- a device with the properties of a semiconductor, the design of which is based on a single-crystal semiconductor having three or more p-n junctions.
His work implies the presence of two stable phases:
- "closed" (conductivity level is low);
- "open" (conductivity levels are high).
Thyristors are devices that perform the functions of power electronic switches. Their other name is single-operation thyristors. This device allows you to regulate the impact of powerful loads by means of minor impulses.
According to the current-voltage characteristic of the thyristor, an increase in the current strength in it will provoke a decrease in voltage, that is, a negative differential resistance will appear.
In addition, these semiconductor devices can combine circuits with voltages up to 5000 volts and currents up to 5000 amperes (at a frequency of not more than 1000 Hz).
Thyristors with two and three outputs are suitable for operation with both direct and alternating current. Most often, the principle of their operation is compared with the operation of a rectification diode and it is believed that they are a full-fledged analog of a rectifier, in a sense even more efficient.
Varieties of thyristors differ from each other:
- Way of management.
- Conductivity (unilateral or bilateral).
General principles of management
The thyristor structure has 4 semiconductor layers in series connection (p-n-p-n). The contact connected to the outer p-layer is the anode, and the contact connected to the outer n-layer is the cathode. As a result, with a standard assembly, the thyristor can have a maximum of two control electrodes, which are attached to the inner layers. According to the connected layer, the conductors are divided into cathode and anode devices according to the type of control. The first type is more commonly used.
The current in thyristors flows towards the cathode (from the anode), therefore, the connection with the current source is made between the anode and the positive terminal, as well as between the cathode and the negative terminal.
Thyristors with a control electrode can be:
- lockable;
- Unlockable.
An indicative property of non-lockable devices is their lack of response to a signal from the control electrode. The only way to close them is to reduce the level of current flowing through them so that it is inferior to the strength of the holding current.
When controlling a thyristor, some points should be taken into account. A device of this type changes the phases of operation from “off” to “on” and back abruptly and only under the condition of external influence: using current (voltage manipulation) or photons (in cases with a photothyristor).
To understand this point, it must be remembered that the thyristor mainly has 3 outputs (trinistor): anode, cathode and control electrode.
Ue (control electrode) is just the same and is responsible for turning the thyristor on and off. The opening of the thyristor occurs under the condition that the applied voltage between A (anode) and K (cathode) becomes equal to or exceeds the voltage volume of the trinistor. True, in the second case, the impact of a pulse of positive polarity between Ue and K will be required.
With a constant supply of voltage, the thyristor can be opened indefinitely.
To put it in a closed state, you can:
- Reduce the voltage level between A and K to zero;
- Lower the value of the A-current so that the values of the holding current are greater;
- If the circuit is built on the action of alternating current, the device will turn off without outside interference when the current level itself drops to zero;
- Apply a blocking voltage to Ue (relevant only for lockable varieties of semiconductor devices).
The closed state also lasts indefinitely until a triggering impulse occurs.
Specific control methods
- Amplitude .
Represents the supply of a positive voltage of varying magnitude to Ue. The opening of the thyristor occurs when the voltage is sufficient to break through the control transition of the rectification current (Isp.). By changing the voltage value on Ue, it becomes possible to change the thyristor opening time.
The main disadvantage of this method is the strong influence of the temperature factor. In addition, each type of thyristor will require a different type of resistor. This moment does not add convenience in operation. In addition, the opening time of the thyristor can only be corrected while the first 1/2 of the positive half-cycle of the network lasts.
- Phase.
It consists in changing the phase Ucontrol (in relation to the voltage at the anode). In this case, a phase shifting bridge is used. The main disadvantage is the low steepness of Ucontrol, therefore, it is possible to stabilize the opening moment of the thyristor only for a short time.
- Phase-pulse .
Designed to overcome the shortcomings of the phase method. For this purpose, a voltage pulse with a steep front is applied to Ue. This approach is currently the most common.
Thyristors and safety
Due to the impulsivity of their action and the presence of a reverse recovery current, thyristors greatly increase the risk of overvoltage in the operation of the device. In addition, the danger of overvoltage in the semiconductor zone is high if there is no voltage at all in other parts of the circuit.
Therefore, in order to avoid negative consequences, it is customary to use TFTP schemes. They prevent the appearance and retention of critical voltage values.
Two-transistor thyristor model
It is quite possible to assemble a dinistor (thyristor with two leads) or a trinistor (thyristor with three leads) from two transistors. To do this, one of them must have p-n-p conductivity, the other - n-p-n-conductivity. Transistors can be made of both silicon and germanium.
The connection between them is carried out through two channels:
- Anode from the 2nd transistor + Control electrode from the 1st transistor;
- Cathode from the 1st transistor + Control electrode from the 2nd transistor.
If you do without the use of control electrodes, then the output will be a dinistor.
The compatibility of the selected transistors is determined by the same amount of power. In this case, the current and voltage readings must necessarily be greater than those required for the normal functioning of the device. The breakdown voltage and holding current data depend on the specific qualities of the transistors used.
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