So what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of 4 levels of semiconductor elements, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are the critical parts in 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 working status. Therefore, thyristors are commonly 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 generally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition in the thyristor is the fact that whenever a forward voltage is used, the gate will need to have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is used between the anode and cathode (the anode is attached to the favorable pole in the power supply, and the cathode is connected to the negative pole in the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), and the indicator light does not light up. This shows that the thyristor will not be 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 towards the control electrode (called a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is switched on, even when the voltage on the control electrode is taken away (that is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. At the moment, to be able to 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 towards the control electrode, a reverse voltage is used between the anode and cathode, and the indicator light does not light up currently. This shows that the thyristor will not be conducting and may reverse blocking.
- In conclusion
1) Once 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) Once the thyristor is subjected to a forward anode voltage, the thyristor will only conduct when the gate is subjected to a forward voltage. At the moment, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is, the controllable characteristic.
3) Once the thyristor is switched on, as long as you will find a specific forward anode voltage, the thyristor will stay switched on regardless of the gate voltage. That is, following the thyristor is switched on, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, and the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for that thyristor to conduct is the fact that a forward voltage should be applied between the anode and the cathode, plus an appropriate forward voltage should also be applied between the gate and the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode has to be shut down, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode composed of three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) plus an NPN transistor (BG1).
- In case a forward voltage is used between the anode and cathode in the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor continues to be switched off because BG1 has no base current. In case a forward voltage is used towards the control electrode currently, BG1 is triggered to produce a base 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 sent to BG1 for amplification and after that sent to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A big current appears within the emitters of these two transistors, that is, the anode and cathode in the thyristor (how big the current is in fact dependant on how big the stress and how big Ea), so the thyristor is totally switched on. This conduction process is done in a very short time.
- Following the thyristor is switched on, its conductive state will be maintained through the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to transform on. After the thyristor is switched on, the control electrode loses its function.
- The best way to switch off the turned-on thyristor would be to decrease the anode current that it is not enough to keep up the positive feedback process. The way to decrease the anode current would be to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is known as the holding current in the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor may be switched off.
Exactly what is the difference between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of any transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage along with a trigger current in the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mostly found 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 accomplish current amplification.
The thyristor is switched on or off by managing the trigger voltage in the control electrode to realize the switching function.
The circuit parameters of thyristors are related to stability and reliability and usually have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications sometimes, because of the different structures and working principles, they have noticeable variations in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within 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 towards the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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