Specifically what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four levels of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are the critical parts of the thyristor, allowing it to control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their operating status. Therefore, thyristors are popular in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a silicon-controlled rectifier is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The operating condition of the thyristor is the fact each time a forward voltage is applied, the gate needs to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used involving the anode and cathode (the anode is attached to the favorable pole of the power supply, and the cathode is connected to the negative pole of the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), and the indicator light does not light up. This implies that the thyristor is not really conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied towards the control electrode (referred to as a trigger, and the applied voltage is known as trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is excited, even when the voltage around the control electrode is removed (which is, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, in order to cut off the conductive thyristor, the power supply Ea has to be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied towards the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light does not light up at the moment. This implies that the thyristor is not really conducting and can reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is in a reverse blocking state no matter what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct once the gate is subjected to a forward voltage. Currently, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will always be excited whatever the gate voltage. That is, after the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is the fact a forward voltage needs to be applied involving the anode and the cathode, as well as an appropriate forward voltage also need to be applied involving the gate and the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode has to be cut off, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made from three PN junctions. It could be equivalently viewed as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is applied involving the anode and cathode of the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is applied towards the control electrode at the moment, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will be introduced the collector of BG2. This current is brought to BG1 for amplification then brought to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, which is, the anode and cathode of the thyristor (the size of the current is in fact based on the size of the stress and the size of Ea), and so the thyristor is completely excited. This conduction process is finished in a very short period of time.
- After the thyristor is excited, its conductive state will be maintained from the positive feedback effect of the tube itself. Even when the forward voltage of the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is always to decrease the anode current that it is not enough to keep the positive feedback process. The way to decrease the anode current is always to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep the thyristor in the conducting state is known as the holding current of the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor can be turned off.
What is the difference between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of a transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current on the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, along with other aspects of electronic circuits.
Thyristors are mainly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Way of working
The transistor controls the collector current by holding the base current to accomplish current amplification.
The thyristor is excited or off by managing the trigger voltage of the control electrode to realize the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors may be used in similar applications in some cases, due to their different structures and operating principles, they have got noticeable differences in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors may be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow towards the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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