Thyristors are power electronic keys that are 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 transferred to one state - conducting. Specifically, it includes a chain. 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 when closed, it can withstand not only direct, but also reverse voltage. The thyristor structure is four-layer, there are three conclusions:
- Anode (indicated by the letter A).
- Cathode (letter C or K).
- Control electrode (Y or G).
Thyristors have a whole family of current-voltage characteristics, which can be used to judge the state of an 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 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 should be positive (with respect to the cathode). The duration of the transition process depends on the nature of the load (inductive, active), the amplitude and the slew rate in the control circuit of the current pulse, the temperature of the semiconductor crystal, as well as the applied current and voltage to the thyristors 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 voltage rise rate 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.
Shutdown Methods
Two types of thyristor switching can be distinguished:
- Natural.
- Forced.
And now in more detail about each type. Natural occurs when the thyristor operates in an alternating current circuit. Moreover, this switching occurs when the current drops to zero. But the forced switching can be a large number of different ways. Which thyristor control to choose, it is up to the circuit designer, but it's worth talking about each type separately.
The most characteristic way of forced switching is to connect a capacitor that has been pre-charged using a button (key). An LC circuit is included in the thyristor control circuit. This chain contains a fully charged capacitor. During the transient, 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 using any auxiliary thyristor. In this case, a discharge to the main (working) thyristor will occur. This will lead to the fact that the capacitor current directed towards the direct current of the main thyristor will help 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 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 equalizing their values, the thyristor turns off. After the current flows from the oscillatory circuit through the thyristor into a semiconductor diode. In this case, as long as the current flows, a certain 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 alternating current circuit, the following operations can be carried out:
- Turn on or off an active resistive or active circuit.
- Change the average and effective value of the current that passes through the load, thanks to the ability to adjust the moment of supply 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 anti-parallel connection. The current and average current values ββcan vary due to the fact that the moment of supply of the signal to the thyristors is different. In this case, the thyristor power must meet the minimum requirements.
Phase control method
With the phase control method with forced switching, the load is regulated by changing the angles between the phases. Artificial switching can be carried out using special circuits, or it is necessary to use fully controllable (lockable) thyristors. Based on them, as a rule, a charger is made on a thyristor, which allows you to adjust the current strength depending on the charge level of the battery.
Pulse Width Control
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 closure (during the entire transient process), the control signal is not supplied, therefore, the thyristors do not conduct current. When implementing phase control, the current curve is not sinusoidal, a change in the waveform of the supply voltage occurs. Consequently, there is also a malfunction of consumers who are sensitive to high-frequency interference (incompatibility appears). A simple design has a thyristor regulator, which without problems will allow you to change the required value. And no need to use massive LATRs.
Lockable thyristors
Thyristors are very powerful electronic keys, used for switching high voltages and currents. But they have one huge drawback - the 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 decrease to zero.
It is this feature that imposes some restrictions on the use of thyristors, and also complicates the circuits based on them. To get rid of such shortcomings, special designs of thyristors have been 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 structure of r-p-p-p in thyristors has its own characteristics. They give them differences from conventional thyristors. It is now about the complete controllability of the element. The current-voltage characteristic (static) in the forward direction is the same as that of simple thyristors. Here are just a direct current thyristor can pass much greater in value. But the function of blocking large reverse voltages for lockable thyristors is not provided. Therefore, it is necessary to connect it counter-parallel with a semiconductor diode.
A characteristic feature of a lockable thyristor is a significant drop in direct voltage. To make a shutdown, a powerful current pulse (negative, in the ratio of 1: 5 to the direct current value) should be supplied to the control output. But only the pulse duration should be as short as possible - 10 ... 100 ΞΌs. Lockable thyristors have a lower limit voltage and current than conventional thyristors. The difference is approximately 25-30%.
Types of Thyristors
Lockable ones were considered above, but there are still many types of semiconductor thyristors, which are also worth mentioning. In a wide variety of designs (chargers, switches, power regulators), certain types of thyristors are used. Somewhere, it is required that control be carried out by applying a stream of light, which means that an optothyristor is used. Its peculiarity lies in the fact that a light-sensitive semiconductor crystal is used in the control circuit. The parameters of the thyristors are different, all have their own characteristics, characteristic only for them. Therefore, it is necessary, at least in general terms, to imagine 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:
- Thyristor diode. The equivalent of this element is a thyristor, to which a counter-parallel semiconductor diode is connected.
- Dinistor (diode thyristor). It can become fully conductive if a certain voltage level is exceeded.
- Triac (symmetric thyristor). Its equivalent is two thyristors connected in counter-parallel.
- The inverter high-speed thyristor is characterized by a high switching speed (5 ... 50 ΞΌs).
- Thyristors with field effect transistor control . Often you can find designs based on MOS transistors.
- Optical thyristors that are controlled by the flow of light.
Protecting an item
Thyristors are devices that are critical to the slew rate of forward current and forward voltage. For them, as well as for semiconductor diodes, such a phenomenon as the occurrence of reverse recovery currents, which very quickly and sharply drops to zero, is typical, thereby increasing the likelihood of overvoltage. This overvoltage is a consequence of the fact that the current in all elements of the circuit that have an inductance (even the ultra-small inductances characteristic of the installation - wires, circuit boards) stops abruptly. To implement the protection, it is necessary to use a variety of schemes that allow in dynamic operating modes to be protected from high voltages and currents.
As a rule, the inductive resistance of a voltage source, which is included in the circuit of a working thyristor, has such a value that it is more than enough to not include any additional inductance in the circuit in the future. For this reason, in practice, the switching path formation chain is often used, which significantly reduces the speed and level of overvoltage in the circuit when the thyristor is turned off. Capacitive resistive chains are most often used for these purposes. They turn on 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 path of the lockable thyristor will be the same as that of transistors.