Тиристоры – это силовые электронные ключи, not fully managed. 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 transformed into one state — conductive. More specifically, it includes a circuit. To turn it off, it is necessary to create special conditions that ensure the forward current in the circuit drops to zero.
Thyristor features
Тиристорные ключи проводят электрический ток only in the forward direction, and in the closed state it can withstand not only direct, but also reverse voltage. The structure of the thyristor is four-layer; there are three conclusions:
- Anode (denoted by the letter A).
- Cathode (letter C or K).
- Control electrode (V or G).
У тиристоров есть целое семейство вольт-амперных characteristics, it is possible 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 intensity is 5000 amperes (the frequency does not exceed 1000 Hz).
Thyristor operation in DC circuits
An ordinary thyristor is turned on by applying a currentimpulse to control output. Moreover, it should be positive (in relation to the cathode). The duration of the transient depends on the nature of the load (inductive, active), the amplitude and rate of rise in the control circuit of the current pulse, the temperature of the semiconductor crystal, as well as the applied current and voltage on the thyristors present in the circuit. The characteristics of the circuit are directly dependent 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.
А теперь более подробно о каждом виде.Natural occurs when the thyristor is operating in an alternating current circuit. Moreover, this switching occurs when the current drops to zero. But to force the switching can be a large number of different ways. What control of the thyristor to choose, it is up to the developer of the circuit, but it is worth talking about each type separately.
The most characteristic way to forceswitching is the connection of a capacitor, which was pre-charged with a button (key). The LC circuit is included in the thyristor control circuit. This chain also contains the charged completely condenser. During a transient, current fluctuations occur in the load circuit.
Forced Switching Methods
There are several other types of forcedcommutation. Often, a circuit is used in which a switching capacitor having a reverse polarity is used. For example, this capacitor can be included in the circuit with the help of some auxiliary thyristor. When this happens the discharge to the main (working) thyristor. This will lead to the fact that the current in the capacitor, directed towards the forward current of the main thyristor, will reduce the current in the circuit down to zero. Consequently, the thyristor will turn off. This happens for the reason that the thyristor device has its own characteristics, characteristic only for it.
There are also circuits in which to connectLC chains. They are discharged (and with vibrations). At the very beginning, the discharge current flows towards the worker, and after equalizing their values, the thyristor is turned off. After the current flows from the oscillatory circuit through the thyristor into the semiconductor diode. At the same time, as long as the current flows, some voltage is applied to the thyristor. It is equal in magnitude to the voltage drop across the diode.
Thyristor operation in AC circuits
If the thyristor is included in the AC circuit, you can perform the following operations:
- Enable or disable an electrical circuit with an active resistive or active load.
- Change the average and effective value of the current that passes through the load, due to the ability to regulate the time of the control signal.
The thyristor keys have one feature -they conduct current only in one direction. Therefore, if it is necessary to use them in AC circuits, it is necessary to apply counter-parallel connection. The current and average values of the current may vary due to the fact that the time of the signal to the thyristors is different. The power of the thyristor must meet the minimum requirements.
Phase control method
При фазовом методе управления с коммутацией forced type, the load is adjusted by changing the angles between the phases. Artificial switching can be done using special circuits, or else it is necessary to use fully controlled (lockable) thyristors. On their basis, as a rule, a charger is made on the thyristor, which allows you to adjust the amperage 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. Transitions are open, and there is some voltage on the load. During closing (during the entire transition process) the control signal is not given, therefore, the thyristors do not conduct current. When performing phase control, the current curve is not sinusoidal; the voltage waveform changes. Consequently, there is also a disruption in the work of consumers who are sensitive to high-frequency interference (incompatibility appears). A simple design has a regulator on the thyristor, which without problems will allow you to change the required value. And you do not need to use massive LATRY.
Lockable thyristors
Thyristors are very powerful electronic keys,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 direct current will drop to zero.
It is this feature that imposes somerestrictions on the use of thyristors, and also complicates schemes based on them. To get rid of this kind of flaws, special designs of thyristors were developed, which are locked by a signal across one control electrode. They are called two-step, or lockable, thyristors.
Lockable thyristor design
Four-layer structure of pnp thyristorshas its own characteristics. They give them differences from ordinary thyristors. We are now talking about the complete controllability of the element. Volt-ampere characteristic (static) in the forward direction is the same as that of simple thyristors. Here are just a direct current thyristor can skip to a much greater value. But the function of blocking large reverse voltages of lockable thyristors is not provided. Therefore, it is necessary to connect it in parallel with a semiconductor diode.
Характерная особенность запираемого тиристора – This is a significant drop in direct voltage. To make a shutdown, it is necessary to supply a powerful current pulse to the control terminal (negative, in a ratio of 1: 5 to the direct current value). But only the pulse duration should be as short as possible - 10 ... 100 μs. Lockable thyristors have a lower value of the limit voltage and current than conventional ones. The difference is about 25-30%.
Types of thyristors
Выше были рассмотрены запираемые, но существует There are still many types of semiconductor thyristors, which are also worth mentioning. Certain types of thyristors are used in various designs (chargers, switches, power regulators). Somewhere it is required that the control be carried out by supplying a stream of light, which means that the optothyristor is used. Its peculiarity is that the control circuit uses a semiconductor crystal sensitive to light. Thyristor parameters are different, all have their own characteristics, characteristic only for them. Therefore, it is necessary at least in general terms to represent what types of these semiconductors exist and where they can be used. So, here is the entire list and the main features of each type:
- Diode-thyristor. The equivalent of this element is a thyristor to which a semiconductor diode is connected anti-parallel.
- Dinistor (diode thyristor). It can go into full conduction if a certain voltage level is exceeded.
- Triac (symmetrical thyristor). Its equivalent is two thyristors connected in parallel.
- Inverter high-speed thyristor is characterized by high switching speed (5 ... 50 μs).
- Thyristors with field-effect transistor control. Often you can find designs based on MOS transistors.
- Optical thyristors that are controlled by streams of light.
Security element
Тиристоры – это приборы, которые критичны к direct current and direct voltage rise rates. For them, as well as for semiconductor diodes, such a phenomenon as the flow of reverse recovery currents, which very quickly and sharply drops to zero, is characteristic, thus aggravating the likelihood of overvoltage. This overvoltage is a consequence of the fact that the current in all elements of the circuit that have inductance (even ultralow inductances typical for installation — wires, circuit boards) abruptly cease. For the implementation of protection, it is necessary to use a variety of schemes that allow in dynamic modes of operation to protect against high voltages and currents.
Typically, the inductive impedance of the sourcevoltage, which is included in the circuit of a working thyristor, has such a value that it is more than enough to no longer include in the circuit some additional inductance. For this reason, in practice, a switching path formation chain is often used, which significantly reduces the speed and level of overvoltage in the circuit when the thyristor is disconnected. Hollow chains are most often used for this purpose. They are included with the thyristor in parallel. There are quite a few types of circuit design modifications of such circuits, as well as methods of their calculation, parameters for the operation of thyristors in various modes and conditions. But the chain of formation of the switching trajectory of the locked thyristor will be the same as that of transistors.
