn it off, it is necessary to create special conditions that ensure that the direct current in the circuit drops to zero.
Features of thyristors
Thyristor keys conduct electric current only in the forward direction, and in the closed state it can withstand not only forward, but also reverse voltage. The structure of the thyristor is four-layer, there are three outputs:
- Anode (denoted by letter A).
- Cathode (letter C or K).
- Control electrode (U or G).
Thyristors have a whole family of current-voltage characteristics, they can be used to judge the state of the element. Thyristors are very powerful electronic keys, they are capable of switching circuits in which the voltage can reach 5000 volts and the current strength - 5000 amperes (while the frequency does not exceed 1000 Hz).
Thyristor operation inDC circuits
A conventional thyristor is turned on by applying a current pulse to the control output. 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 rate of rise in the current pulse control circuit, the temperature of the semiconductor crystal, as well as the applied current and voltage to the thyristors available in the circuit. The characteristics of the circuit directly depend 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, such a value at which the element spontaneously turns on (even if there is no signal in the control circuit). But at the same time, the control signal must have a very high slope.
Ways to turn off
Two types of thyristor switching can be distinguished:
- Natural.
- Forced.
And now in more detail about each species. Natural occurs when the thyristor operates in an alternating current circuit. Moreover, this switching occurs when the current drops to zero. But to implement forced switching can be a large number of different ways. Which thyristor control to choose is up to the circuit designer, but it is worth talking about each type separately.
The most characteristic way of forced switching is to connecta capacitor that was pre-charged using a button (key). The LC circuit is included in the thyristor control circuit. This circuit contains a fully charged capacitor. During the transient process, the current fluctuates in the load circuit.
Methods of forced switching
There are several other types of forced switching. Often a circuit is used that uses a switching capacitor with reverse polarity. For example, this capacitor can be connected to the circuit using some kind of auxiliary thyristor. In this case, a discharge will occur on the main (working) thyristor. This will lead to the fact that at the capacitor, the current directed towards the direct current of the main thyristor will help to reduce the current in the circuit down to zero. Therefore, the thyristor will turn off. This happens for the reason that the thyristor device has its own characteristics that are characteristic only for it.
There are also schemes 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 equalizing their values, the thyristor is turned off. After that, from the oscillatory chain, the current flows through the thyristor into a semiconductor diode. In this case, while current flows, a certain 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, it is possible to carry out suchoperations:
- Turn on or off an electrical circuit with an active-resistive or resistive load.
- Change the average and effective value of the current that passes through the load, thanks to the ability to adjust the moment of the control signal.
Thyristor keys have one feature - they conduct current in only one direction. Therefore, if you need to use them in AC circuits, you have to use back-to-back connection. The effective and average current values may change due to the fact that the moment the signal is applied to the thyristors is different. In this case, the thyristor power must meet the minimum requirements.
Phase control method
In the forced-type phase control method, the load is adjusted by changing the angles between the phases. Artificial switching can be carried out using special circuits, or it is necessary to use fully controlled (lockable) thyristors. On their basis, as a rule, a thyristor charger is made, which allows you to adjust the current strength depending on the level of charge of the battery.
Pulse width control
They also call it PWM modulation. During the opening of the thyristors, a control signal is given. The junctions are open and there is some voltage across the load. During closing (during the entire transient process) no control signal is applied, therefore, the thyristors do not conduct current. When implementingphase control current curve is not sinusoidal, there is a change in the waveform of the supply voltage. Consequently, there is also a violation of the work of consumers that are sensitive to high-frequency interference (incompatibility appears). A thyristor regulator has a simple design, which will allow you to change the required value without any problems. And you don't need to use massive LATRs.
Thyristors lockable
Thyristors are very powerful electronic switches used to switch high voltages and currents. But they have one huge drawback - management is incomplete. More specifically, this is manifested by the fact that in order 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 circuits based on them. To get rid of such shortcomings, special designs of thyristors were developed, which are locked by a signal along one control electrode. They are called dual-operation, or lockable, thyristors.
Lockable thyristor design
The four-layer p-p-p-p structure of thyristors has its own characteristics. They make them different from conventional thyristors. Now we are talking about the full controllability of the element. The current-voltage characteristic (static) in the forward direction is the same as that of simple thyristors. That's just a direct current thyristor can pass a much larger value. Butthe function of blocking large reverse voltages for lockable thyristors is not provided. Therefore, it is necessary to connect it back-to-back with a semiconductor diode.
A characteristic feature of a lockable thyristor is a significant drop in forward voltages. To make a shutdown, a powerful current pulse (negative, in a ratio of 1:5 to the direct current value) should be applied to the control output. But only the pulse duration should be as short as possible - 10 … 100 μs. Lockable thyristors have a lower limiting voltage and current than conventional ones. The difference is approximately 25-30%.
Types of thyristors
The lockable ones were discussed above, but there are many more types of semiconductor thyristors that are also worth mentioning. A wide variety of designs (chargers, switches, power regulators) use certain types of thyristors. Somewhere it is required that the control be carried out by supplying a stream of light, which means that an optothyristor is used. Its peculiarity lies in the fact that the control circuit uses a semiconductor crystal that is sensitive to light. 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 understand what types of these semiconductors exist and where they can be used. So, here is the whole list and the main features of each type:
- Diode-thyristor. The equivalent of this element is a thyristor, to which it is connected in anti-parallelsemiconductor diode.
- Dinistor (diode thyristor). It can become fully conductive if a certain voltage level is exceeded.
- Triac (symmetric thyristor). Its equivalent is two thyristors connected back-to-back.
- The high-speed inverter thyristor has a high switching speed (5… 50 µs).
- Field transistor controlled thyristors. You can often find designs based on MOSFETs.
- Optical thyristors controlled by light fluxes.
Implement element protection
Thyristors are devices that are critical to the forward current and forward voltage slew rates. They, like semiconductor diodes, are characterized by such a phenomenon as the flow of reverse recovery currents, which very quickly and sharply drops to zero, thereby aggravating the likelihood of overvoltage. This overvoltage is a consequence of the fact that the current stops abruptly in all circuit elements that have inductance (even ultra-low inductances typical for installation - wires, board tracks). To implement protection, it is necessary to use a variety of schemes that allow you to protect yourself from high voltages and currents in dynamic operating modes.
As a rule, the inductive resistance of the voltage source that enters the circuit of a working thyristor has such a value that it is more than enough to not include some additionalinductance. For this reason, in practice, a switching path formation chain is more often used, which significantly reduces the speed and level of overvoltage in the circuit when the thyristor is turned off. Capacitance-resistive circuits are most commonly used for this purpose. They are connected with the thyristor in parallel. There are quite a few types of circuit modifications of such circuits, as well as methods for their calculation, parameters for the operation of thyristors in various modes and conditions. But the circuit for forming the switching trajectory of the gated thyristor will be the same as that of transistors.
