What is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure includes four levels of semiconductor components, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely 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 widely used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any semiconductor device is normally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The operating condition of the thyristor is the fact when a forward voltage is used, 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 also the cathode is connected to the negative pole of the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), and also the indicator light does not glow. This implies that the thyristor is not conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used for the control electrode (referred to as a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is turned on, even when the voltage in the control electrode is taken off (that is, K is turned on again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, so that you can shut down the conductive thyristor, the power supply Ea has to be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used involving the anode and cathode, and also the indicator light does not glow at this time. This implies that the thyristor is not conducting and may reverse blocking.
- In conclusion
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is within 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 will simply conduct if the gate is subjected to a forward voltage. At this time, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) If the thyristor is turned on, as long as there is a specific forward anode voltage, the thyristor will always be turned on whatever the gate voltage. Which is, right after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact a forward voltage should be applied involving the anode and also the cathode, plus an appropriate forward voltage also need to be applied involving the gate and also the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode has to be shut down, or the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made up of three PN junctions. It could be equivalently viewed as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is used involving the anode and cathode of the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. If a forward voltage is used for the control electrode at this time, BG1 is triggered to create a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be brought in the collector of BG2. This current is delivered to BG1 for amplification and after that delivered 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 within the emitters of these two transistors, that is, the anode and cathode of the thyristor (how big the current is actually based on how big the stress and how big Ea), therefore the thyristor is totally turned on. This conduction process is done in a very short time.
- Right after the thyristor is turned on, its conductive state is going to be maintained through the positive feedback effect of the tube itself. Whether or not the forward voltage of the control electrode disappears, it is still within the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to change on. When the thyristor is turned on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor would be to reduce the anode current that it is inadequate to keep the positive feedback process. How you can reduce the anode current would be to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to maintain the thyristor within the conducting state is referred to as the holding current of the thyristor. Therefore, as it happens, as long as the anode current is less than the holding current, the thyristor could be turned off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made up of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current at the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mainly utilized in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Means of working
The transistor controls the collector current by holding the base current to achieve current amplification.
The thyristor is turned on or off by managing the trigger voltage of the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors can be utilized in similar applications sometimes, due to their different structures and operating principles, they have noticeable differences in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors can be utilized in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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