Thyristors for Dummies
Good evening habr. Let's talk about such a device as a thyristor. Thyristor is a semiconductor device with two stable states, having three or more interacting rectifying transitions. In terms of functionality, they can be correlated to electronic keys. But there is one feature in the thyristor, it can not go into the closed state, unlike the usual key. Therefore, it can usually be found under the name - not fully managed key.
The figure shows the usual type of thyristor. It consists of four alternating types of electrical conductivity of the semiconductor regions and has three terminals: an anode, a cathode, and a control electrode.
The anode is a contact with the outer p-layer, the cathode is with the outer n-layer.
Refresh the memory of the pn junction here .
Depending on the number of outputs, you can derive a classification of thyristors. In fact, everything is very simple: a two-pin thyristor is called a dynistor (respectively, it only has an anode and a cathode). A thyristor with three and four leads is called triode or tetrode. There are also thyristors with a large number of alternating semiconductor areas. One of the most interesting is a symmetrical thyristor (triac), which is turned on for any voltage polarity.
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Typically, the thyristor is represented as two transistors connected to each other, each of which operates in active mode.
In connection with such a pattern, extreme regions can be called emitter regions, and the central transition is a collector region.
To understand how the thyristor works, you should look at the current-voltage characteristic.
A small positive voltage was applied to the thyristor anode. Emitter transitions are included in the forward direction, and the collector in the opposite direction. (in fact, all the voltage will be on it). The section from zero to one on the current-voltage characteristic will be approximately analogous to the reverse branch of the diode characteristic. This mode can be called - the closed state of the thyristor.
As the anode voltage increases, the main carriers are injected in the base region, thereby accumulating electrons and holes, which is equivalent to the potential difference at the collector junction. With increasing current through the thyristor, the voltage at the collector junction will begin to decrease. And when it decreases to a certain value, our thyristor will go into a state of negative differential resistance (in Figure 1, section 1-2).
After that, all three transitions will shift in the forward direction, thereby turning the thyristor into the open state (section 2-3 in the figure).
In the open state, the thyristor will remain until the collector junction is displaced in the forward direction. If the thyristor current is reduced, the number of nonequilibrium carriers in the base regions will decrease as a result of recombination and the collector junction will be shifted in the opposite direction and the thyristor will go to the closed state.
When the thyristor is switched back on, the current-voltage characteristic will be similar to that of two series-connected diodes. The reverse voltage will be limited in this case to the breakdown voltage.
1. The turn - on voltage is the minimum anode voltage at which the thyristor goes into the on state.
2. Forward voltage is the forward voltage drop at the maximum current of the anode.
3. Reverse voltage is the maximum allowable voltage on the thyristor in the closed state.
4. The maximum allowed direct current is the maximum current in the open state.
5. Reverse Current - current at maximum reverse voltage.
6. Maximum electrode control current
7. Delay on / off time
8. Maximum permissible power dissipation
Thus, there is a positive current feedback in the thyristor - an increase in current through one emitter junction leads to an increase in current through another emitter junction.
Thyristor - not fully control key. That is, going into the open state, it remains in it even if you stop giving a signal to the control transition, if a current is applied above a certain value, that is, the holding current.
Sources:
ru.wikipedia.org
electricalschool.info
The figure shows the usual type of thyristor. It consists of four alternating types of electrical conductivity of the semiconductor regions and has three terminals: an anode, a cathode, and a control electrode.
The anode is a contact with the outer p-layer, the cathode is with the outer n-layer.
Refresh the memory of the pn junction here .
Classification
Depending on the number of outputs, you can derive a classification of thyristors. In fact, everything is very simple: a two-pin thyristor is called a dynistor (respectively, it only has an anode and a cathode). A thyristor with three and four leads is called triode or tetrode. There are also thyristors with a large number of alternating semiconductor areas. One of the most interesting is a symmetrical thyristor (triac), which is turned on for any voltage polarity.
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Principle of operation
Typically, the thyristor is represented as two transistors connected to each other, each of which operates in active mode.
In connection with such a pattern, extreme regions can be called emitter regions, and the central transition is a collector region.
To understand how the thyristor works, you should look at the current-voltage characteristic.
A small positive voltage was applied to the thyristor anode. Emitter transitions are included in the forward direction, and the collector in the opposite direction. (in fact, all the voltage will be on it). The section from zero to one on the current-voltage characteristic will be approximately analogous to the reverse branch of the diode characteristic. This mode can be called - the closed state of the thyristor.
As the anode voltage increases, the main carriers are injected in the base region, thereby accumulating electrons and holes, which is equivalent to the potential difference at the collector junction. With increasing current through the thyristor, the voltage at the collector junction will begin to decrease. And when it decreases to a certain value, our thyristor will go into a state of negative differential resistance (in Figure 1, section 1-2).
After that, all three transitions will shift in the forward direction, thereby turning the thyristor into the open state (section 2-3 in the figure).
In the open state, the thyristor will remain until the collector junction is displaced in the forward direction. If the thyristor current is reduced, the number of nonequilibrium carriers in the base regions will decrease as a result of recombination and the collector junction will be shifted in the opposite direction and the thyristor will go to the closed state.
When the thyristor is switched back on, the current-voltage characteristic will be similar to that of two series-connected diodes. The reverse voltage will be limited in this case to the breakdown voltage.
General parameters of thyristors
1. The turn - on voltage is the minimum anode voltage at which the thyristor goes into the on state.
2. Forward voltage is the forward voltage drop at the maximum current of the anode.
3. Reverse voltage is the maximum allowable voltage on the thyristor in the closed state.
4. The maximum allowed direct current is the maximum current in the open state.
5. Reverse Current - current at maximum reverse voltage.
6. Maximum electrode control current
7. Delay on / off time
8. Maximum permissible power dissipation
Conclusion
Thus, there is a positive current feedback in the thyristor - an increase in current through one emitter junction leads to an increase in current through another emitter junction.
Thyristor - not fully control key. That is, going into the open state, it remains in it even if you stop giving a signal to the control transition, if a current is applied above a certain value, that is, the holding current.
Sources:
ru.wikipedia.org
electricalschool.info
Source: https://habr.com/ru/post/165111/
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